Citation: Pande, S., Bock, C.H., Bandyopadhyay, R., Narayana, YD., Reddy, B.V.S., Lenne, J.M., and Jeger, M.J. 1997. Downy mildew of sorghum. (in En., summaries in Fr., Es.) Information Bulletin no. 51. Patancheru 502 324, Andhra Pradesh, India. International Crops Research Institute for the Semi-Arid Tropics. pp. 32. ISBN 92-9066-379-0. Order code: IBE 051

Abstract

Downy mildew of sorghum (Peronosclerospora sorghi) is one of the most important diseases of sorghum and maize. It is distributed widely in Africa, Asia, and the Americas, where serious epidemics occur. Systemic infection causes complete or partial sterility of the panicle, resulting in yield loss roughly proportional to the disease incidence. The seriousness of the problem has resulted in significant investment to increase knowledge of the pathogen and to investigate various disease control measures. Several alternatives for the control of sorghum downy mildew are now available including cultural and chemical control, and the deployment of resistant varieties. The current state of knowledge of the pathogen's biology, epidemiology, variability, and control are described in this bulletin, together w i th practical aspects of disease management.

Resumen

The research activities were partially supported by the Asian Development Bank, and the Governments of Australia, Germany, Italy, Norway, South Africa, Switzerland, UK, and USA.

Copyright© 1997 by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT).

All rights reserved. Except for quotations of short passages for the purposes of criticism and review, no part of this publication may be reproduced, stored in retrieval systems, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior permission of ICRISAT. It is hoped that this Copyright declaration will not diminish the bona fide use of its research findings in agricultural research and development in or for the tropics.

Downy Mildew of Sorghum

Information Bulletin no. 51

S Pande, C H Bock, R Bandyopadhyay, Y D Narayana, B V S Reddy, J M Lenne

and M J Jeger

ICRISAT

International Crops Research Institute for the Semi-Arid Tropics Patancheru 502 324, Andhra Pradesh, India

1997

About the Authors

Suresh Pande Senior Scientist (Patho logy) , ICRISAT, Patancheru 502 324, A n d h r a Pradesh, I n d i a .

Cl ive H Bock Formerly Research Fel low, C r o p Protec t ion D i v i s i o n , ICRISAT, Patancheru 502 324, A n d h r a Pradesh, I nd ia . Presently, Research P lant Patho log is t , U n i t e d States D e p a r t m e n t o f A g r i c u l ­ture, A g r i c u l t u r a l Research Service, Sou thern Regiona l Research Center ( U S D A - A R S - S R R C ) , 1100 Rober t E Lee B o u l e v a r d , N e w Or leans, LA 70115, USA.

Ranaj i t Bandyopadhyay Senior Scientist (Patho logy) , ICRISAT, Patancheru 502 324, A n d h r a Pradesh, I nd ia .

Y D Narayana Formerly Senior Research Associate, C r o p Protec t ion D i v i s i o n , ICRISAT, Pa tancheru 502 324, A n d h r a Pradesh, I nd ia . Presently, Associate Professor o f P lant Pathology, U n i v e r s i t y o f A g ­r i c u l t u r a l Sciences, A g r i c u l t u r a l Research Sta t ion , A l a n d Road , Gu lba rga 585 101 , Karna taka , I nd i a .

Belum V S Reddy Senior Scientist (Breeding) , ICRISAT, Patancheru 502 324, A n d h r a Pradesh, I nd ia .

Ji l l ian M Lenne Formerly Director, C r o p Protect ion D i v i s i o n , ICRISAT, Patancheru 502 324, A n d h r a Pradesh, Ind ia . Presently, Consu l tan t , In ternat iona l G e r m p l a s m Associates, 13 H e r o n ' s Quay , Sandside, M i l n t h o r p e , C u m b r i a , L A 7 7 H N , U K

M i k e J Jeger Professor of Ecological P lant Pathology, D e p a r t m e n t o f Phy topa tho logy , A g r i c u l t u r a l U n i v e r s i t y o f Wagen ingen , POB 8025,6700 EE, Wagen ingen , The Ne the r lands .

About the Reviewer

Richard A Frederiksen Professor, Depa r tmen t of P lant Pa tho logy and M ic rob io logy , Co l lege of A g r i c u l t u r e and L i fe Sciences, R o o m 120, L F Peterson B u i l d i n g , Co l lege Sta t ion , Texas 77843-2132, U S A .

ICRISAT thanks Professor Freder iksen for c r i t i ca l l y r e v i e w i n g the manusc r i p t .

The designations employed and the presentation of the material in this publication do not imply the expression of any opinion whatsoever on the part of ICRISAT concerning the legal status of any country, territory, city, or area, or of its authorities, or concerning the delimitation of its frontiers or boundaries. Where trade names are used this does not constitute endorsement of or discrimination against any product by the Institute.

Contents I n t r o d u c t i o n

Economic damage

O r i g i n s a n d geograph ic d i s t r i b u t i o n

Causa l o rgan i sm

Disease s y m p t o m s

Systemic in fec t ion

Local lesions

E p i d e m i o l o g y and b i o l o g y

C o n i d i a p r o d u c t i o n , d ispersa l , and in fec t ion

Oospore p r o d u c t i o n , d ispersa l , and in fec t ion

Co l la te ra l hosts

Seed t ransmiss ion

Seasonal disease d e v e l o p m e n t

Var iab i l i t y w i t h i n P. sorghi

C o n t r o l

Chemica l con t ro l

C u l t u r a l con t ro l

C r o p ro ta t i on

Deep t i l lage

O v e r - s o w i n g and r o u g i n g diseased p lants

S o w i n g date

Effect o f host p l an t n u t r i t i o n

B io log ica l con t ro l

Hos t -p lan t resistance

Iden t i f i ca t i on of resistance

Screening techniques

Inher i tance of resistance

Resistance sources in b reed ing , and resistant var iet ies used

In tegra ted con t ro l

Techno logy Transfer a n d A d o p t i o n

References

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Introduction S o r g h u m (Sorghum bicolor L. Moench.) is an i m ­p o r t a n t c rop for b o t h h u m a n c o n s u m p t i o n and a n i m a l fodder . M u c h o f the crop's p r o d u c t i o n area is in the semi -a r id reg ions of the less-deve loped w o r l d , i n c l u d i n g A f r i ca , As ia , and Sou th A m e r i c a , a l t h o u g h there is also substan­t ia l s o r g h u m p r o d u c t i o n in the USA. The cur­ren t w o r l d w i d e t rends i n s o r g h u m p r o d u c t i o n since 1961 suggest that s o r g h u m w i l l r ema in an i m p o r t a n t c rop in these regions.

The s o r g h u m d o w n y m i l d e w (SDM) fungus (Peronosclerospora sorghi [Weston a n d Uppal (Shaw)], infects b o t h s o r g h u m and ma ize (Zea mays L.), and is w idesp read in m a n y t rop ica l and sub- t rop ica l regions o f the w o r l d whe re s o r g h u m and ma ize crops are g r o w n . But ler (1907) was the f i rs t to repo r t d o w n y m i l d e w o f s o r g h u m . Howeve r , the t rue taxonomic status of the pa thogen as P. sorghi was sat is factor i ly establ ished m u c h later (Weston and U p p a l 1932; Shaw 1976,1978, and 1980). I t has caused severe ep idemics i n b o t h s o r g h u m and ma ize crops i n m a n y count r ies (Kenne th 1976, W i l l ­i ams 1984).

Ef fect ive disease con t ro l t h r o u g h w e l l -i n f o r m e d disease managemen t strategies can he lp con t r ibu te to sustainable f o o d p r o d u c t i o n in endemic areas, pa r t i cu la r l y those that sup­p o r t a b u r g e o n i n g p o p u l a t i o n . Th is bu l l e t i n covers m a n y aspects of the b io logy, e p i d e m i ­ology, and con t ro l o f S D M - b u t does no t c la im to be comprehens ive . I t is i n t ended to act as an i n t r o d u c t i o n to the disease and to p r o v i d e i n ­f o r m a t i o n of interest and use to fa rmers , exten­s ion agents, students, and researchers. A l ­t h o u g h P. sorghi infects b o t h s o r g h u m and ma ize th is b u l l e t i n concentrates on the disease o f s o r g h u m , and o n l y consider i n fec t ion o f ma ize w h e r e i t is deemed necessary, pa r t i cu ­l a r l y i f i n f o r m a t i o n is l ack ing on a par t i cu la r aspect fo r s o r g h u m . The reader r e q u i r i n g mo re de ta i led i n f o r m a t i o n is d i rected t owards the source l i te ra ture .

Economic damage S o r g h u m d o w n y m i l d e w i s p a r t i c u l a r l y de ­s t ruc t ive since systemic i n fec t i on of the host results in a bar ren inf lorescence (Freder iksen et a l . 1973). There are m a n y repor ts of the effect of S D M on y i e l d . In a s ingle season in the U S A , a S D M ep idemic i n g r a i n s o r g h u m i n the coastal count ies of Texas caused an es t imated loss of US$ 2.5 m i l l i o n (Freder iksen et a l . 1969). Payak (1975) repor ted that in par ts o f I nd i a a n n u a l y i e ld loss due to S D M was at least 1.0 x 105

metr ic tons. In Venezuela, c rop losses were so severe in the ear ly 1970s that a na t i ona l emer­gency was dec lared (Freder iksen and Renf ro 1977). In Israel b o t h forage s o r g h u m and ma ize have been severely in fec ted w i t h inc idences up to 50% (Kenne th 1976), a n d in the U S A the i nc i ­dence can even be as h i g h as 90% in some f ie lds (Freder iksen et a l . 1969). M o d e l s have been deve loped that s h o w a l inear re la t i onsh ip ex­ists be tween the inc idence o f systemic S D M a n d y i e l d loss a t n o r m a l s o w i n g densi t ies (Frederiksen et a l . 1973, Tuleen and Freder iksen 1981, C r a i g et a l . 1989).

Origins and geographic distribution The S D M pa thogen has been c o n f i r m e d on a l l cont inents in t rop ica l and sub- t rop ica l zones o f the w o r l d (W i l l i ams 1984). Coun t r i es f r o m w h i c h i t has been repo r ted are l is ted in Table 1 , and its geograph ic d i s t r i b u t i o n is i l l us t ra ted in F igure 1. Peronosclerospora sorghi is cons idered a n 'O ld W o r l d ' pa thogen , h a v i n g o r i g i na ted i n A f r i ca or As ia (Shaw 1981, W i l l i a m s 1984). I t subsequent ly spread to the Amer i cas in the late 1950s, whe re i t was p r o b a b l y i n t r o d u c e d (Toler et a l . 1959, Freder iksen 1980a).

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Figure 1. Geographical distribution of Peronosclerospora sorghi showing regions where the sorghum/maize and maize infecting strains occur (the maize strain in Thailand has recently been designated Perono­sclerospora zeae (Yao 1991).

Figure 2. The asexual phase of Peronosclerospora sorghi. Conidia are sub-orbicular, hyaline, and thin-walled (x350).

Causal organism The f o l l o w i n g desc r ip t i on of P. sorghi is based on that o f Weston and U p p a l (1932). The f u n ­gus produces asexual con id ia (Fig. 2) and sexu­a l l y p r o d u c e d oospores (Fig. 3). Con id i a are p r o d u c e d on the leaf surface on erect con id io -phores that g r o w o u t t h r o u g h stomata. The con id i opho re compr ises a basal cel l and a mo re or less comp lex , usua l l y d i c h o t o m o u s l y b ranched , expanded apex (Fig. 4). The basal cell is knobbed or bu lbous at the base, then of fa i r l y u n i f o r m d iameter (7-9 μm) for a leng th of approx ima te l y 100-150 urn. I t is usua l ly d e l i m ­i ted f r o m the m a i n axis by a complete sep tum.

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Maize - infecting strains

Sorghum/maize - infecting strain

Table 1. Countries from which Perono-sclerospora sorghi has been reported1.

Region or continent Country

Africa Benin, Botswana, Burundi, Egypt, Ethiopia, Ghana, Kenya, Malawi , Mozambique, Mauritania (Frison and Sadio 1987), Nigeria, Rwanda, Somalia, South Africa, Sudan, Swaziland, Tanzania, Uganda, Zimbabwe, Zambia (de Mi l l iano 1992)

Asia2 Bangladesh, China, India, Japan, Pakistan, Philippines

Australia Australia (Queensland, Western Australia)

Nor th America Mexico, USA (Alabama, Arkansas, Georgia, Il l inois, Indiana, Kansas, Kentucky, Louisiana, Minnesota, Montana, Nebraska, New Mexico, Oklahoma, Tennessee, Texas)

Central America and El Salvador, Guatemala, the West Indies Honduras, Panama, Puerto

Rico

South America Argentina, Bolivia, Brazil, Colombia (Buritica et al. 1992), Uruguay, Venezuela

Midd le East Israel, Iran, Yemen

1. Unless otherwise indicated, all reports were obtained from the Commonwealth Mycological Institute Distribution Maps of Plant Diseases, Map no. 179, Edition 5, Issued 1 April 1988.

2. Previously a maize-infecting strain of P. sorghi was thought to occur in Thailand (Bonman et al. 1983). This race is now considered a separate species, P. zeae (Yao 1991).

Figure 3.The sexual phase of Peronosclerospora sorghi. Oospores are spherical and thick-walled (x 475).

Figure 4. A mature conidiophore of Peronosclerospora sorghi showing the

basal cell, the main body of the conidiophore, and the conidia

attached to the sterigmata (x375).

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The m a i n axis has a d iameter of 15-20 μm, and is usua l l y less t han , or equa l to , the basal cell in leng th (80-150 μm f r o m the s e p t u m of the basal cel l to the b e g i n n i n g of the b ranch sys­tem). B ranch ing of the con id iopho re is by a succession o f shor t , s tou t d i cho tomies usua l l y i n v o l v i n g p r ima ry , secondary, and te r t ia ry branches t e r m i n a t i n g i n taper ing s ter igmata a p p r o x i m a t e l y 13 μm l ong . The branches are a r ranged so the con id ia bo rne on the s ter ig­ma ta l ie app rox ima te l y in a hemispher ica l p lane. The con id ia are sub-orb icu la r (21-24.9 μm x 19.0-22.9 μ m ) , hya l ine , w i t h a t h i n w a l l , a n d ge rm ina te d i rec t l y by a h y p h a l ge rm- tube (F ig . 5).

Figure 6. Oospores of Peronosclerospora sorghi are typically produced in parallel bands between the fibro-vascular strands of the leaf (x 150).

Oospores are p r o d u c e d w i t h i n the leaf m e -sophy l l be tween the f ib ro-vascu lar b u n d l e s (Fig. 6). They are spher ica l , the m a j o r i t y b e i n g 31.0-36.9 μm in d iameter . The oospore w a l l is 1.1-2.7 μm th ick and o range -ye l l ow in color. The oospore conta ins f i ne ly g ranu la r ma te r i a l w i t h masses o f o i l g lobu les. The oospore ger­minates by means o f an nonsepta te , u s u a l l y b ranched , hya l i ne ge rm- tube , ave rag ing 4.4 μm in w i d t h .

Peronosclerospora sorghi is an ob l iga te pa ra ­site. Howeve r , i t has been successful ly g r o w n in d u a l cu l tu re w i t h host t issue on a m o d i f i e d Wh i te ' s m e d i u m (Kaver iappa e t a l . 1980). Bhat and G o w d a (1985) later descr ibed an i m ­p r o v e d m e t h o d for o b t a i n i n g con taminan t - f ree d u a l cu l tu res , b u t the inheren t p r o b l e m s o f m a i n t a i n i n g these cu l tures has p reven ted t h e m be ing w i d e l y used. M o s t cu l tu re ma in tenance therefore depends on i nocu la t i ng seedl ings o f the host w i t h con id ia o r oospores and u s i n g i n ­fected p lan ts as a source of i n o c u l u m (Cra ig 1976).

Figure 5. Conidia germinate by means of a single, unbranched hyphal germ-tube (x300).

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Disease symptoms

Systemic infection

Systemic in fec t ion (Fig. 7) can man i fes t i tsel f at any stage f r o m abou t one week after seed l ing emergence. The s y m p t o m s f i rs t appear as ch lo­rot ic areas emana t i ng f r o m the base of the f i rs t leaves s h o w i n g the in fec t ion . Th is o f ten covers hal f the lamina (the 'ha l f - lea f s y m p t o m , Fig. 8). Progress ive ly greater p ropo r t i ons of the l a m i n a o f y o u n g e r leaves s h o w this s y m p t o m u n t i l the who le lamina becomes chlorotic (Fig. 9). In cool , h u m i d weather the asexual reproduct ive s t ructures of the f ungus , i.e., the con id iophores

Figure 7. Sorghum plants showing typical symp­toms of systemic sorghum downy mildew infection. The leaves are chlorotic with some pale streaking (arrowed) on the younger leaves; the leaves tend to be narrow and the plants have an upright habit.

Figure 8. The typical 'half-leaf symptom first manifested in a sorghum plant systemically in­fected with sorghum downy mildew. The lower part of the leaf is infected and chlorotic, while the upper portion remains green and uninfected.

and con id ia , f o r m d u r i n g the n i g h t o n the leaves, pa r t i cu la r l y on the abax ia l surfaces. Th is g ives a w h i t e d o w n - l i k e appearance to the infected leaves (Fig. 10). As the p l a n t ages w h i t e , ch loro t ic streaks deve lop f r o m the base of the younge r leaves (F ig . 11), these t u r n pale to r e d d i s h - b r o w n as the i n te rve ina l t issue dies (Fig. 12), and oospores deve lop . As the streaks t u r n b r o w n they start t o shred i n t o l o n g s t r ips , the l am ina d is in tegrates a l ong the f i b ro -vascu-lar s t rands o f the leaf thus resu l t i ng in t yp i ca l ' l ea f -shredd ing ' s y m p t o m s (F ig . 13).

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Figure 9. The whole-leaf chlorosis that develops as the systemic infection progresses.

Figure 10. The downy appearance of leaves infected with sorghum downy mildew resulting from the asexual sporulation of Peronosclerospora sorghi.

Figure 11. The typical pale, chlorotic streaking of the younger leaves of a sorghum plant systemically infected with sorghum downy mildew. This symp­tom is indicative of the early stages of oospore pro­duction.

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Figure 12. As the oospores mature the leaf streaking darkens and becomes brown in color.

Figure 13. 'Leaf-shredding' typically observed in older sorghum plants infected with sorghum downy mildew. As the oospores reach maturity the leaves start to shred along the fibro-vascular strands, releasing the oospores into air currents that probably allow them to be dispersed from the host.

Plants that are systemica l ly in fected as seed­l ings r e m a i n s tun ted and o f ten d ie. Sy temica l l y in fec ted p lants are u p r i g h t i n hab i t , w i t h nar­r o w fo l iage, and are genera l ly bar ren (Fig. 14), a l t h o u g h some g r a i n m a y be p r o d u c e d (Fig. 15). Occas iona l ly a p lan t m a y recover and p r o ­duce heal thy, v iab le g ra in ( s y m p t o m remis ­s ion) , b u t the basis for th is p h e n o m e n o n is u n ­k n o w n (S ingh and de M i l l i a n o 1989b). The p ro ­d u c t i o n of a n o r m a l g ra in -bear ing panic le on a sys temica l ly in fected p l an t has also been re­p o r t e d (S ingh and de M i l l i a n o 1989a).

Local lesions

The local les ion phase (F ig . 16) can occur on any leaf o f an in fec ted s o r g h u m p lan t . Lesions deve lop as discrete ch loro t ic areas, var iab le in size, b u t genera l l y e longate w i t h para l le l edges (1-4 mm x 5-15 m m ) . Asexua l spores are p r o ­duced m o s t l y on the abaxia l surface o f leaves d i s p l a y i n g these lesions.

Epidemiology and biology The l i fe cycle of P. sorghi is s h o w n in F igure 17. Con id ia of P. sorghi are cop ious l y p r o d u c e d , t h i n - w a l l e d , and ephemera l , they can cause the r a p i d b u i l d - u p o f an ep idemic . Oospores are

Figure 14. Generally plants systemically infected with sorghum downy mildew have barren panicles (arrowed), hence no yield is produced.

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per iod of about 6 h (Lal 1981). D u r i n g this t ime h i g h relat ive h u m i d i t y (RH) is essential. Shetty and Safeeulla (1981) showed that sys temica l l y in fected s o r g h u m leaves he ld in the d a r k a t 20°C p r o d u c e d a m a x i m u m of 10,800 con id ia cnv2 a t 100% R H , b u t o n l y 3,600 con id ia c m 2 a t 85% R H , and none a t 80% R H . Leaf wetness i s another i m p o r t a n t factor i n f l u e n c i n g con id ia p roduc t ion . The o p t i m u m tempera ture fo r sporulat ion of P. sorghi on ma ize is 15-23°C (Bonde et a l . 1985). M a x i m u m spore ge rm ina ­t ion occurs at 15°C, and the greatest ge rm- tube g r o w t h a t 22°C, a l t h o u g h g e r m i n a t i o n is g o o d a t 10-19°C, and ge rm- tube g r o w t h is r a p i d a t 14-22°C. A d e w p e r i o d of abou t 4 h a n d t em­peratures o f 10-33°C s u p p o r t sys temic infec­t i o n (Bonde et a l . 1978). C o n i d i a are u s u a l l y p roduced be tween m i d n i g h t and 0500 h w h e n the tempera ture is abou t 20°C and the RH ex­ceeds 85% (Shenoi a n d R a m a l i n g a m 1979). Con id i a are d ispersed in air cur rents (Rajasab et a l . 1979). On a leaf, the c o n i d i u m germina tes

Figure 15. Systemically infected plants may have panicles that are partially fertile and produce some grain (arrowed). The amount of grain produced will depend on the proportion of the panicle colonized by the fungus.

t o u g h - w a l l e d and l o n g - l i v e d , and p r o v i d e b o t h a pe renna t i ng stage for the pa thogen and a m echan i sm for long-d is tance t ranspor t .

Conidia production, dispersal, and infection

The infected p lan t m u s t be subject to a precon­d i t i o n i n g pe r i od of l i gh t for at least 4 h p r i o r to p r o d u c i n g conid ia in the da rk (Schmit t and Freytag 1974). The conid iophores subsequent ly f o r m in the da rk f r o m w i t h i n the stomata over a

S

Figure 16. Typical local lesions on sorghum caused by infection with conidia of Peronosclerospora sorghi. The lesions can be seen as discreet tan areas on the leaf lamina.

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w h e n ma tu re , and the ge rm- tube g r o w s a t r i g h t angles to the l o n g axis of the leaf u n t i l i t encounters a s tomata. An app resso r ium fo rms over the s tomata l open ing , and the pene t ra t ing s t ructure enlarges to f o r m an ova l -shaped sub-s tomata l vesicle, that then gives r ise to one or mo re in fec t ion hyphae (Jones 1971a). S o r g h u m p lants are suscept ib le to systemic in fec t ion for o n l y 2 -3 weeks after emergence (Jones 1978). I f a systemic in fec t ion deve lops, in terce l lu lar hy ­phae proceed to the apical me r i s tem of the p lan t and i nvade the d e v e l o p i n g leaves and f l o w e r i n g par ts . S y m p t o m s can mani fes t t hem­selves a t any t ime up to f l o w e r i n g . Local les ion in fec t ions can occur at any stage, and take ap­p r o x i m a t e l y 7 days to deve lop (Cohen and Sherman 1977).

A t the p o i n t o f host co lon iza t ion , dif ferences are observed be tween resistant and susceptible cu l t i va rs ( M a u c h - M a n i et a l . 1989). In resistant cu l t i va rs necrosis occurs at the pene t ra t ion site. In suscept ib le cu l t i vars systemic co lon iza t ion proceeds w i t h the progress o f h y p h a l g r o w t h t h r o u g h the in terce l lu la r spaces of the meso-p h y l l cel ls, and the d e v e l o p m e n t o f haustor ia w i t h up to e igh t f inger - l i ke tubes.

Oospore production, dispersal, and infection

Oospores o f S D M deve lop subsequent to the f us i on o f oogon ia and an ther id ia in i t ia ls in the m e s o p h y l l o f s o r g h u m leaves (Safeeulla and Th i r uma lacha r 1955). Oospores m a y be d is ­persed by m a n or on f a r m animals in soil adher­ing to their feet, or on f a rm implements (Wi l l ­iams 1984). They can surv ive passage th rough the bov ine digest ive tract, so dispersal in manure is imp l ica ted (Safeeulla 1976). Oospores can be seed-transmitted (Bain and A l f o r d 1969), or m a y also be w i n d - (Bock et al. 1997) or water-d is­persed (Rajasab et al . 1979). They can surv ive ad­verse condi t ions for several years (Safeeulla 1976). The highest incidence of infect ion is f ound in sandy-textured soils at temperatures of 2 4 -29°C, and soil moisture contents of about 0.2 bar (Schuh et al. 1987). Host and nonhost roots can

st imulate oospore germina t ion (Pratt 1978). The germ-tube g rows towards the mer istemat ic re­g ion of the root where i t fo rms an appressor ium and infect ion peg (Safeeulla 1976).

Collateral hosts

Col la tera l hosts, c o m m o n in m a n y areas w h e r e s o r g h u m and ma ize crops are g r o w n , can act as reservoirs o f b o t h con id ia l and oospore i n -ocula. Several species of G r a m i n a e f r o m the t r ibes A n d r o p o g o n a e , M a y d a e , a n d Panicae can be in fected w i t h P. sorghi (Table 2). W h e r e col lateral hosts are a source of i n o c u l u m for crops i t m a y be feasible to cons ider r o g u i n g those p lants in the v i c i n i t y o f the c r o p p i n g area.

Seed transmission

Oospores can be dispersed on the surface of sor­g h u m seed (Bain and A l f o r d 1969). In s o r g h u m and ma ize there is also ev idence that oospores ( U p a d h y a y 1987) and m y c e l i u m (Kave r i appa and Safeeulla 1978, Prabhu et al . 1983) of P. sorghi m i g h t be i n te rna l l y seedborne. Freder iksen (1980b) discussed w a y s in w h i c h oospore con tam ina t i on a n d i n te rna l myce l i a l t ransmiss ion o f S D M cou ld be a v o i d e d . D r y i n g seed to be l ow 20% mo is tu re content , u s i n g hea l thy seed, p r o d u c i n g seed in areas n o t p rone to ep idemics o f S D M , b reed ing resistant hyb r i ds , and observ ing st r ic t qua ran t i ne mea ­sures are a l l pract ices that can be used to a v o i d seed in fec t ion . In laborator ies w h e r e the tech­no logy is ava i lab le , mo lecu la r probes and D N A h y b r i d i z a t i o n can be used to test for seed t ransmiss ion o f g raminaceous d o w n y m i l d e w s (Yao et a l . 1990).

Seasonal disease development

To r e c o m m e n d con t ro l me thods , an unde r ­s tand ing of the factors that con t r i bu te to d is ­ease i n i t i a t i on a n d ep idemic d e v e l o p m e n t is necessary. Oospores and con id ia m a y v a r y i n their impo r tance i n ep idemic i n i t i a t i o n a n d

10

Table 2. Host range of Peronosclerospora sorghi.

Host Author

Panicum trypheron Shult. McRae (1934) Pennisetum americanum (L.) Leeke Castellani (1939) Para-sorghum sp. Karunakar et al. (1994) Sorghastrum rigidifolium Stapf. Karunakar et al. (1994) Sorghum aethiopicum (Hack.) Stapf. Karunakar et al. (1994) Sorghum almum Perodi. Tarr (1962) S. arundinacium (Willd.) Stapf. Karunakar et al. (1994) S. bicolor x S. sudanense (Piper) Stapf. Futrell and Bain (1967) S. bicolor (L.) Moench Bonde and Freytag (1979). S. controversum (Steud.) Snowden Karunakar et al. 1994 S. drummondii (Steud.) Millsp. & Chase Karunakar et al. (1994) S. halepense (L.) Pers. Frederiksen et al. (1965) S. hewisonii (Piper) Longley Bonde and Freytag (1979) S. lanceolatum Stapf. Bonde and Freytag (1979) S. miliaceum (Roxb.) Snowden Karunakar et al. (1994) S. niloticum (Stapf. ex Piper) Snowden Bonde and Freytag (1979) S. plumosum (R. Br.) Beauv. Nagarajan et al. (1970) S. propinquum (Kunth.) Hitch. Bonde and Freytag (1979) S. pugionifolium Snowden Bonde and Freytag (1979) S. purpureo-serecium (A. Rich.) Aschers. & Schwerf. Karunakar et al. 1994 S. sudanense (Piper) Stapf. Nagarajan et al. (1970) S. verticilliflorum (Steud.) Stapf. Tarr (1962) S. controversum (Steud.) Snowden Bonde and Freytag (1979) S. usamberance Snowden Karunakar et al. (1994) S. versicolor Anderss. Bonde and Freytag (1979) S. virgatum (Hack.) Stapf. Nagarajan, et al. (1970) Zea mais ssp. mexicana (L.) (Schrad.) litis. Uppal and Desai (1932) Zea mais (L.) Bonde and Freytag (1979)

b u i l d - u p , d e p e n d i n g on the par t i cu la r reg ion . In some areas i n i t i a l in fect ions m a y be t h r o u g h so i lborne oospores. P rov i ded cond i t ions are fa­vorab le fo r con id ia l p r o d u c t i o n , these ear ly i n ­fect ions lead to an increased i n o c u l u m load of con id ia later in the season, and this results in an increased inc idence o f S D M on la te -sown crops. An increased r i sk o f in fec t ion in late-s o w n crops has been i l l us t ra ted in several re­g ions , i n c l u d i n g par ts o f I nd ia (Rama l i ngam and Rajasab 1981) and Israel (Cohen and She rman 1977). In other areas, i n c l u d i n g the U S A , oospores are the major source of infec­t i o n (Freder iksen et a l . 1980a, Tuleen et a l . 1980). Soi l tempera tu re , mo is tu re and texture p r o b a b l y in f luence the inc idence of systemic in fect ions in these regions (Schuh et a l . 1987).

The presence of in fec ted col la tera l hosts can also in f luence ep idemic deve lopmen t .

Variabi l i ty w i t h i n P. sorghi

Peronosclerospora sorghi can in fect b o t h sor­g h u m a n d maize . H o w e v e r , s t ra ins that in fec t o n l y ma ize , b u t no t s o r g h u m are recogn ized . The af f i l ia t ions of these so-cal led ma ize st ra ins ' is unclear. General ly , as mo re i n f o r m a ­t i on becomes avai lab le they are des ignated as separate species. A ma ize - in fec t i ng s t ra in of the S D M pa thogen i n Rajasthan was even tu ­a l l y s h o w n to be a d i f fe ren t species and des ig ­nated P. heteropogonii (S i radhana et a l . . 1980). S imi lar ly , recent studies suggest that a ma ize pa tho type f r o m T h a i l a n d s h o u l d be g i v e n

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specif ic r ank as P. zeae (Micales et a l . 1988, Yao 1991, Bonde et a l . 1992). A n o t h e r maize- in fect ­i n g d o w n y m i l d e w cu r ren t l y cons idered a ma ize p a t h o t y p e o f the S D M pa thogen occurs in N i g e r i a (Fajemis in 1980, Anaso et a l . 1987).

M o r p h o l o g i c a l va r i a t i on be tween strains o f P. sorghi is l i m i t e d (Bock 1995). There is l i t t le adap ta t i on to specif ic env i ronmen ts as sug­gested by the env i r onmen ta l requ i rements of a range o f isolates f r o m d iverse locat ions (Bonde et a l . 1985, Bock 1995).

The f i rs t i nd i ca t i on o f pathogenic va r i ab i l i t y on s o r g h u m was observed i n the U S A i n the late 1970s (Cra ig a n d Freder iksen 1980). A pre­v i o u s l y resistant h y b r i d became suscept ib le to S D M . Subsequent ly , three d is t inc t pa tho types have been i den t i f i ed in the U S A by the di f fer­ent ia l react ion of the var iet ies Tx 412, Tx 430, CS 3541, a n d QL 3 (Table 3, C r a i g 1983, C ra ig and Freder iksen 1983). O ther pa tho types do occur, and have been ident i f ied in Brazi l (Fernandes and Schaffert 1983), Honduras (Craig and O d v o d y 1992), and Z i m b a b w e (de M i l l i ano and Veld 1990). Pawar et al. (1985) tested 75 sor­g h u m varieties for their reaction to 16 isolates f r o m di f ferent geographic regions. They found a di f ferent ial reaction that ident i f ied each isolate as a di f ferent pathotype. Those f r o m Afr ica (Niger ia and Eth iop ia) and As ia had greater v i ru lence than those f r o m the Amer icas .

Control Chemical control

Pr ior to the late 1970s several d i f fe ren t f u n g i ­cides had been tested in a t tempts to con t ro l the graminaceous d o w n y m i l d e w s (S ingh e t a l . 1970). N o n e of these had p r o v e d ef fect ive. Howeve r , the d iscovery of the acy l -a lan ine fung ic ide me ta laxy l [N - (2 -me thoxyace t y l ) -N -(2 ,6-xy ly l ) -DL-a lan inate ] r e v o l u t i o n i z e d the chemica l con t ro l o f S D M ( S c h w i n n 1980). I n I nd ia , seed t reatments of 1 g ai kg -1 of seed p l u s a fo l iar spray of 1 g ai L -1 40 days after emer­gence ( D A E ) or fo l iar sprays of 2 g ai L -1 at 10, 20 or 40, and 50 D A E gave comp le te con t ro l o f systemic S D M (Venugopal a n d Safeeul la 1978, A n a h o s u r and Pat i l 1980). Seed t rea tment alone d i d no t f u l l y pro tect the p l an t o r n o d a l t i l lers f r o m systemic i n fec t i on , and the sp ray reg ime was requ i red to p reven t a l o w i n c i ­dence of late systemic in fec t ion and to con t ro l local lesions. A t some locat ions concent ra t ions of meta laxy l as l o w as 0.05 g ai kg -1 seed can g ive comple te con t ro l o f systemic S D M , and in some cases concentrations greater than 1 g ai kg-1

seed can cause seed l ing dea th ( O d v o d y a n d Freder iksen 1984). There is a r i sk that S D M m a y deve lop resistance to me ta l axy l , a v i e w suppo r ted by the fact tha t o ther oomycetes have deve loped resistance to th is f ung i c i de

Table 3. Identification of pathotypes i, II and III of Peronosclerospora sorghi in the USA by the differential reaction of four sorghum inbred lines'.

Reaction to infection

Sorghum variety Pathotype I2 Pathotype II Pathotype I I I

Tx 412 S S S

Tx 430 R R S

CS 3541 R s s

QL 3 R R R

1. Source: Craig and Frederiksen 1983 2. S = susceptible, and R = resistant to infection.

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(Georgopou los a n d G r i g o r i u 1981, Bruck e t a l . 1982).

Soi l s te r i l i za t ion has also been s h o w n to be effect ive i n r e d u c i n g in fec t ion t h r o u g h so i l -bo rne oospores, b u t i s p robab l y no t economic (Matocha et a l . 1974).

Cultural control Crop rotation

Roots of host a n d non-host p lants t r igger ger­m i n a t i o n of oospores (Pratt 1978) and ba i t crops ' (e.g., Linum usitatissimum) g r o w n in i n ­fested soi l can reduce the inc idence of in fec t ion in suscept ib le s o r g h u m crops s o w n in the same soi l (Tuleen et a l . 1980). Th is m a y be of va lue w h e r e oospores are the p r i n c i p a l source of i n ­o c u l u m and soils are heav i l y in fested.

Deep tillage

B o t h the inc idence o f S D M and the oospore content in the u p p e r strata of in fested soi l are reduced by deep t i l lage (Tuleen et a l . 1980, Janke et a l . 1983). Howeve r , i t is an expensive ope ra t i on , and p robab l y no t cost-effect ive as a means of con t ro l .

Over-sowing and rouging diseased plants

S o w i n g at seed rates o f up to 50% more than the r e c o m m e n d e d agronomic o p t i m u m can lead to an acceptable p l an t dens i ty o f hea l thy p lan ts r e m a i n i n g at harvest after s tand losses d u e to 20 -30% disease inc idence (Freder iksen et a l . 1973).

Sowing date

In areas w h e r e the asexual i n o c u l u m r a p i d l y increases as the season progresses it is exped i ­ent to s o w early. In Israel ea r l y - sown crops a v o i d e d the disease (Cohen and Sherman 1977). H o w e v e r , Tu leen et a l . (1980) f o u n d a l owe r inc idence o f the disease in l a te r -sown crops in the U S A , whe re oospores are the p r i n ­c ipa l source of in fec t ion .

Effect of host plant nutrition

There are con f l i c t i ng repor ts abou t the effect of n u t r i t i o n on the inc idence o f systemic S D M (Ba lasubramanian 1973, G u p t a and S i radhana 1978). The effect of n u t r i t i o n is p r o b a b l y associ­ated w i t h such other factors as p l an t age, i n ­o c u l u m type and pressure, and o ther var iab les.

Biological control

A c h y t r i d f u n g u s (Gaertennomyces sp.) w a s f o u n d to be effect ive in r e d u c i n g the inc idence o f systemic in fec t ion in soi ls heav i l y in fes ted w i t h oospores by as m u c h as 58% (Kunene et a l . 1990). Howeve r , f i e ld app l i ca t i on of th is or­gan i sm has no t been inves t iga ted a n d i t i s u n ­l i ke l y b io -con t ro l o f S D M by th is means w i l l become a pract ica l rea l i ty in the near f u tu re . Other po ten t ia l b io -con t ro l agents have also been repor ted a l t h o u g h they have no t been s tud ied in de ta i l (de D iaz and Polanco 1984, L a k s h m a n a n et a l . 1990).

Host-plant resistance

The c u l t i v a t i o n of disease-resistant var ie t ies is p robab l y the mos t cost-effect ive a n d feasible l ong - t e rm m e t h o d for the con t ro l o f S D M . Breed ing disease-resistant var iet ies requi res b o t h the i den t i f i ca t i on of a stable source of re­sistance, and i ts subsequent u t i l i z a t i o n in a b reed ing p r o g r a m . Breed ing p r o g r a m s a i m to incorpora te resistance i n t o a va r i e t y w i t h h i g h y i e l d po ten t i a l , g o o d g r a i n qua l i ty , a n d other ag ronomic t ra i ts that are acceptable in the re l ­evant consumer marke t .

Identification of resistance

By 1988 the In te rna t iona l Crops Research Ins t i ­tu te for the S e m i - A r i d Tropics ( ICRISAT) had screened 13,101 s o r g h u m accessions f r o m 73 count r ies u s i n g a f ie ld-based screening sys tem at D h a r w a d , Ind ia . Of these 46 accessions f r o m geograph ica l l y d iverse sources were resistant to P. sorghi (Table 4). U s i n g s im i la r f ie ld-screen-

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Table 4. Origin and number of accessions from the world collection of sorghum germplasm screened in the field from 1981-88 and found to be resistant to sorghum downy mildew at Dharwad, Karnataka, India1.

Number of Number of Number of Or ig in countries lines screened lines resistant

Eastern Africa 7 3345 18 Western Africa 14 3324 4 Southern Africa 9 930 2 Middle East 8 252 0 Indian Subcontinent 5 3403 12 Southeast Asia and the Far East 8 218 0 Nor th and Central America 7 1476 4 South America 3 19 0 Europe 9 71 0 Eastern Europe - 40 0 Australia and Oceania 2 23 6 Total 73 13101 46

1. Eastern Africa: Ethiopia, Kenya, Sudan, Somalia, Tanzania, Uganda, Zaire. Western Africa: Benin, Burkina Faso, Cameroon, Congo, Central African Republic, Chad, Ghana, Gambia, Cote d'lvoire, Mali, Nigeria, Niger, Sierra Leone, Senegal. Southern Africa: Angola, Botswana, Lesotho, Malawi, Malagasy Republic, South Africa, Swaziland, Zambia, Zimbabwe. North Africa and the Middle East: Egypt, Israel, Iran, Iraq, Lebanon, Syria, Saudi Arabia, Yemen. Indian Subcontinent: Afghanistan, Bangladesh, India, Nepal, Pakistan. Southeast Asia and Far East: China, Indonesia, Japan, Myanmar, Philippines, South Korea, Taiwan, Thailand. North and Central America: Cuba, El Salvador, Guatemala, Mexico, Nicaragua, USA, West Indies. South America: Argentina, Uruguay, Venezuela. Europe: Belgium, Cyprus, France, Greece, Hungary, Italy, Portugal, Spain, Turkey. Eastern Europe. Australia and Oceania: Australia, Papua New Guinea.

i n g techniques more than 3000 g e r m p l a s m ac­cessions and 2000 b reed ing l ines were again eva lua ted by ICRISAT a t D h a r w a d . Of these, 151 g e r m p l a s m accessions and 334 b reed ing l ines we re resistant t o S D M . A t Ma topos , Z i m ­babwe , ICRISAT screened 4500 s o r g h u m en­tr ies, and f o u n d 1008 resistant. Over 5000 sor­g h u m and ma ize entr ies were screened a t G o l d e n Valley, Z a m b i a , whe re 928 so rghums were f o u n d resistant (S Pande, unpub l i shed ) .

The In te rna t iona l S o r g h u m D o w n y M i l d e w N u r s e r y ( I S D M N ) was establ ished in 1976 to screen s o r g h u m g e r m p l a s m to i den t i f y stable resistance and dif ferences in pa thogen v i r u ­lence be tween locat ions (Wi l l i ams et a l . 1980). The results of mu l t i l oca t i ona l test ing of se­lected resistant s o r g h u m accessions a t I S D M N test locat ions are s h o w n in Table 5. A l t h o u g h the results o f the I S D M N d i d no t i n i t i a l l y i n d i ­

cate pa thogen va r iab i l i t y of P. sorghi on sor­g h u m , the existence of pa tho types of P. sorghi on s o r g h u m was subsequent ly repor ted (Cra ig and Freder iksen 1983, Pawar et a l . 1985). Pathogen va r i ab i l i t y underscores the need to i den t i f y stable resistance to th is pa thogen .

Screening techniques

Field screening

Several f ie ld-screening techniques have been used to i d e n t i f y resistance to S D M in s o r g h u m and ma ize (Anahosur and H e g d e 1979, C a r d w e l l et a l . 1993). A m e t h o d that chal lenges resistance to b o t h con id ia and oospores is o p t i ­m a l i f env i r onmen ta l cond i t i ons p e r m i t . I n places whe re oospores are the m o s t i m p o r t a n t componen t o f ep idemics , soils heav i l y in fes ted w i t h oospores are used to screen b reed ing m a -

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Table 5. Sorghum downy mildew incidence in selected resistant accessions in the International Sorghum Downy Mildew Nursery during, 1976-86.

Highest disease incidence at locations (% plants infected)

Entry Or ig in Manfredia Pergaminoa Jaboticabalb Samaruc Dharwadd Mysored Texase

IS 1317 Tanzania -2 0(1)3 - - 3(5) 0(3) 0(1)

IS 2132 USA - 0(1) - - 0(5) 3(3) 0(1) IS 2204 India - 0(1) - - 0(5) 6(5) 0(1) IS 2473 USA - 0(1) - - 3(5) 6(3) 0(1) IS 2482 USA - 0(1) - - 2(5) 6(5) 0(1) IS 3443 Sudan 0(3) 3(4) 0(1) 0(1) 7(5) 6(6) -IS 3546 Sudan - 0(1) - - 1(5) 6(3) 0(1) IS 3547 Sudan 0(3) 0(4) 0(1) 0(2) 0(6) 0(3) -IS 4696 India - 0(1) - - 0(5) 0(2) 0(1) IS 5616 India - 0(1) - - 4(5) 0(3) 0(1) IS 5628 India - 0(1) - - 5(5) 0(2) 0(1) IS 5651 India - 0(1) - - 4(5) 0(2) 0(1) IS 5665 India - 0(1) - - 0(5) 0(2) 0(1) IS 5743 India - 0(1) - - 0(5) 0(2) 0(1) IS 7528 Nigeria 0(3) 0(4) 0(2) 0(2) 5(5) 14(3) -IS 8185 Uganda 0(3) 0(4) 0(2) 0(2) 3(6) 15(5) -IS 8283 Uganda 6(2) 0(2) 0(1) 0(1) 2(5) 3(5) -IS 8607 Uganda 3(3) 0(4) 0(2) 0(2) 5(6) 13(6) -IS 14387 Zimbabwe - 0(1) - - 0(5) 2(2) 0(1) IS 18757 Australia 0(4) 0(5) 0(2) 0(2) 0(11) 0(11) -IS 22227 Australia 0(4) 0(5) 0(1) 0(2) 0(7) 12(4) -IS 22228 Australia 0(4) 0(5) 0(1) 0(1) 4(7) 11(4) -IS 22229 Australia 0(4) 0(5) 0(1) 0(1) 0(7) 28(4) -IS 27042 India 0(2) 0(3) - 0(1) 0(5) 7(5) -Susceptible control

DMS 652 India 100(4) 36(5) 9(2) 100(2) 100(5) 100(5) 45(1)

1. Locations: a. Argentina, b. Brazil, c. Nigeria, d. India, and e. USA. 2. - not tested at that location. 3. Numbers in parentheses indicates number of years tested at each location.

ter ia l . The oospore content of the soi l is b u l k e d up by i n c o r p o r a t i n g in fected crop residues f r o m the p rev ious season. Howeve r , even in some of these areas host react ion to con id ia l i n o c u l u m i s checked by ar t i f ic ia l i nocu la t i on i n the greenhouse (Freder iksen 1980a). A large-scale f ie ld-screen ing technique u t i l i z i n g p r i ­m a r i l y w i n d b o r n e con id ia of P. sorghi was de­ve loped at ICRISAT (Pande and S ingh 1992). The sys tem was des igned so that con id ia l showers were b l o w n b y w i n d f r o m in fector r o w s on to test mater ia ls (Fig. 18). The t w o c r i t i ­

cal aspects fo r the successful use of th is tech­n ique are the es tab l ishment of disease in fector r ows , and favorab le temperatures and h u m i d i ­ties for abundan t con id ia l p r o d u c t i o n f r o m the systemica l ly in fec ted s o r g h u m p lants i n the i n ­fector rows . Idea l l y therefore, screening shou ld be conduc ted at locat ions k n o w n to be favor­able fo r the disease, as at D h a r w a d , G o l d e n Valley, and Ma topos . I t is desi rable to use the same f i e ld fo r several years to encourage an accumula t ion o f oospore i n o c u l u m in the soi l that w i l l act as an add i t i ona l source of in fect ion.

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Figure 18. The spreader-row technique employed by breeders to ensure effective screening of sorghum germplasm against sorghum downy mildew. Note the older infector rows sown to the left and right of the four rows of test material. The infector rows were planted about three weeks prior to the test material.

The f ie ld-screening technique has the f o l l o w i n g components :

1 . Source a n d o f f -season ma in tenance o f i n ­o c u l u m . In fec ted mate r ia l fo r i nocu la t i on o f the spreader r o w s m a y come f r o m d i f ferent sources. In fected vo lun teers or col latera l hosts m a y be read i l y avai lable in the v i c i n i t y (Pande and S ingh 1992). A l te rna t ive ly , i n ­fected p lan ts can be ma in ta i ned by the b reed ing p r o g r a m t h r o u g h o u t the season. A s imp le and effect ive w a y of m a i n t a i n i n g in fec ted p lan ts i s to inocu la te s o r g h u m seedl ings u s i n g the ' s a n d w i c h ' technique. The p rocedure can be done in pe t r i dishes l i ned w i t h mo i s t f i l ter paper t o m a i n t a i n h i g h R H . Pre-germinated seedl ings ( w i t h 0.5-1.0 mm l o n g p l u m u l e s and radicles) are

incubated by p lac ing t h e m on the adax ia l surface of a piece of sys temica l ly in fec ted leaf, and cover ing t h e m w i t h another sec­t i o n of sys temica l ly in fec ted leaf (adax ia l surface touch ing the seeds), thus ' s a n d w i c h ­i n g ' the seedl ings (Fig. 19). They are p laced in pe t r i d ishes and incuba ted in darkness a t 18-20°C for 12-16 h . A f t e r th is p e r i o d spo ru la t i on , g e r m i n a t i o n , and i n fec t i on w i l l have occurred. I f incubators and pe t r i dishes are no t avai lab le, b u t e n v i r o n m e n t a l c o n d i ­t ions a t n i gh t are favorab le fo r S D M , th is seed l ing i nocu la t i on techn ique can be m o d i ­f ied (Pande and S ingh 1992). G e r m i n a t e d seeds can be ei ther spread on a p o l y t h e n e sheet (1-2 m x 0.75 m) or on seedbeds (2 m x 1 m ) . Systemica l ly in fec ted leaves are l a id abaxia l surface d o w n over the seed-

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Figure 19. Inoculation of pre-germinated sorghum seedlings using the 'sandwich' technique. The seedlings are placed between the abaxial sides of sorghum leaves systemically infected with sorghum downy mildew and incubated in conditions conducive to sporulation and infection.

l i ngs , and covered w i t h mo is t newspaper o r g u n n y bags and po l y thene sheets to m a i n ­ta in h i g h R H .

2. Establ ishing infector rows. The mos t i m ­p o r t a n t componen ts o f the ICRISAT S D M screening technique are the infector rows . To establ ish these, seedl ings of a suscept ib le va r ie t y ( D M S 652, IS 643, Marupan tse , Suga rd r i p , or a loca l ly avai lab le suscept ible va r ie t y or landrace) are inocu la ted us ing the s a n d w i c h technique. I m m e d i a t e l y after i n ­ocu la t i on the p re -ge rm ina ted seedl ings are s o w n in the spreader r o w area, as every 5 th o r 10th row , d e p e n d i n g o n p reva i l i ng w i n d cond i t ions .

3. Sowing test materials and indicator rows. Once the sys temica l ly in fected p lants in i n ­fector r o w s are establ ished and show heavy spo ru la t i on (20-25 days after sow ing ) , the test ma te r ia l s h o u l d be s o w n . Test l ines are

s o w n i n the i n t e r v e n i n g r o w s be tween the spreaders. The ind ica to r r o w s are s o w n to the same SDM-suscept ib le va r i e t y as the i n ­fector r o w s , a longs ide the test ma te r ia l at regu lar in terva ls (genera l ly every 10-20 rows ) . Ind ica to r ma te r i a l p r o v i d e s a mea ­sure of the disease pressure in the nursery.

I t i s also usefu l to i nc l ude genotypes w i t h var iab le levels of resistance as cont ro ls in screening tr ia ls. W i l l i a m s et a l . (1981) used a sp r ink le r sys tem to increase h u m i d i t y and p r o v i d e o p t i m a l cond i t i ons for asexual spo ru la t i on o f the in fec tor r o w s w h e n screening for resistance to pear l m i l l e t d o w n y m i l d e w . Th is m e t h o d can also be ap­p l i ed to S D M screening.

4. Evaluat ing resistance. To compare host re­act ions i t is necessary to deve lop an effec­t ive (bo th accurate a n d precise) assessment m e t h o d . Scor ing systemic in fec t ion is s t r a i gh t f o rwa rd . The inc idence o f sys temi ­cal ly in fected p lan ts in the ind ica to r r o w s and test mate r ia l s h o u l d be recorded on a t least t w o occasions d u r i n g the season (at seed l ing and f l o w e r i n g stages) to p r o v i d e a real ist ic est imate of disease inc idence ( W i l l ­iams 1984). S o r g h u m l ines w i t h no m o r e than 5% sys temica l ly in fec ted p lan ts are re­ga rded as resistant. Var ious sever i ty scales have been deve loped to assess local lesions. A 1 - 5 scale has been used to score th is t ype o f i n fec t ion ( S i n g b u r a u d o m and W i l l i a m s 1978, Freder iksen 1980a) a n d Shenoi a n d R a m a l i n g a m (1976) deve loped a 1-4 scale to assess the sever i ty of local les ion i n fec t i on on i n d i v i d u a l leaves.

Greenhouse and laboratory screening

Several greenhouse and labora to ry screening techniques u s i n g b o t h oospores a n d con id ia have been s tandard ized (Jones 1971b, C r a i g 1983, N a r y a n a et a l . 1996). Oospores can be i n ­corpora ted i n to p o t so i l a t a k n o w n rate, the test mate r ia l then s o w n in the container, a n d the subsequent d e v e l o p m e n t o f systemic in fec t ion assessed (Cra ig 1983). A l te rna t i ve l y ,

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con id ia cou ld be used as the source of i nocu ­l u m to in fect the p lants . Con id i a are p r o d u c e d on pieces o f sys temica l ly in fected s o r g h u m leaves incubated in the d a r k fo r 6 -7 h at 1 8 -20°C a n d >90% RH (Fig. 20). C o n i d i a are har­vested f r o m the leaves by gen t l y b r u s h i n g t h e m in to co ld d i s t i l l ed wa te r ( to p reven t con id ia l ge rm ina t i on ) us ing a pa in t b r u s h (Fig. 21). The con id ia l suspens ion is ad jus ted to 1x10 5 con id ia mL - 1 . The suspension is genera l ly a p p l i e d to seedl ings at the coleopt i le to one-leaf stage u s i n g a hand -he ld sprayer (Fig. 22), a l t h o u g h seedl ings m a y s i m p l y be d i p p e d i n the suspens ion (Naryana et a l . 1996). Sp ray ing seedl ings is a use fu l w a y to mass-screen breed­i n g mater ia ls i n the off-season. A l te rna t i ve ly , i f i n o c u l u m q u a n t i t y is c ruc ia l to the study, t hen a sy r inge- inocu la t i on m e t h o d can be used to accurate ly a p p l y a k n o w n n u m b e r o f con id ia (Fig. 23). In this technique a d r o p of i n o c u l u m is p laced precisely at the apex of the 12-24 h o l d co leopt i le and a l l o w e d to r u n off to the base. Once the seedl ings have been inocu la ted they are incuba ted a t 18-20°C and RH >90%

Figure 21. Preparing a conidial inoculum of Pero­nosclerospora sorghi in cold water by brushing sporulated infected leaves.

Figure 20. Sporulation of Peronosclerospora sorghi conidia on pieces of systemically infected leaves incubated in a moist chamber.

Figure 22. The seedling spray inoculation method. When the coleoptiles are 1-2 cm long the seedlings are spray inoculated with a conidial suspension of Peronosclerospora sorghi until they are evenly coated with droplets.

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Figure 23. The seedling droplet inoculation method. A pipette is used to administer a known volume of conidial suspension of Peronosclerospora sorghi to each coleoptile.

Inheritance of resistance

K n o w l e d g e of the inher i tance of resistance, or at least a w o r k i n g k n o w l e d g e of the ease of re­sistance transfer, is essential to a b reed ing p r o ­g ram. S o r g h u m is a se l f -po l l ina ted species w h i c h means genetic u n i f o r m i t y can be at­ta ined (Freder iksen et a l . 1973). H o w e v e r , i t can be i nduced to cross-pol l inate. Studies of the inher i tance o f resistance in s o r g h u m under ­taken by va r ious au thors suggest that resis­tance is d o m i n a n t to suscept ib i l i t y (Rana et a l . 1982, Sifuentes and Freder iksen 1988, Rosenow 1992, T h a k u r a n d Pande 1995) a l ­t h o u g h some ear l ier w o r k e r s f o u n d d o m i ­nance of suscept ib i l i t y ( P u t t a r u d r a p p a et a l .

for 12-16 h. They are subsequent ly t ransferred to greenhouse benches. S y m p t o m s of systemic in fect ions start to appear 8-12 days after i nocu­la t ion , after 15 days they are obv ious (Fig. 24).

A compa r i son of f ie ld and labora tory tech­n iques u n d e r t a k e n at ICRISAT indicates con­t ro l led i nocu la t i on w i t h con id ia i s the more d i sce rn ing test for pu ta t i ve resistance to S D M (Table 6). M a n y sources of resistance have been repo r ted u s i n g b o t h f ie ld-screening and labo­r a t o r y techniques (Freder iksen et a l . 1973, W i l ­l i ams et a l . 1982, N a r y a n a et a l . 1996).

Figure 24. Ten to fifteen days after inoculation of the seedlings the symptoms of systemic infection are clearly visible in susceptible lines and assess­ments can start.

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Table 6. The incidence of sorghum downy mildew (SDM) on selected resistant varieties and a sus­ceptible control using different methods of inoculation at ICRISAT Patancheru.

Downy mi ldew incidence (% plants systemically infected)

Field exposure to Seedlings sprayed Sprouted seeds dipped

Sorghum conidia and oospores1 wi th conidial suspension in conidial suspension

accessions Mean Max2 Mean Max3 Mean Max3

IS 1032 0 0 6 8 36 42 IS 1317 1 3 4 5 4 8 IS 1331 0 0 0 0 0 0 IS 2132 0 0 5 8 26 29 IS 2233 2 5 46 47 61 64 IS 2234 3 4 67 82 71 73 IS 2266 0 0 10 13 19 21 IS 2474 0 0 0 0 5 10 IS 3443 0 0 0 0 0 0 IS 3547 0 0 0 0 0 0 IS 3882 4 5 40 42 48 54 IS 4298 3 5 39 44 36 43 IS 5616 2 4 <2 2 27 33 IS 5628 2 5 6 8 10 13 IS 5646 0 0 12 14 38 42 IS 5665 1 2 5 6 63 67 IS 5743 0 0 0 0 0 0 IS 6094 1 2 18 23 14 18 IS 6918 1 4 2 4 6 12 IS 7179 0 0 0 0 0 0 IS 7528 2 3 4 5 8 9 IS 8185 0 0 0 0 0 0 IS 8247 0 0 3 4 10 13 IS 8274 2 4 5 9 8 8 IS 8276 0 0 0 0 0 0 IS 8283 0 0 0 0 0 0 IS 8589 1 3 7 8 18 23 IS 8607 1 2 2 3 1 2 IS 8615 <1 1 2 4 10 13 IS 8864 0 0 0 0 0 0 IS 8906 0 0 0 0 0 0 IS 8954 0 0 0 0 2 4 IS 14387 0 0 11 12 15 21 IS 22228 0 0 0 0 0 0 IS 22229 0 0 0 0 0 0 IS 22230 0 0 0 0 0 0 IS 27042 0 0 7 10 27 38 Susceptible control

DMS 652 87 88 100 100 100 100

1. Mean percentage incidence of SDM over 5-7 years. 2. Mean max imum incidence in 5-7 years. 3. M a x i m u m percentage incidence from 4 replications.

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1972, M i l l e r 1966). C r a i g and Schertz (1985) i l ­lus t ra ted that the resistance to S D M expressed by the i n b r e d l ine SC 414-12 was confer red by a s ingle d o m i n a n t gene. Th is resu l ted in an i ncompa t i b l e hos t -pa thogen in te rac t ion that i n h i b i t e d pa thogen deve lopmen t and sporu la -t i o n on inocu la ted leaves. Gimenes-Fernandes et a l . (1984) f o u n d that resistance was con­fer red by one o r t w o d o m i n a n t o r pa r t i a l l y d o m i n a n t genes that were d i f ferent . Sifuentes and Freder iksen (1988) invest iga ted the inher­i tance of resistance to three pa tho types of P. sorghi. The i r resul ts i nd ica ted that the resis­tant va r ie t y QL 3 has t w o d o m i n a n t genes con­d i t i o n i n g resistance to each of the three pa tho types . Reddy et a l . (1992) f o u n d resis­tance in QL 3 d o m i n a n t to suscept ib i l i ty : a t w o -loc i m o d e l w i t h i ndependen t segregat ion, and a c o m b i n a t i o n of comp lemen ta r y and i n h i b i ­t o r y in ter-a l le l ic in te rac t ion appeared to be the mos t app rop r i a te in exp la i n i ng the inher i tance pa t te rn they observed. Fur ther invest igat ions are needed to character ize the inher i tance of resistance to S D M in more s o r g h u m l ines, and the mechan isms of th is resistance, w h i c h re­m a i n p o o r l y unde rs tood . Resistance, prefer­ab ly of a du rab le nature needs to be i nco rpo ­ra ted i n to ag ronomica l l y sui table var iet ies. S y m p t o m remiss ion has been observed in sys-temica l l y in fec ted p lants . Th is m i g h t be an­other resistance mechan ism that cou ld be u t i ­l i zed (S ingh and de M i l l i a n o 1989a and b) .

Resistance sources in breeding, and resistant varieties used by farmers

S o r g h u m d o w n y m i l d e w resistant l ines are be­i n g successful ly used at ICRISAT to breed SDM-res is tan t var iet ies and hyb r i ds . Other b reed ing p rog rams i n c l u d i n g the one at Texas A & M U n i v e r s i t y i n the U S A also make use o f SDM-res is tant sources in their s o r g h u m breed­i n g p rog rams (Frederiksen et al . 1973). In the U S A variet ies resistant to S D M have been com­merc ia l ly avai lable since the early 1970s. At ICRISAT large numbers of crosses i n v o l v i n g QL 3, IS 3443, and IS 8283 as resistant parents

have been made. Several SDM-res is tant var iet ­ies and popu la t ions have been deve loped at D h a r w a d (Anahosur 1992) and in var ious sou thern A f r i c a n countr ies (de M i l l i a n o 1992).

Integrated control

S o r g h u m d o w n y m i l d e w lends i tsel f t o in te ­gra ted con t ro l that i nvo l ves the use o f t w o or mo re me thods of con t ro l to b r i n g abou t a re­d u c t i o n in disease inc idence ( O d v o d y e t a l . 1983). A g o o d k n o w l e d g e of disease e p i d e m i ­o logy a n d con t ro l op t ions ava i lab le to fa rmers in a g i v e n area are needed before an in teg ra ted package can be i m p l e m e n t e d (F ig . 25). In te ­gra ted con t ro l m i g h t i n v o l v e chemica l con t ro l (e.g., me ta laxy l seed t rea tment ) , cu l t u ra l con ­t ro l (e.g., deep p l o w i n g or use o f c rop ro ta t ion ) , and the use of hos t -p lan t resistance. The c o m ­b i n a t i o n o f con t ro l me thods can be m u t u a l l y benef ic ia l . For examp le , O d v o d y and Freder iksen (1984) suggested the use of a resis­tant va r ie ty and seed t rea tment c o u l d ex tend the l i fe o f the host resistance a n d p reven t de­v e l o p m e n t o f f ung i c i de resistance in the pa tho ­gen. The f i na l me thods chosen m u s t d e p e n d on their effectiveness in a pa r t i cu la r s i t ua t i on , a n d on the fa rmers ' ab i l i t y to use t h e m .

Technology Transfer and Adoption Technology transfer i s an ex tens ion ac t i v i t y in w h i c h p roduc ts o f scient i f ic i nves t i ga t i on i n any area of c rop i m p r o v e m e n t are a p p l i e d to pract ica l use in other, usua l l y d is tan t places. Technology transfer i s an ICRISAT manda te . In th is case the i n t ended p roduc ts are the S D M resistance screening techniques tha t have been t ransfer red to o ther locat ions by the f o l l o w i n g methods :

1 . Scientists f r o m locat ions where the S D M re­sistance screening technology is requ i red are t ra ined in the methods a t ICRISAT where the screening techniques were deve loped. The scientists then establ ish the technology at their o w n locations on their re tu rn .

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2. The S D M resistance-screening techniques are demons t ra ted at the loca t ion where the scientists are based and the techno logy is re­q u i r e d . Loca l l y avai lab le resources are used and adap ted as necessary. The second m e t h o d has p r o v e d the more

successful i n t rans fe r r ing S D M screening tech­n iques. I n d i v i d u a l s rece iv ing t ra in ing in a fa­m i l i a r e n v i r o n m e n t appear to ga in conf idence by m o d i f y i n g , as needed, the loca l ly avai lable faci l i t ies to establ ish the technique. Techniques have been successful ly establ ished in Z i m b a ­

b w e and Zamb ia (Pande and S ingh 1992). The s a n d w i c h i nocu la t i on c o m p o n e n t o f the tech­n ique has been used to establ ish in fec tor r o w s for in R w a n d a . Componen t s o f the techn ique have been adop ted by several o ther na t i ona l p rog rams, such as the A l l I nd i a C o o r d i n a t e d S o r g h u m I m p r o v e m e n t P r o g r a m a t the U n i ­versity o f A g r i c u l t u r a l Sciences, D h a r w a d , Karnataka (K H Anahosur , persona l c o m m u ­nicat ion) . The reasons for the w i d e a d o p t i o n o f this technique are i ts s imp l i c i t y , f lex ib i l i t y , a n d effectiveness.

Figure 25. Sorghum downy mildew infected crop in a farmer's field near Kigali, Rwanda. An integrated disease management package could reduce crop losses in such situations.

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About ICRISAT

The semi-arid tropics (SAT) encompasses parts of 48 developing countries including most of India, parts of southeast Asia, a swathe across sub-Saharan Africa, much of southern and eastern Africa, and parts of Latin America. Many of these countries are among the poorest in the wor ld . Approxi ­mately one-sixth of the world's population lives in the SAT, which is typif ied by unpredictable weather, l imited and erratic rainfall, and nutrient-poor soils.

ICRISAT's mandate crops are sorghum, pearl millet, finger millet, chickpea, pigeonpea, and groundnut; these six crops are vital to life for the ever-increasing populations of the semi-arid trop­ics. ICRISAT's mission is to conduct research which can lead to enhanced sustainable production of these crops and to improved management of the l imited natural resources of the SAT. ICRISAT communicates information on technologies as they are developed through workshops, networks, training, l ibrary services, and publishing.

ICRISAT was established in 1972. It is one of 16 nonprofit, research and training centers funded through the Consultative Group on International Agricultural Research (CGIAR). The CGIAR is an informal association of approximately 50 public and private sector donors; it is co-sponsored by the Food and Agriculture Organization of the United Nations (FAO), the United Nations Development Programme (UNDP), the United Nations Environment Programme (UNEP), and the World Bank.

ICRISAT International Crops Research Institute for the Semi-Arid Tropics

Patancheru 502 324, Andhra Pradesh, India

1997

ISBN 92-9066-379-0 Order code: IBE 051 158-97