Potato Res. 21 (1978) 291-300

The distribution of sclerotia ofRhizoctonia solani Ki.ihn on the surface of the potato tuber

D O R O T H Y S P E N C E R a n d R.A. FOX

Scottish Horticultural Research Institute, Invergowrie, Dundee, Scotland

Accepted for publication: 11 May 1978

Zusammel!]hssung, Rksum~; p. 299

Additional keywords: haulm destruction dates, harvest intervals, tuber maturity

Summary

Seed tubers ofcv. Majestic harvested from untreated plants in August had fewer sclerotia of R. solaniat the rose than at the heel end. As plant growth ceased in September and tubers matured, sclerotia formed on the rose end so that all regions of the tuber had similar concentrations. In a preliminary experiment in the previous year, presumably inflUenced by seasonal factors, the onset of maturity was such that the rose end had the highest and the heel end significantly the lowest concentration of sclerotia. In the second season destroying haulms with acid decreased both the time necessary for tuber maturation and the accompanying change of distribution of sclerotia when spraying was done in August but not if it was delayed until September. The results can be explained, in part, by postulating tuber exudates as a source of nutrition for the fungus.

Introduction

E x a m i n a t i o n o f severa l s tocks o f seed p o t a t o tubers i nd ica t ed tha t the d i s t r i bu t ion on

t h e m o f sc le ro t ia o f Rhizoctonia solani K t i h n was n o t r a n d o m . Th i s p a p e r p resen t s the ex t ens ion o f these o b s e r v a t i o n s and cons ide r s poss ib le e x p l a n a t i o n s o f the results .

Materials and methods

Field e x p e r i m e n t s wi th cv. Ma je s t i c were d o n e at the Sco t t i sh H o r t i c u l t u r a l R e s e a r c h

Ins t i tu te in two seasons .

Planting material T u b e r s were s p r o u t e d for two weeks be fo re p lan t ing . In the first season , the seed used was

na tu ra l l y infes ted wi th sc le ro t ia o f R. solani, the sever i ty o f wh ich was var iab le . In the

s econd season , a s t a n d a r d i n o c u l u m was used to lessen var iab i l i ty . T u b e r s v is ib ly free

f r o m b lack scurf , were i n o c u l a t e d at the rose end jus t p r io r to p l a n t i n g by inse r t ing f o u r

ba r l ey seeds each in fec ted wi th a d i f fe ren t i so la te o f R. solani o b t a i n e d o r ig ina l ly f r o m

sc le ro t i a on p o t a t o e s .

291

D O R O T H Y S P E N C E R A N D R. A. F O X

T r e a t m e n t s

A b lock o f th ree h u n d r e d tube r s was p l an t ed on 20 A p r i l in the first year , the h a u l m s o f

s o m e o f the p lan t s were sp r ayed wi th ac id on 14 S e p t e m b e r , the c r o p h a r v e s t e d on 12 O c t o b e r , and a s a m p l e t a k e n o f 430 seed t ube r s ( < 6.4 cm, > 3.2 cm). In the second

season the e x p e r i m e n t was laid d o w n as a spli t p lo t des ign wi th f o u r repl ica tes , each p lo t

cons i s t ing o f t e n p lants . T u b e r s were p l an t ed on 24 Apr i l , h a u l m s sp r ayed on o n e o f f ive

da te s and p lan t s h a r v e s t e d at in te rva l s t he rea f t e r (Tab le l) , twe lve seed tube r s be ing t aken at r a n d o m f r o m each plot . T h e acid spray c o m p r i s e d 15 pa r t s c o m m e r c i a l 'B l ack

Oil o f V i t r io l ' ( 7 7 ~ H E S O 4) p lus 85 par t s wa t e r app l i ed at ca. 1125 l i t res /ha .

Table 1. Dates of haulm treatment (D) and harvest (H) in the second season.

Haulm Dates of harvest 2 (H) treated

(Ho) HI H2 H3 H4 Hs H6

DI 5Aug. 12 Aug. 19 Aug. 26Aug. 2Sep. - - D2 14Aug 21 Aug. 28 Aug. 4Sep. 11Sep. - - D3 21 Aug. 28 Aug. 4Sep. 11Sep. 18 Sep. 25 Sep. - D4 2 Sep. 9 Sep. 16 Sep. 23 Sep. 1 Oct. 8 Oct. - D5 12 Sep. 16 Sep. 26 Sep. 3 Oct. 10 Oct. 17 Oct. 24 Oct.

l Krautbehandlung - Traitement des fanes; 2Erntedaten - Dates de rOcolte

Tabelle 1. Datum der Krautbehandlung (D) und der Ernte (H) im zweiten Jahr. Tableau 1. Dates de traitement des fanes (D) et de r6colte (H) la seconde ann&.

Table 2. Scoring levels used in two seasons to determine the distribution of sclerotia on the tuber surface.

Exposed area covered by sclerotia t (~o)

l s t season 2

0 nil 1 < 30 2 > 30

2rid season

0 nil 1 < 3 2 > 3-20 3 > 20-60 4 > 60

1Mit Sklerot ien bedeckten Fliiche - Surface couvert par des sc l&otes , 2 Jahr - Annie .

Tabelle 2. In beiden Jahren verwendete Bewertungsskalen zur Bestimmung der Sklerotienverteilung auf der Knollenoberfl~iche. Tableau 2. Echelles de notation utilis~es les deux ann6es pour d6terminer la distribution des scl+rotes a la surface des tubercules.

292 Pota to Res . 21 ( 1 9 7 8 )

D I S T R I B U T I O N OF R H I Z O C T O N I A S O L A N I S C L E R O T I A ON T U B E R S

In both years, each tuber was divided into four a pp rox ima te ly equal regions, the first at the rose and the last at the heel end. A rectangle 10 m m wide was cut from within a str ip of pape r which was placed a long the length of each tuber in turn. The concent ra t ion of sclerot ia on each region within the rectangle was scored as in Table 2. The first year ' s results indica ted that a more refined scoring system was required : in the second year an extended scale was used, bo th sides of a tuber were scored and the values for a.region tota l led. In the second season, each tuber had a 5 m m wide cut m a d e on its long axis and the percentage o f pe r ide rm noted which, s tar t ing at the rose end, could be removed with tweezers. The result was taken as an assessment o f the 'ma tu r i t y ' o f the per iderm, less being removed f rom older than younger tubers.

A n a l y s i s

Significance levels are for P < 0.05 unless otherwise stated. D u n c a n ' s Mul t ip le Range Test was appl ied to the da ta when necessary (Duncan , 1955).

R e s u l t s

The resul ts for the first year (Table 3) showed that there was a significance re la t ionship between the region of a tuber and the level of infection on it. The rose end had the greatest incidence of infect ion, the heel end had signif icantly less (Table 3, line 1 of data) . The m a x i m u m score o f 2 occurred most of ten at the rose end ; the f requence o f occurrence o f sc lerot ia then was a p p r o x i m a t e l y halved f rom one tuber region to that ad jacent in each o f the three c o m p a r i s o n s (Table 3, last line of data) .

The h i s tograms in Fig. 1 and 2 are for the second season and indicate the concen t ra t ion o fsc le ro t i a on ~icid-sprayed and un t rea ted plots respectively. At each harves t fol lowing

Table 3. N umber of times (ofa possible 430) in which four regions of the tuber surface were infected by sclerotia of R. solani in the first season.

Score t Region 2

l(rose) ~ 2 3 4 (heel) ~

0 75 129 161 226 1 148 207 211 177 2 207 94 58 27

Z 2 values for P < 0 .01 : incidence of infection 123 .3"* , severity of infection 1 5 0 . 9 * * - z2-Vv'ertefiirP < 0.01."

lnfi, k t i onsvorkommen 123.3"*, S t i i rke der lnfi, k t ion - 150.9"* - Valeurs du Z 2 pour <0 .01: incidence d" in-

J~,ction 123.3"*, skv~ritO d ' in/~,ct ion- 150.9"*.

Ska la - Echelle de notation; z Abschni t t R~;gion; ~ Kronenende - Couronne ; a Nabelende - Talon.

Tabelle 3. Anzahl Male (von m6glichen 430 Malen), in welchem im ersten Jahr vier Abschnitte der Knollen- oberfliiche durch Sklerotien yon R. solani infiziert waren. Tableau 3. Nombre de fois (430 possibilit6s) pour lesquelles les 4 r6gions de la surface du tubercule sont infect~es par les sclerotes de R. solani la premi6re ann+e.

P o t a t o Res . 21 ( 1 9 7 8 ) 293

Fig . 1. D i s t r i b u t i o n o f s c l e r o t i a a t h a r v e s t o n f o u r r e g i o n s o f seed t u b e r s f o l l o w i n g d i f f e r e n t d a t e s o f h a u l m

d e s t r u c t i o n a n d i n t e r v a l s to h a r v e s t .

D ~ - D 5 : D a t e o f h a u l m d e s t r u c t i o n - Datum der Krautvernichtung - Date de dc!fanage H o - H s : I n t e r v a l to h a r v e s t - Zeitliche Abstiinde his cur Ernte - lntervallesjusqu'a la r#colte.

Score

2so 7

125

tO0

50

0

! !

H 0 H 1 H 2 H 3 H 4

Score

250

D 3 e 125

100

5O

0

H 0 H I H 2 H 3 H 4

i H 5

25or

D 2 ~ 125

I00

5O

H 0 H 1 H 2 H 3 H 4

250

125

100

25O

12

i0

50

D 4

H 0 H I H 2 H 3 H 4 H 5

D 5 Tuber region

4 (heel end)

3

5O 2

I (rose end)

0

H 0 H I H 2 H 3 H 4 H 5 H 6

H e e l e n d - Nabelende - Talon R o s e e n d - Kronenende- Couronne

Abb. 1. Verteilung yon Sklerotien bei der Ernte auf vier Abschnitten yon Pflanzknollen nach versehiedenen Daten der Krautvernichtung und Ernteabstiinden. Fig. 1. Distribution des scl6rotes sur 4 rOgions des tubercules de plant suivant di/]~rentes dates de destruction des [hnes et de r~'colte.

294 Potato Res. 21 (1978)

D I S T R I B U T I O N OF R H I Z O C T O N I A SOLANI SCLEROTIA ON TUBERS

Fig. 2. D i s t r i b u t i o n o f s c l e ro t i a at h a r v e s t on f o u r r e g i o n s o f seed t u b e r s f r o m u n t r e a t e d c h e c k p lo t s co r -

r e s p o n d i n g to the a c i d - t r e a t e d p lo t s in Fig . l .

D I - D 5, H o - H 5 : As in Fig. I - Wie in Abh. 1 - Comme sur lafigure 1.

Score Score

125 125

100 100

50

D I

50

H 2 H 3 H 4

0 0

H 0 H 1

n

H 0 H 1 H 2 H 3 H 4 H 5

125

100

50

D 2

125

100

5O

::::::::::: :~:~:i:i:i -:~:i:i:i:~:-

:i:iiii:i ~ Inllll - 0

H 0 H 1 H 2 H 3 H 4 H 0 H 1 H 2 H 3 H 4 H 5

1 5 0

tO0

50

H 0 H 1 H 2 H 3 R 4 H 5

Tuber regio_ n

4 (heel end)

3

2

1 (rose end)

H 6

Abb. 2. Verteilung yon Sklerotien bei der Ernte a,tf vier .4bschnitten yon Pflanzknollen aus unhehandelten Versuchsparzellen entsprechend den mit Sdure behandehen Parzellen (,4 bb. 1). Fig. 2. Distribution des scl~rotes cJ la r&'olte sur 4 r~gions des tubercules des parcelles tOmoins correspondant aux parcelles trait~;es fi I'acide sur la figure 1.

Potato Res. 21 (1978) 295

Removal (%)

~oo i~i

50

0

100

50

C

I00

50

0

i00

50

0

100

50

0

I

D O R O T H Y S P E N C E R A N D R. A. FOX

d 0 H I

D 1

i{il .... id spray

acid spray

H 2 H 3 H 4

~H 0 H I H 2 H 3 H 4

~ H 0 H 1 H 2 H 3 H 4 H 5

D 2 D 3 D 4

August ~

~0 HI H2 H3 H4 H5

~H 0 H I H 2 H 3

I I I D 5

September >

H 4 H 5

i

R 6

I

October �9

296 Potato Res. 21 (1978)

D I S T R I B U T I O N O F R H I Z O C T O N I A S O L A N I S C L E R O T I A O N T U B E R S

the three August kills, D l, D2 and D3, and up to H 2 following the early September kill D4, the acid-treated plants had fewest sclerotia on region 1 o f the tuber. The con- centrat ion increased markedly towards the heel end. At all other harvests there was little variation between concentra t ions on regions. The differences between regions were not significant at either harvests H o or H 1 following D l, at H x following Dz, and at H o, H ~, H z, H 5 following D3. The remaining harvests from the August kills had significantly more sclerotia in regions 3 and 4 than there were in regions 1 and 2. At no harvest fo l lowingD4 and D5 were there any significant differences between the concentrat ion of sclerotia on the four regions.

In general, the longer the interval to harvest, after each of the five dates of haulm destruction, the greater the concentra t ion of sclerotia on any one region. Differences, however, were significant only following the three August kills where, for all the regions of the tuber, the following results were true : H o and H l did not differ significantly ; between H 1 and H 2 there was a significant rise in concentra t ion of sclerotia; there was another rise between H z and the later harvests which was significant following D 3 but not always so for earlier times of destruction.

Check tubers varied little f rom the corresponding treated ones where haulms had been destroyed in September at D4 or D5 (Fig. 2). For treatments D 1 and D2, corresponding check tubers had little scurf. At all harvests, region 4 had two to three times the concentra t ion on region 1. Destruct ion at D3 induced little scurf, other than at the last harvest.

The ease with which the periderm could be removed from tubers is illustrated in Fig. 3. There was an apparent negative relationship between tuber matur i ty and the percentage of periderm removed. Invariably, the longer tubers remained in the ground, the more difficulty there was in removing a periderm strip f rom any tuber. U p to H 2 following each of the August kills, strips were removed easily from tubers f rom both acid-killed and check plots. Between H 2 and H 3, and again between H 3 and H 4 there was a marked decrease in ease of removal on treated tubers which was not reflected on those from check plots. Fol lowing D4 and D5 the tubers f rom the acid-sprayed and check plots behaved

Fig. 3. Percentage of periderm strips removable from tubers following different dates of haulm treatment ano intervals to harvest.

Dr-D5 : Date of haulm treatment - Datum der Krautbehandhmg - Date de d~/hnage H o H 6 : Interval to harvest - Zeitliche Abstdnde bis zur Ernte - Intervallesjusqu'6 la r&'olte

No acid spray - Keine Siiurespritzung Pas de puh'~;risation d I'aeide Acid spray - S ~ i u r e s p r i t z u n g - Pulw;risation 6 l'acide Removal En(/k, r n t - Enlev~

Abb. 3. Prozentsatz yon Peridermstre~l'en, ~#r nach den verschiedenen Daten der Krautbehandhmg und der Ernteabstffnde yon den KnolleJz eJl(/&nl werdeJl komlte. Fig. 3. Pourcentage de pellicules tie pOriderme enlev~ sur les tubercules en.[onetion tit, dil]~;rentes dates de d)[cmge

et tie rbcolte.

Potato Res. 2 l (1978) 297

D O R O T H Y S P E N C E R A N D R. A. F O X

similarly, the greatest decreases occurring between H t and H 2 and again between H 2 and H 3. By the later harvest the periderm could not be removed.

Discussion

The nutritional req uirements for the initiation ofsclerotia of R. solaniin vitro differ fi'om those for their maturation. Thus Hunt (1957) and Townsend (1957) found that although initiation would occur in a given environment, some alterations to it, such as a lowering of the concentration of carbon compounds, was necessary before the sclerotia could mature. Hunt (1957) varied the carbon to nitrogen ratio in media and he found that an increase in the concentration of one, or of both, dealyed both the time for the initiation of sclerotia and for their maturation.

The concept of the tuber as a source of nutrition for the fungus was first suggested by Allington (1936) although his results were not convincing. His idea can be considered in relation to work on plant exudates and fungal reaction to them. Zentmyer (1961) and Royle & Hickman (1964) noted that the greatest concentration of exudates is from regions of tissue elongation. For the tuber, initially this is over the whole surface but later elongation is mainly at the rose end (Artschwager, 1924: Hooker et al., 1960). The distribution of sclerotia on a tuber can partly be explained by combining the principles behind these observations.

Until early September in the second season, check tubers, as judged by periderm strips, were immature and had more sclerotia towards the heel than the rose end. By mid September the rose end showed an increase in concentration of sclerotia so that all regions were similar. Assuming there are exudates at the rose end up to early September. then fungal mycelia grew and formed sclerotial initials but few sclerotia. In later Septem- ber, as the rose end matured, the intials developed into sclerotia concomitant with the loss and/or changes of available nutrient. Haulm destruction causes tubers to cease growth and their periderms to mature. Thus the gradual development ofsclerotia during the latter part of the growing season seen on check tubers was accelerated so that tubers harvested two to three weeks after each of the August kills were similar to those from check plots in mid September, both in maturity and in the distribution of sclerotia.

There is an apparent conflict between the results from the two season for distribution of the sclerotia on the tuber surface when comparing the date of haulm destruction and interval to harvest in the first season (14 September to 12 October) with a corresponding haulm kill and harvest in the second season (12 September to 10 October). In the first season the rose end had the highest and the heel end significantly the lowest score, whereas in the second season there were no such differences (Fig. 1, D s, H~). It is apparent from both figures 1 and 2 that with increasing age and maturity there is a general ttrend for the proport ion of sclerotia at the rose end to increase also. It thus appears that in the first year the onset of the age/maturity affect was more rapid than in the second year, presumably because of seasonal differences which would also influence the potential modifying factors noted below.

The hypothesis of tuber exudates can account for the change in the pattern of distribution of sclerotia with time, but the size of the increments recorded are not so

298 Pota to Res. 21 r 1978J

D I S T R I B U T I O N O F R H I Z O C T O N I A S O L A N I S C L E R O T I A O N T U B E R S

satisfactorily explained. A factor which must both modify and complicate the hy- pothesis will be nutrient available to the fungus from parasitized stolons and roots especially when they start to decay following death of the haulm: their role will be examined in another paper.

Acknowledgments

The work reported here was supported by a grant from the Potato Marketing Board.

Zusammenfassung

Die

Kartq[felknolle

Die Untersuchung verschiedener Lagerposten yon Pllanzkartoffelknollen liess vermuten, dass die Ver- teilung yon Sklerotien yon Rhizoctonia solani Kfihn auf den Knollen nicht zu[iillig ist. Diese Annahme wurde gepriift, indem Knollen aus zwei Feldversu- chen untersucht wurden.

Im ersten Jahr waren die Pflanzknollen auf nat~ir- liche Weise mit Sklerotien infiziert. Im zweiten Jahr wurde jede Knolle vor dem Auspflanzen mit einem Standardinokulum behandelt, lm ersten Jahr wurde das Kraut Mitre September mit S~iure (ca. 11.5",, H_,SO.~, 1125 I/ha) behandelt oder blieben unbehan- delt. Der Bestand wurde vier Wochen spfiter geern- tet. hn zweiten Jahr wurde das Kraut an einem yon fi3nf Daten gespritzt oder nicht gespritzt und nachher in Abst~inden geerntet (Tabelle 1 ). Jede Knolle war in vier gleiche A bsch nitre eingeteilt, der erste beim K ro- nen- und der letzte beim Nabelende, und wurde, wie in Tabelle 2 aufgefi.ihrt, auf Sklerotien bonitiert, hn zweiten Jahr wurde durch jede Knolle ein Lfings- schnitt gemacht und der Prozentsatz des Periderms notiert, das beginnend am Kronenende - mit einer Pinzette entfernt werden konnte. Das Ergebnis wur- de als Beurteilung der 'Reife" verwendet.

Die Ergebnisse des ersten Jahres (Tabelle 3) zeig- ten, dass ein signifikanter Zusammenhang zwischen dem Knollenabschnitt und dem Grad der Infektion besteht. Das Kronenende wies das gr6sste Infek- tionsvorkommen auf, das Nabelende signifikant we-

Verteihmg yon Sklerotien yon Rhizoctonia solani Kiihn auf der ObeiJliiche der

niger. Die Histogramme in den Abb. I und 2 sind ['iir das zweite Jahr und zeigen die Konzentration der Sklerotien auf s~iure- bzw. unbehandelten Parzellen. Die Leichtigkeit, mit welcher das Peridenn von die- serf Knollen entfernt werden konnte, wird in Abb. 3 dargestellt, hn August geerntete Saatknollen yon unbehandelten Pflanzen wiesen am Kronenende we- niger Sklerotien aufals am Nabelende. AIs das Pflan- zenwachstum im September auFh6rte und die Knol- len reiften, bildeten sich am Kronenende Sklerotien, so dass alle Knollenteile gleiche Konzentrationen aufwiesen. Durch die Krautvernichtung mit Sfiure nahmen sowohl die ffir das Reifen der Knollen not- wendige Zeit als auch der damit einhergehende Wechsel der Sklerotienverteilung ab, so fern das Sprit- zen im August geschah, aber nicht, wenn es bis Sep- tember verschoben wurde. Unterschiede in der Skle- rotienverteilung zwischen den Ergebnissen desersten Jahres und einem entsprechenden Datum der Kraut- vernichtung und der Ernte im zweiten Jahr diirften saisonbedingten Unterschieden. welche die Reife- verh~ilmisse beeinllussten, zuzuschreiben sein.

Die Ergebnisse k6nnen teilweise erkliirt werden. wenn man voraussetzt, dass Knollenausscheidungen dem Pilz als Nahrungsquelle dienen, die auf einer jungen Knolle das Pilzwachstum anregt, wS.hrend deren Entzug bei der Reifung der Knolle Sklerotien- bildung verursaeht.

Potato Res. 21 11978J 299

D O R O T H Y S P E N C E R A N D R. A. FOX

R~sum~

La distribution des sclbrotes de Rhizoctonia

pomme de terre

Un examen de plusieurs lots de tubercules de pomme de terre de semence sugg6rait que la distribution fi leur surface des scl6rotes de Rhizoctonia solani Kfihn n'etait pas au hasard. Cette suggestion a 6t~ contr616e en examinant des tubercules r~:coltbs dans deux champs exp6rimentaux.

La premiere annee, les tubercules de semence 6taient infect+s naturellement par des scl~rotes, mais la seconde ann6e un inoculum standard 6tail appli- qut~ fi chaque tubercule pr6alablement ~i la planta- tion. La premi6re ann6e, les fanes +taient trait6es ou non ~i l'acide sulfurique ( 11,5~ H2SO, ~: I 125 I/ha) fi la mi-septembre et la r6colte faite 4 semaines plus tard: la seconde annee, les fanes +talent traitees ou non ~ I'une des 5 dates mentionn6es dans le tableau 1 et r6colt6es a i ntervalles r6guliers par la suite. Chaque tubercule 6tait divis6 en 4 r6gions 6gales, la premiere 6tant fi la couronne etla derni+re au talon etla pr6sen- ce de scl~rotes not6e conform6ment au tableau 2. La seconde annee seulement, chaque tubercule 6tail coup6 selon I'axe longitudinal et on notait le pourcen- tage de p6riderme, se trouvant fi l'extr6mite de la cou- ronne, qui pouvait &re enlev6 fi l'aide de petites pin- ces; le r+sultat +tait consider6 comme une re&bode d'appr6ciation de la maturit6.

Les resultats de la premi6re annie (tableau 3) ont montr+ qu'il y avait une relation significative entre la r6gion du tubercule et le niveau d'in fection fi sa surfa- ce. l_'extremite de la couronne avail la plus grande

solani Kiihn d la smJhce des tubercules de

contamination, l'extr6mit6 du talon significative- ment moins. Les histogrammes des figures I et 2 concernent la seconde ann6e el indiquent la concen- tration de scl6rotes respectivement sur les parcelles trait6es et non trait~es ~i I'acide. La facilit6 avec la- quelle le p+riderme peut ~tre enlev6 de ces tubercules est montr6e 5, la figure 3. Les tubercules de semence, recoltes ;~ partir de plantes non trait6es au mois d'aofit, avaien t moins de scl~rotes ~i ta couronne q u'au talon. D6s que la pousse des plantes cessa en septembre et que les tubercules mflrirent, les scl6rotes se form6rent tellement ~i la couronne que toutes les parties du tubercule eurent des concentrations identiques. En d6truisant les fanes avec de I'acide, on obtint fi la fois une r+duction du temps necessaire fi la maturation des tubercules et une modification de distribution des sclerotes avec la pulv~risation en aofit mais en la repoussant jusqu'fi septembre. Les differences dans la distribution des scl6rotes entre les r+sultats de la premi+re annee et une date correspondante de des- truction des fanes et de recolte dans la deuxieme annee peuvent en ~tre attributes aux differences de saisons, afl'ectant la vitesse de maturation.

Les resultats peuvent &re expliques en pattie en postulant que l'exudation des tubercules est une sour- ce de nourriture pour les champignons, ce qui, sur un jeune tubercule, stimule la croissance des cham- pignons tandis que son absence, lorsqu'un tubercule mfirit, provoque la formation des scl~rotes.

References

Allington, W. B., 1936. Sclerotial formation in Rhizoctoniasolanias affected by nutritional and other factors. Phytopathology 26:831-844.

Artschwager, E. F., 1924. Studies on the potato tuber. J. Agric. Res. 27:809 836. Duncan, D. B., 1955. Multiple range and multiple F tests. Biometrics 1 I : 1~12. Hooker, W. J. & O. T. Page, 1960. Relation of potato tuber growth and skin maturity to infection by common

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