J. C. Tu Harrow Research Station, Agriculture Canada, Harrow, Ont.

Integrated Control of the Pea Root Rot Disease Complex in Ontario

Southern Ontario is a prime agricultural district in Canada, and processing g m n peas are gown there to supply the domestic market. In 1983, Ontario growers farmed 8,600 ha of peas (3). A survey of 550 ha showed that 26.5% of the plants had root rot disease (Fig. I), and many fields showed total losses. When root rots were kept under control, a yield of 5 tlha could be achieved. In Ontario in 1983, average yields were 2.2 tiha. The value of this yield loss of 2.8 t/ha, or approximately 24,000 t, was estimated at $6 million U.S. (3).

Pea root rot is a worldwide probIem. and more than 20 different fungi can cause thedisease (20). In Canada. Basu et at (5) reported that the most frequently isolated root rot fungi of peas were Fusarium solmi (Mart.) Appel & Wr. f. sp.pisi (F. R, Jones) Snyd. & Hans., F. oxysporum Schlecht. t sp.pisi Snyd. 6t Hans., and Pythium spp.; Ascochyta pinadella t. R. Jones and Rhizoctonio solani KIIhn were isolated occasionally. The report was based on limited disease surveys and pathogen isolation and was insufficient to develop an integrated disease control (IDC) strategy for the Ontario pea root rot complex. A detailed survey was therefore undertaken to obtain information about epidemiology, cultural practices, and environmental factors so that necessary research could be conducted and an IDC program developed.

Epld~mlology Etiology. Approximately 50% of the

commercial fields in Essex and Kent counties of southwestern Ontario were surveyed during late May and mid-June in 1983, and 782 cultures were isolated and identified. Frequencies of isolation of F. sohni, F, oxyspowm. Aphanomyces euteiches D ~ c h s . . and Pyrhium spp.

were 7:4:1:1 (17). Other fungi, e.g.. R. solani. A. pinodella, and Thielaviopsis basicola (Berk. & Br.) Ferraris, were isolated only occasionally. Overall disease severity and yield loss are the sum contributed by each component fungus.

The incidence of pea root rots in southern Ontario averaged 26.5%. Disease severity of each root rot was determined on a scale of 0 to 9, where 0 = < 10% of root with symptoms, I = 10-20% of root with symptoms, etc., 9= plant dead, The severity of disease caused by F, soloni (root rot), E oxysporum (wilt), Pythium, and Aphanomyces averaged 3.2,8.7,2.6, and 4.0, respectively. Isolates of F. oxysporum were identified as races 1 and 2 by means of a comparative differential scheme (12) and known races provided by J. M. Kraft of Prosser, Washington. The ratio of race 1 to race 2 was 7:4.

On the basis of this infomation, and assuming that hosts, environmental conditions. and cultural uractices are random relative to isolatibn, a simple disease damage index (DDI) can be developed to rank the root rot fungi according to etiologic importance. The DDI of a root rot equals the total amount of root rot X the frequency of occurrence of each root rot fungus X the severity of disease mused by each fungus. The DDIs (Table 1) show that Fwsarivm wilt is the most serious problem, followed by Fusariurn root rot. Plants with Fusariurn wilt usually die, whereas plants with Fusariurn root rot show various degrees of stunting and yellowing and often droop during warm, sunny afternoons. but rarely die. Fythium and Aphanornym root rots are considered to be minor proMems in Ontario.

Symptoms. F. solani f. sp. pfsiroutinely infects the cotyledon, epicotyl, and hypocotyl (Fig. 2), and the disease progresses to the soil line and down the root system. Early symptoms are stunting and epinasty. Reddish brown

streaks on the roots coalesce as the disease progresses. and the infected areas later appear dark to chocolate brown. Severely affected plants are stunted and chlorotic, and lower leaves die.

F. oxysporum f. sp, pisi, races 1 and 2, infects peas during the early stages of plant growth. Early symptoms are yellow to light orange discoloration in the vascular tissue of the taproot in an otherwise normal looking root system. Later, the discoloration extends upward to the basal stem [Fig, 3). Yellowing of the leaves starts from the basal part of the plant and progresses to the tip. The disease progresses quickly on a warm sunny day, and the plants die rapidly.

Pyrhiurn spp. arc the same fungi that cause damping-off in young seedlings. When the plants grow older and become more lignified, however, the infection tends to be directed toward young feeder roots. The roots break off at areas where soft rot develops, which usually appear tan to light brown (Fig. 4). Diseased plants appear to lack vigor and often show some stunting and yeltowiag, with reduced yields.

A. euteiches infects the epicotyl of peas at the early seedling stage and the roots at

< <

Fungus

Fusorium soluni

F. oxyspom Pyrhbm spl Aphurromyr

eureiches

r a m I. DE! ,f each Fun# :omplex (a%

Ease damal pls in the C zrage incidl

re index (D lntario root ence = 26.5

Dl) rot 96) - DI -

.S 'E 5

8

'Frequency 'Disease sev where 0 symptoms,

of occurrer trity, based = <lo%

1 = 10-20 . a ,

ICC.

I o n a W s c of root v % of Foot . . .

b ale. vith wi th

symptoms. etc.. Y = plant acaa.

Plant DiseaseIJanuary 1987 9

any growth stage. Infected arms of ttre epicotyl flrst become waw-smked, then turn brown. The cortiwl tissue softelw and is easily sloughed. The epicotyl of severely diseased seedlidp may rot completely, and the Wlings collapse and die. If the roots become infected, light brown Liom develop a& U r g e and spread into the cortex. The cortex tis8ues turn dark chocolatc brown, kcom soft, md am easily slougbad (Fk 5). Surviving plan& usually ahow pltowing of the tower leaves, poor growth. and sevm atunring. Yield is scwrtly curtailed.

DIwwre el&. Ihe four main root rot fungi in Ontario hew similar d i cych. F. o x y m and F. sdaAi produce maidla and aeptatc m p l & during the infeetiow stage and cbtamydo- spores during the reti- stage. przhitm spp. and A. cuteichm produce zaospoies and ascptatc m p i a during the iddous stage and oospom during the rating stage. P. rrl~imum, however, d m not praduce -ores. Botb chhmydmpom and ocwpoma can survive in soil for many years. Fungal populations increase rapidly in soils on which peas and other susccptibk legumes have kengrown but

d- gradually over time in the a h = of ausccptibk hosts. Furmum spp. may ako sunrive on the roots of nonhost MOP. pvthium and Aphmro- mycef spp. have a wide host range and may survive pathogmically or sapropby- tieally by colonizing other crops and d (10). Any of the main root rot fungian kdwminatod bymovementai contamimted so& d i d plant debris, a d contamiaatcd or i n f d sad

In Ontuio, p a am grown on medium to heavy clay barn soil. Fields are chid- plowed, smoothed, and cultipacked before d i n g . Seeds an mtcd with a chemical formulation consisting of -pun, dhzinon, and lindarre to control maggots, wirmrorms, and W y . Succes- sive mwiw arc made during April and carly May on the bmii of aceumulatud beat units. lkc seeds art sown in 2 k r n rows at a rate of 250-400 kglha, dependig on seal h e end p e r # n w of germination. For w e d comrol, a paste- bwbide, w d y MCPB or MCPAIMCPB (I: IS), M applied at the thrrs-trifolirtc stw. Oraa peas are Iuwkd in late June and most of July, and fields are then left bare until the next spine (4).

Most cultivars in use are highly susceptibk to the root rot complex. A 3-year rotation with two of four crops, i.k, wheat, aoybeaa, corn, or snap h n , is -rally pmtbd .

H a r b l d k ~ a n o C p c r t o ~ rot, The eflect of MCPB or MCPAI MCPB on p a mot rot wrs invdgattd in a heavily infested fi ld, uing a split- plot design. When the p h t s mchcd the threparifoliau stage, one-half d each

plot war sprayed with MCPAl MCPB at a reeommmded raw (1.7 kglba). One week after spraying, plants in the spmycd area t ~ m more chlototicaud wilted than those in the cuntrol area. Spraying with MCPB or MCPAl MCPB apmred to be r stress factor that i n c d root rot swcrity and quickened the expression of root rot symptom.

Many herbicides are s t d u l and predispose plants to d i including root rots (I). Dimitmadint harbieidea. however, have bem found to h a the severity of some root rob (1,16). Scvctal diniuoaoitiac and triazine berbieides, including pendimcihalin, ayanazine, trifluralin. and orydn, are rtported to have fewer advent effsets on peas with root rot than th phcnoxy herbicides, eg. MCPA. MCPB, sad MCPAI MCPB ( f 9). For example, when c o m p d with trifluralin, MCPAl MCPB promoted disease dcvdopmmt that multed in kmd d y a d q u k h c d symptom exprdon: this adverse cfftct iacmscd with temperatures from 20 to 28 C (19). H d t h y plants, on the other hand, did not appear to b adversely a f f d by either trifluralin or MCPA/ MCPB treatment. In fact. MCPA/ MCPB slightly stimulated plant growth. Appar- ently, pea plants with F d u m wilt and Fumium root rot we= mort mwcptibk than haltby p b b to pbenoxy herbi ia , whereas no such difference was obscrwd io tdl~ralin-mttd pear (19).

Dinivoaniliw h e r b i e s d h p t the flaseUaofzoosporw(I6)aadtharrsdwe the h t y of root rots csussd by phyeomyatous fungi such as Whrm. Aphunomy#~, a d PIarmodIopItom ( I). b u r e most fields in Ontario haw a high d d t y of m u m spp. and a low density of p v ~ h spp. (S,I7), the

F@ Z Frtllrkrm root Pol (F. mhnl it. ~p Fig. 8. Fw#lurn wlR (F. DX-IR L mp. pkl) of plu (A) Wlllsd pl.ntl on rlgM, h l t h y pill) of- p l ln t~k lC ; (B )~ .ymptorm.

10 Plant DbeasoNol. 71 No. f

reduetiom in severity of pea root rot may bt attributable to something other than disruption d the flsplia of zoospores. Tht ~ n o x y ~ o m p o d s MCPA MCPR 4 MCPAl MCPBaresorrsmrbat similar in structure and function to 2,eDand am growth promoters (2), whereas the d i a i t m d h htrbiddea inhibit protein ryntbcsis and tend to reduce root ebngatiom (2). PomiMy, the demase in m t i t y of mot rot in plants trtated with dinitrodhe h w b i i w is W t s d with reduced mtdccllulardonption, ia contrast to the increased root elonption and plant growth indud by P ~ ~ X Y h b i c k , Rapidpwthuually mulu in thianw, more tender dl waUs and also r physiological dcmaad on the roots. Furtber m h is DeFdGd to elucidate the mechanism -using this diffennee.

S t d trt.hasal. Trutment of 8 4 s witb aptan done d w not control tk fuugi that =use p a raat rot in Ontario. Rmults arc bctrcr with new formulatioas that include fungicides specific to Aphanomyces rpp. and/ or Pyrhium spp., such as metalaxyl (Ridomil), furakxyl(Fongarid), benalaxyl (Galben), and Dowco 444.

Field testa with t m t d #sd (60 g a.i.ll00 kg) of the ptr cultivar GO 313 {wrinkWcd) s h o d that captan + Galbea + Ridomil (formulation a) arsd captan f Dowco 444 + Ridomil (formulation b) wen more dkt ivc than 0 t h combinatio~. Thtle two formu- latiom not only p ~ d seeds from rou and dampinpoff but also had rwiduaI effects @mat the root rot complex. For seeds treated with formulation a, fomuhtion b, or aptan or not tc##d, germination was 84, 82, 78, and 64% r w r p c c t i v e l y ; t h e r m t ~ ~ ( ~ ~ ~

I mouth after gtrmirution was 7.1. 7.5, 8.6. and 8.8. respectively. The high root rot i a d h wcn tx ja td . hecause of the residual effects of seed treatment c b m h b and because GG 313 is highly susceptible to root rots.

W eomp.dlon. Pea planu grown in artaa of soil compacted by tracton and tillage equipment havt ohcn been reported as being stunted and baving h c m d incidenct and severity of root rot (9,21). We haw found hat plants m w mlallex and have sign8mtly lower root rot incidence and di#ase severity in m i d (0.2 m) &beds rhnn in wnven- tionally prepared flat sewlkls. Elmer plant growth and reduced root rots am cclrnlstad witb lower mil peaetromcttr readings (Tabk 2). fmproved soil drainage and acration in raised bcda may also contribute to reduction of root rot.

Lack 0 t h drainy*. Pea fields in wuthwattra Ontario have medium to heavy day loam soil d most are tiled. k m s c the f~ldsare normally smoothed and cultipacked before seeding to prepare a flat ground surface for hawcsting, no surface drainage i s provided. Tdc drains often w o t , drain water quickly after a heavy rain, so temporary puddh form in the lower areas of a field. Plants grown in the f l d e d n a r c muaUystuated and hrw a bigher iPcide~a of root rot. For pas in high and low anarm plant heights ayetap 56.2 and 14.5 cm and rot rot indices are 7.8 and L7, respoedvely. Graaa ~#l#ao aopa S o h in south-

wlesrern Ontario are generally low in organic matter owing to continuow a h p farming. Low wil organic matter affects mil texture in t e rm of porosity and crumb formation and consequently hinden root development

and plant gtowth. Root rots arc known to k greater in numbtr and mote severe in soil with Low organic matter. lncorporatioa of organic matter reportedly d d root 101 in wveral crops (7,8,14). In an attempt to increase soil organicmatter and prevent wind snd soil erosion during the winter, various g m a manure crops wtm sown immo- diately a h peas were hamsled. By mid- or htc Oetober, the mps were at full- blossom mgt and wwe choppod and cWed into the mil. W were planted t k next spring, and the severity of mot rot war assmud in late Jum. Oats, sorghum, sudsngrass. and corn significantly reduced the *verity of o f t rot, when compared with fitlds Mt hnre: oats war the most eliseriw, followed by sorghum, then sudapgmu (18).

TilImge praeticn. Stwral tillage practices were evaluated for effectiveness in aUeviathg soil eompdoa. Fall c h i 1 plow + spring flat scadbad pmparatioa and fall plow + spring raised s d b o d prcpsration si@f!kantly reduced nwrt rot &ty. compared with conventional fall plow f spring flat seedbed pnpa- ration. Root rot was mast wvere with fsll plow + cornpaction, followsd by fall plow + fail &kl pmration ( 181.

Cdliru s t k t h . Most cultiwrs wed in Ontario arc highly susceptible to both Fusarium wilt and root rot. To determine if any dtivprs are ~~t to the Ontario root rot complex, several o b t s d from various research or&- zations and seed companies were evaluated for specific resistance to Fusarium wilt and nonspecific mistance to Fusariuln, Pythium, 8ndAphPmlnym root row Somc pthotypcr of F. sdanl, P. ultimum, a d A. mreichcs have betn

- -Fm P b t ~ ( o m ) 65.1anH.6b Diosus-

(%I 1 m 8 tw. Dircue &y' 7.7 r 8.8 b s a i t p - m d i

mdhgat1Qom: (tl-3 f lA 18.0

d r w t w h b l y m m I =ID-maf rool vltb S y m m a, 9 = p b l dud. %dl Remdh& %il Peawmmaor mdel Muk 1 d I 2 W a a m

reported (6.1 1.13). but these are mot believed to Be highly specialized yet.

Each year from 5983 to 1986, between 100 and 158 lines were tested in four replications of single-row plots in a completely randomized plot. Root rot severity was rated on a 0-9 scale during the last week of June: 20 plants were examined in each plot. All cultivars with a rating higher than 4.0 were eliminated from Further testing. and new cultivars

Table 3. Cultivars and lincs showing rcsistancc to pea root rot throughout a 3-year (1983-1985) trial in which more than 220 were tested

Root rot fndtx* C a l v m and I h

1.1-2.0 389171, RS-7 2.1-3.0 Frontier, Gmn

Giant 53 1 (Harrow selection). Melody, Minn 108, Puget, SNS, WSU R22

3.1-4.0 Alpha 1, Bolero, Early Frosty. Early Perfeaion, Frosty, Haral. Home Guard. Little Sweetie. M-129, Mara, Massettc, Mercurio, New Era. Olympia, Parlay. Perfection W. R., Sprite

'Based on aQ-9 scale, whcrc 0=< 1095 of raot with symptoms, 1 = 10-20%of root with symptoms, ctc., 9 = plant dead. Any cultivaror linewitharating higher than 4.0 was eliminated from further testing.

were added to the test in subsequent years. Table 3 lists the cultivars and lines that showed resistance throughout the test years. Although most of the cultivars were susceptible (raot rot index > 4.0), many were moderately to highly resistant (root rot index G4.0). Bolero and Sprite, two cultivars with moderate resistance, were introduced as part of an integrated disease control program in southwestern Ontario and have contributed significantly to improved yields. Many cultivars have a better root rot index (i.e., 1.1-3.0) than Bolero and Sprite (Table 3) and offer further potential for controlling root rot.

Soll lndexlng Root rot fungi are soil inhabitants, and

the incidence and severity of root rots are related to the density of inocula in the soil. Because fields in which root rots have been severe are likely to have the same problem the next time a susceptible cultivar is planted, the density of inocula in a field should be determined before peasare planted. Sherwood and Hagedarn (1 5) developed a soil-indexing method for common root rot (A. eureiches). WE adapted the method to the Ontario pea root rot complex (predominantly F. solaniand F. oxysporum)and determined the relationship between the root rot index and yield (Fig. 6). The method is laborious and costly, however, and we are recommending an on-site program in which growers test selected cultivars in a small area 1 year before the pea crop is grown.

Conclusions The results of our survey and research

led to the following conclusions: I. Many commercial cultivan have

Fig. 6. Relatlonahlp belwemn root rat Index and yleld of green p e a

12 Plant DiseaseiVeI. 71 No. 1

6 -

5 -

- 4 - r

2 --. 6 3 - t fE 0 s 2 -

1 -

0 0

good tolerance or resistance to Ontario's root rot complex.

2. Phenoxy herbicides such as MCPA, MCPB, and MCPAl MCPB predispose

a

m

. ' "

. r . * . . . . . *

* t 2 0 . . .

am

* l

. I 1 I 1 1 1 1 I 1 1 2 3 4 5 6 7 8 9

peas to root rot, whereas dinitroanilinc and triazine herbicides such as cyanazine, oryzatin, pendimethalin, and trifluralin do not.

3. Peas grown in raised seedbeds have significantly lower root rot incidence and severity than those grown i n flat seedbeds.

4. A green manure crop such as oats, sorghum, or sudangrass reduces root rot severity in subsequent pea crops.

Root rot inden

5. Soil compaction increases root rot incidence and severity.

6. Fall chisel plow or fall plow + spring raised seedbed reduces root rot severity mere than other tillage practices. 7. Seed treatment with captan +

GaIben + Ridornil is more effective than captan alone in reducing the incidence of rots and damping-off.

8. An on-site soil indexing exercise reliably determines the level of field infestation as well as cultivar susceptibility.

These conclusions form the foundation of aur IDC program: planting resistant cuftivars, using seed treatment formu- lations, avoiding MCPAI MCPB, reduc- ing soil compaction, practicing fall chisel

I

*.I 4 ,

II

I

i '4

(*: , 4 ,A

J. C.Tu

Dr. Tu Is a research plant pathologist wlth Agriculture Canada at the Harrow Research Station in Ontaria. He obtained hls B.Sc. and M.S. degrees from National Talwan University and hFs Ph.0. degree in 1966 from WashingtonStateY nivanity at Pullman. H e investigates diseases of whrte beans, soybans. and peas. Hiscurrent Interests center on epidemiology, etiology, and integrated pest manege ment of these crops.

verage pea itario, for 1

GG 512 Mini 38 1 GG 45 1 E. J. 235 - . . bparKle GG313 All

$sex and

1m GC 512 306.8 3,334.5 Mtni 381 12I.8 2,837.7 GG 451 400.5 3.063.9 E. J. 235 90.9 1,841.1

66.4 986.4

Sparkle All

GG 512 Mini 381 GG 451 Bolero

plow or fall plow + spring raised seedbed, using soil indexing, and planting green manure crops in the intervals between pea crops. The average yield per hectare increased from 1,42 1 kg in 1983 to 2,974 kg in 1984 to 4,150 kg in 1985 (Table 4). In Essex County alone, this yield increase was valued at about $500,000 U.S. in 1984and % I million in 1985. The increase is attributable to selecting fields with low root rot infestation, discontinuing use of MCPBand MCPAI MCPB, and replacing susceptible cultivars such as GG 31 3 and E.J. 235 with resistant cuItivars such as Bolero and Sprite; the increase in 1985 is also partially attributable to favorable weather conditions. Practicing the recommended cultural practices should stabilize yields in the 3,00@4,500 kglha range. Yields of 5,000-6,000 kg/ ha could be achieved by adopting other IDC measures, which largely depends on efforts by extension workers.

Literature Clted 1. Altman, J. 1985. Impact of herbicidts on

plant diseases. Pages 227-231 in: Ecology and Management af Soitborne Plant Pathogens. C. A. Parker, A. D. Rovira, K. J. Moore, and P. T. W. Wong, eds. American Phytopathological Society, St. Paul, MN. 358 pp.

2. Anonymous. 1974. Herbicide Handbook of tbe Weed Science Society of America. 3rd ed. Weed Science Society OF America, Champaign, IL. 430 pp.

3. Anonymous. 1983. Agricutturc statistics for Ontario. Ont. Agric. Food Minist.

PubI. 20. Toronto, Ont. 80 pp. 4. Anonymous. 1983. VegetabIe production

recommendations. Ont. Agric. Food Minist. Publ. 363. 62 pp.

5. Basu, P. K., Crete, R., Donaldson, A. G.. Gourley,C. O., Haas, J. H., Harper, F. R., Lawrence, C. H., Seaman. W. L.. Toms. H. N. W., Wong, 1.. and Zimmer, R. C. 1973. Prevalence and severity of diseases of processing peas in Canada, 1970-71. Can. Plant Dis. Surv. S3:49-57.

6. BoIton. A. T., Donaldson. A. G.. and Nuttall, Y. W. 1470. Variation in isolates of Fusarium solani F. pisi collected from processing peas in Ontario. Can. Plant Dis. Surv. 50:108-109.

7. Borst, G . 1983. Organic matter heips control root rot spread (Phyrophrhora cinnamomi avocode). Avocado Grower 7(12):38-39.

8. Bouhot, D. 1981. Induction of a biological resistance to Pythium in soils by the contribution of an organic matter. Soil Biol. Biochem. 133269-274.

9. Burke, D. W., Hagedorn, D. J., and Mitchell. J. E. 1970. Soil conditions and distributions of pathogens in relation to pea root rot in W~sconsin soils. Phyto- pathology 60:403-406.

10. Hagedorn, D. J..ed. 1984. Compendiumof Pea Diseases. American Phytopathological Society. St. Pad, MN. 57 pp.

11. Kraft, J. M., and Burke, D. W. 1970. Pylhium ultimum as a root pathogen of beans and peas in Washington. Plant Dis. Rep. 55: 1056- 1060.

12. Kraft, J. M., and Aaglund, W. A. 1978. A reappraisal of the race classification of Fu~arium oxysporurn f. sp. pisi. Phyto- pathology 68973-275.

13. Pfender. W. F.,and Hagedocn, D. 1. t983. Disease progress and yield Ioss in Aphanomyces root rot of peas. Phylo- pathology 73: 1 109-1 I 13.

14. Pittman, U. J.. and Horricks, J. S. 1972. Influence of crop residue and fertilizers on stand, yield and root rot of barley in southern Alberta. Can. J . Plant Sci. 523463469,

IS. Sherwood, R. T., and Hagedorn, D. J . 1958. Determining common root rot potential of pea fields. Wis. Agric. Exp. Stn. Bull. 53 1. 12 pp.

16, Teasdale, J. R., Harvey, R. G., and Hagedorn. D. J. 1979. Mechanism for the suppression of pea (Pisum sativum) root rot by dinitroaniline herbicide. Weed Sci. 27: I95-20 1.

17. Tu, J . C. 1986. Incidence and etiology of pea root rots in southwestern Ontario. Can. Plant Dis. Suw. In press.

18. Tu, J, C., and Findlay, W. 1. 1986. The effects of different green manure crops and tillage practices on pea root rots. Proc. Br. Crop Prot. Conf. Pesrs Dis. In pwss.

19. Tu, J . C., and Hamill. A. S. 1986. Differential effect of two groups of herbicides, dinitroaniline and phenoxy. on pea root rat. Proc. Br. Crop Prot. Conf. Pests Dis. In press.

20. U.S. Department o f Agriculture. 19M. Index of Plant Diseases in the United States. Agric. Handb, 165. U.S. Govern- ment Printing Office, Washington, DC. 531 pp.

2 I. Vigier, B.. and Rag havan, G. S. V. 1980. Soil compaction effect in clay soils on common root rot in canning peas. Can. Plant Dis. Surv. 60:43-45.

Plant Disease/January 1987 73