Host sources, virulence and overwinter survival of Rhizoctonia solani anastomosis groups isolated from field lettuce with bottom rot symptoms

Host sources, virulence and overwinter survival of Rhizoctonia solani anastomosis groups isolated from field lettuce with bottom rot symptoms Leonard J. Herr

Department of Plant Pathology, The Ohio State University, Ohio Agricultural Research and Development Center, Wooster, OH 44691-4096, USA

Lettuce plants, with symptoms of Rhizoctonia bottom rot, were collected from two organic soil lettuce production areas (Celeryville in 1989 and Hartville in 1990) in Ohio. The numbers, anastomosis groups (AG), intraspecific groups (ISG), and virulence (greenhouse assays) of the Rhizoctonia solani isolates are reported by specific lettuce type (Boston, bibb, green leaf, red leaf, romaine; and two similarly used leafy vegetables, endive and escarole), plant organs of origin, and production areas. At Celeryville, 58% of total isolates were from green leaf, 20% from escarole, 14% from red leaf and 8% from romaine. Of green leaf isolates, 93% were AG-i-IB and 90% of isolates from escarole were AG-I-IC; 64% of all isolates from Celeryville were AG-I-IB. At Hartville, 16 of 54 total isolates (30%) came from romaine, whereas Boston, endive and bibb had 20, 20 and 19%, respectively; 48% of all isolates were AG-I-IB. AG-2-1 isolates (isolated at Hartville only) were obtained primarily from Boston and bibb lettuce and were eitl~er of low virulence or were non-pathogenic in greenhouse assays on romaine lettuce. AG-I-IB isolates collectively were the most virulent, A G - I - I C were intermediate and AG-2-1 least virulent of the major AGs. Single isolates of three AGs of R. solani overwintered significantly better on the soil surface than when buried in soil in the field. AG-4 inoculum recovered from a soil depth of 15 cm was significantly less virulent on romaine lettuce than were the other buried AG inocula. It is suggested that the particular A G - I S G isolates occurring on lettuce are determined mostly by preceding crops and that, if so, it may be possible to shift the 'mix' of AGs towards less virulent groups by crop rotation and thus to reduce yield losses. Keywords" lettuce; overwinter survival

bottom

rot;

Recently, H e r r (1992) r e p o r t e d on the anastomosis groups ( A G s ) and intraspecific groups (ISGs) of Rhizoctonia solani Kfihn, t e l e o m o r p h Thanatephorus cucumeris (Frank) D o n k , associated with b o t t o m rot disease of lettuce (Lactuca sativa L.) in organic soils in Ohio. O f the R. solani isolates from diseased lettuce plants collected at the two lettuce production areas assayed (Celeryville, 1989 and Hartville, 1990), a total of six different A G - I S G were isolated ( A G - I - I B , A G 1-1C, AG-2-2, A G - 4 f r o m Celeryville, and A G - I - I B , A G - I - I C , AG-2-1, A G - 4 , A G - 5 f r o m Hartville). Isolates of A G - I - I B p r e d o m i n a t e d in each production area. Isolates from each area were assayed for virulence in greenhouse tests on romaine lettuce. [Nomenclature and concepts of pathogenicity, virulence and related t e r m usage are based on those of Shaner et al. (1992)]. O f the numerically m a j o r A G s collected, A G - I - I B isolates collectively were the most virulent, isolates of A G - I - I C were intermediate, and isolates of AG-2-1 were least virulent. [Previously, the AG-2-1 isolates were tentatively separated into three provisional I S G s

Rhizoctonia solani;

anastomosis

groups;

host

sources;

based on cultural types (Herr, 1992); however, for this study no distinctions are m a d e a m o n g the AG-2-1 isolates.] A m o n g the numerically minor A G s , AG-2-2 isolates were highly virulent, and the remaining A G s were of lesser degrees of virulence. Additionally, low numbers of binucleate Rhizoctonia spp. and Laetisaria arvalis Burdsall isolated from both production areas were non-pathogenic in all tests. In the preceding report (Herr, 1992), the A G - I S G isolates were grouped by, and reported according to, the two lettuce production areas; however, in the study reported here, the same A G - I S G isolates are grouped by host source (field host, mostly lettuce types) f r o m which they were isolated for each production area. Further, the greenhouse virulence assay results (Herr, 1992) have b e e n simplified and are reported, similarly to the isolation data, by host sources. Separate reports were chosen because the two sets of data are distinct and the combined report was excessively long. Townsend (1934) investigated overwinter survival of R. solani (field studies) in diseased lettuce plant debris

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Bottom rot of lettuces, Rhizoctonia solani AG: L.J. Herr

and reported good survival in debris on the soil surface, but poor or no survival in debris buried 15 cm deep in soil. Further, Townsend (1934) reported that the R. solani isolates from lettuce included different strains, which differed in virulence, growth at various temperatures and cultural characteristics. However, his studies do not provide any information on the relative (differential) overwinter survival of such strains. Few, if any, comparative studies of overwinter survival of different AG-ISG of R. solani have been reported. The objectives of this study were (a) to investigate the relationships among host plants (lettuce types) and the AG-ISG of R. solani isolated, (b) to investigate the relative overwinter survival of some different AG-ISG of R. solani, and (c) to attempt to identify factors governing the occurrence of particular AG-ISG of R. solani in given lettuce fields in relation to potential disease control measures.

Materials and methods Field collections of symptomatic lettuce plants

As previously reported (Herr, 1992), the two Ohio lettuce production areas sampled were CeleryviUe and Hartville. Both areas have the same soil type, namely Rifle peat, an euic Typic Borohemists (Anonymous, 1975). Lettuce types including Boston, bibb, green leaf, red leaf, romaine and two similarly used leafy vegetables, endive and escarole (the latter two types both Cichorium endivia L.) with bottom rot symptoms were collected from growers' fields on 19 July and 14 August 1989 at Celeryville, and on 21 June, 18 July, 26 July and 16 August 1990 at Hartville. Sampling date isolation data were pooled and are reported by host, season and production area. Isolation and characterization of isolates

Plants were washed, separated into leaves, crowns and roots, and pieces of tissue were plated on 2% water agar (Herr, 1992). Stock cultures were maintained on potato dextrose agar (PDA). The AG-ISG of the isolates was determined by the technique and criteria of Ogoshi (1985), as previously reported (Herr, 1992). Host and plant organ sources of AG-ISG

At the time of plating lettuce plant samples, the isolation plates were labelled by host source and plant organs plated. Colonies recognized as R. solani (also colonies of binucleate Rhizoctonia spp. and L. arvalis, a basidiomycete not Rhizoctonia, isolated at low frequencies) were transferred to PDA stock cultures. Subsequently, these cultures were classified to A G ISG and assayed for virulence in greenhouse assays (Herr, 1992). Listings of these data were made by host source (lettuce type), plant organ of origin, AG-ISG, and production area. Additionally, for each host a composite virulence rating (CVR) was determined.

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This value was obtained by summing the disease ratings (DR) (two assays combined, eight replicates per isolate) of labelled isolates, from previous greenhouse assays on romaine lettuce (Herr, 1992), according to host source, regardless of AG-ISG designations, and dividing this sum by the number of isolates obtained from the given host. These values gave useful relative measures for comparison of isolate virulence among the different hosts. CVR values are reported separately for each production area. Overwinter survival of isolates

Moistened barley grain (30 ml distilled water per 50 g barley grain in 250 ml flasks), autoclaved at 121°C for 60 min (Gaskill, 1968) was seeded with two discs (diameter 7 mm) cut from 7-day-old PDA cultures of selected single isolates of AG-I-IB, AG-I-IC and AG4. Culture flasks were incubated for 14 days at 24°C, harvested, air-dried for several days in a chemical fume hood, then screened (mesh screen, 4.76 mm) to separate grains, and stored in paper bags at room temperature (--24°C) for future use (Herr, 1988). After 6 days of storage, an initial viability test of the inocula was made by plating colonized grain of each AG isolate on 2% water agar (WA) in plates (five grains per plate, six replicate plates per anastomosis group isolate). Barley grains with hyphal growth were recorded following 24 and 48 h of incubation at 24°C. For the overwinter survival test, 150 colonized barley grains were placed in packets made from plastic screen squares (10 X 10 cm, mesh 1.0 mm) stapled around the edges. Four replicate screen packets were prepared for each AG isolate and for each of two survival treatments (burial depths). Screen packets of colonized grain were placed between two square pieces of wire screen (30 X 30 cm, mesh 12 mm), four replicate screen packets per wire screen unit. The two wire screen squares, one placed over the other, were wired together to retain the screen packets and then were appropriately labelled with moisture-proof tags for later identification. Three assembled units were placed on the soil surface and three units were buried 15 cm deep in Rifle peat soil at Hartville in a grower's field on 27 November 1990. Units were recovered on 23 April 1991, after 116 days in the field. Upon recovery, the screen packets of colonized, overwintered grain were washed to remove soil, and 100 grains from each replicate packet of each AG isolate and depth of burial were plated on WA plates (ten colonized grains per plate, ten replicate plates per screen packet of grain), incubated at 24°C and numbers of grains with R. solani hyphal growth were recorded after 24 and 48 h. Data were analysed by analysis of variance (ANOVA). Virulence assays

The colonized barley grains that remained after viability had been tested were used to determine the virulence

Bottom rot of lettuces, Rhizoctonia solani AG: L.J. Herr

of the surviving A G isolates on romaine lettuce. Romaine cv. Tall Guzmaine MF lettuce plug transplants were produced and transplanted as previously reported (Herr, 1992), and the soil in each pot (diameter 10 cm) was infested with ten grains of overwintered, colonized barley, five plants (pots) per replicate of each of the three A G isolates and the two burial depths. Five holes (2 cm deep) were made in the potting soil mix of each pot adjacent to the lettuce transplant plug and two grains of colonized barley were placed in each hole. The holes were then filled by adding 50 ml sterile soil per pot. G r e e n h o u s e temperatures were maintained at 26 + 3°C day and 20 + 2°C night. Supplemental fluorescent lighting (16 ~tmol m -2 s-1) was provided daily for 14 h. Disease evaluations were made 10 days after infestation. A D R scale (1, healthy; 2, diseased; 3, dead plants) was used. Data were analysed by A N O V A as a completely randomized design. In greenhouse tests, an isolate of A G - I - I C , isolated from escarole (Celeryville) and highly virulent on romaine was assayed for virulence on escarole cv. Florida D e e p along with an isolate of A G - I - I B from green leaf lettuce (Celeryville) and highly virulent on romaine, to ascertain their relative virulence on escarole. Two similar tests were run using four replications of ten plants per replicate per A G isolate. An appropriate uninfested control treatment was included. Seedling production and soil infestation methods were the same as those in a previous report (Herr, 1992). Soil was infested with two discs (diameter 9 mm) from 7-day-old cultures on P D A , buried in slits (2 cm deep) in potting soil on opposite sides and immediately adjacent to the transplanted escarole plant plug. After 7 additional days, the escarole plants were rated for disease using the D R scale previously given. Results were analysed by A N O V A as a completely randomized experimental design.

Results Isolates from lettuce host s o u r c e s

At Celeryville, 29 of 50 isolates (58%), were obtained from green leaf type lettuce; isolates from escarole constituted 20%, red leaf 14% and romaine 8% of the total isolates (Table 1). Twenty-seven of 29 isolates from green leaf lettuce were A G - I - I B . Moreover, the CVR for green leaf lettuce was the highest (2.8) of all host sources (Table 1). The CVR for escarole ranked second (2.3) and included predominately A G - I - I C isolates, which indicates that A G - I - I C isolates were mostly of intermediate virulence. Red leaf and romaine lettuces ranked lowest in CVRs, reflecting their different, more diverse isolate compositions (R. solani AGs, binucleate Rhizoctonia spp., L. arvalis). The reference AG-2-2 IV isolate from sugar beet, which was highly virulent (killing most plants) on sugar beet (Herr, 1988), was only moderately virulent on lettuce; however, the two AG-2-2 isolates from green leaf killed romaine lettuce plants.

Table 1. Rhizoctonia isolates from field lettuce plants with bottom rot symptoms collected during 1989 at Celeryville, listed by crop, plant organ of origin, anastomosis group-intraspecific group (AG-ISG), and composite virulence rating (CVR) of hosts Crop, plant organ source

AG-ISG

CVR a

Green leaf Leaf (16) Crown (13)

(29) b A G - I - I B AG-2-2

(27) b (2)

2.8

Escarole Leaf (10)

(10) AG-I-IB AG-I-IC

(1) (9)

2.3

Red leaf Leaf (7)

(7)

AG-I-IB AG-I-IC AG-4

(3) (2) (2)

1.6

Romaine Leaf (4)

(4)

AG-I-IB BN c L.a. a

(1) (2) (1)

1.5

Sugar beet Crown (1)

(1) e

AG-2-2IV

(1)

2.2

"Sums of disease ratings (DRs) (two assays combined, eight replicates per isolate, for individual isolates in greenhouse assays on romaine lettuce), according to host source, regardless of AG-ISG designations, divided by the number of isolates obtained from the given hosts; bno. of isolates in parentheses; CBN, binucleate Rhizoctonia spp.; aL.a., Laetisaria arvalis; ereference isolate

A n almost equal number of green leaf isolates were obtained from leaves and crowns (Table 1); however, for all host lettuces combined, more isolates (37) were obtained from leaves than from crowns (13), and no isolates were obtained from roots. At Hartville, the greatest number of isolates was obtained from romaine type lettuce, 16 of 54 (30% of the total) isolates (Table 2). Boston, endive and bibb types each had approximately equal numbers of isolates (20, 20 and 19%, respectively) and escarole had 11% of the total number of isolates. The reference isolates listed from green leaf (Celeryville), the young lettuce seedlings with pinched-off stem symptoms, and garden beet were included in the tests, although they differed from the other isolates in source (Table 2). Of 16 romaine lettuce isolates, 14 (88%) belonged to AG-11B of R. solani. Additionally, the CVR for romaine lettuce ranked highest (2.6) of all host lettuces, even outranking endive isolates, which were all (100%) A G l - l B . Obviously, the virulence of individual A G - I - I B isolates was variable. The CVRs for endive A G - I - I B isolates and the reference A G - I - I B isolates from green leaf (Celeryville) were equal. Boston and bibb lettuce hosts were distinguished by the occurrence of high numbers (nine of 11 and eight of ten isolates, respectively) of AG-2-1 isolates. Six of the AG-2-1 isolates from Boston lettuce were non-pathogenic, whereas five of the AG-2-1 isolates from bibb lettuce were nonpathogenic. Thus, the CVRs for Boston and bibb lettuces were low (1.5 for both) and were equal to that of escarole, which had a more varied isolate composition (Table 2). The binucleate Rhizoctonia spp. isolates from the lettuce seedlings and garden beet were non-

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Bottom rot of lettuces, Rhizoctonia solani AG: L.J. Herr Table 2. Rhizoctonia isolates from field lettuce plants with bottom rot symptoms collected during 1990 at Hartville, listed by crop, plant organ of origin, anastomosis group-intraspecific group (AG-ISG) and composite virulence rating (CVR) of hosts

Crop, plant organ source

AG-ISG

CVRa (14)b (1) (1)

2.6

(11) AG-2-1 AG-4 AG-5

(9) (1) (1)

1.5

Bibb Leaf (1) Crown (9)

(10) AG-I-IC AG-2-1

(2) (8)

1.5

Endive Leaf (8) Crown (3)

(11) AG-I-IB

(11)

2.2

Romaine Leaf (13) Crown (3)

(16)b AG-I-IB AG-I-IC AG-2-1

Boston Leaf (9) Crown (2)

Discussion

Escarole Leaf (4) Crown (2)

(6)

AG-I-IB AG-I-IC AG-2-1 BNc L.a. a

(1) (1) (2) (1) (1)

1.5

Green leaf Leaf (2)

(2)

AG-I-IB

(2)

2.2

Lettuce seedling with pinched-off stem Crown (2)

(2)

BN

(2)

1.0

Garden Beet Root (1)

(1)

BN

(1)

1.0

~aAs in Table1 p a t h o g e n i c ( C V R s 1.0), as w e r e all similar isolates and L. arvalis isolates tested. A g a i n , m o r e isolates w e r e o b t a i n e d f r o m leaves (35) t h a n f r o m crowns (19) for all isolates f r o m the Hartville area. Bibb lettuce was the exception a m o n g the hosts, with nine isolates f r o m crowns and only o n e f r o m a leaf. N o isolates were r e c o v e r e d f r o m roots. Overwinter survival of isolates

All colonized barley grains of each o f the three A G isolates plated in the initial l a b o r a t o r y viability test run b e f o r e the overwintering test in the field h a d g r o w t h of R. solani h y p h a e , which indicated that the barley grains w e r e u n i f o r m l y colonized. Survival of all R. solani A G s was significantly greater for inocula placed o n the soil surface than for inocula buried 15 cm d e e p (Table 3). Survival of A G - 4 i n o c u l u m was significantly less t h a n for A G - I - I B and A G - I - I C inocula, which did n o t differ significantly f r o m o n e a n o t h e r . Virulence assays

In the virulence test o n r o m a i n e lettuce, barley grain i n o c u l u m o f A G - 4 buried 15 c m d e e p in soil h a d a significantly lower D R t h a n all o t h e r A G inocula

(Table 4).

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B e c a u s e nine of ten isolates f r o m escarole w e r e A G 1-1C, the c o m p a r a t i v e virulence o f a selected A G - I - I C isolate, with a selected A G - I - I B isolate (both virulent on r o m a i n e ) , was tested to ascertain w h e t h e r escarole was m o r e susceptible to A G - I - I - C than to A G - I - I B (Table 5). I n each of two tests, l~he A G - I - I B isolate was significantly m o r e virulent on escarole than the A G - 1 1C isolate. N o disease s y m p t o m s were evident on the uninfested escarole controls in either test.

T h e p o p u l a t i o n s o f R. solani isolated f r o m s y m p t o m a t i c lettuce hosts and the o t h e r two leafy vegetables collected (endive and escarole) consisted o f a diversity o f different AGs, differing in frequencies o f isolation f r o m the various hosts and locations, and these A G s possessed variable virulence capabilities a m o n g and within the g r o u p s (Tables 1 and 2; H e r r , 1992). A hypothesis that differences in the virulence o f isolates could a c c o u n t for the differences in n u m b e r s of isolates o b t a i n e d f r o m given hosts and the frequencies of isolation of given constituent A G isolates was examined. O f all R. solani isolates o b t a i n e d at Celeryville, 64% were A G - I - I B (collectively the m o s t virulent of the A G s ) . T w e n t y - s e v e n A G - I - I B isolates, 54% of the 50 total Celeryville isolates, were o b t a i n e d f r o m green leaf lettuce ( C V R 2.8, highest), w h e r e a s 6% were f r o m red leaf lettuce ( C V R 1.6), 2% were f r o m escarole ( C V R 2.3) and 2% were f r o m r o m a i n e lettuce ( C V R 1.5). Thus, a l t h o u g h isolates of A G - I - I B were o b t a i n e d f r o m all host sources at Celeryville, these A G - I - I B isolates w e r e unequally distributed a m o n g hosts (Table 1). In a direct c o m p a r i s o n o f the virulence of an A G - 1 1C isolate f r o m escarole (nine of ten isolates A G - I - I C ) and an A G - I - I B isolate o n escarole, the A G - I - I B isolate was significantly m o r e virulent than the A G - 1 1C isolate (Table 5). I conclude, tentatively, that the high f r e q u e n c y of isolation of A G - I - I C isolates f r o m escarole in the field was n o t attributable to their high virulence capabilities on escarole.

Table 3. Overwinter survival (27 November to 23 April) of single isolates of three anastomosis groups of Rhizoctonia solani on colonized barley grain placed either on the soil surface or buried 15 cm deep in the field in organic soil

Category

Mean number of barley seeds yielding colonies

Placemenff ,c Surface 15 cm depth

80.1 59.4

Anastomosis groupb'd AG-I-IB AG-I-1C AG-4

77.5 81.1 50.0

aMeans of combined isolate-placement depths based on 100 grains per individual replicate (four replicates per AG isolate; three AGs); bmeans of combined isolate placement treatments based on 100 seed per individual replicate (four replicates per AG isolate); *F significant;al.s.d.(005)20.1

B o t t o m rot of lettuces, Rhizoctonia solani AG: L.J. Herr Table 4. Greenhouse virulence assay of overwintered (27 November to 23 April) barley grain inoculum of single isolates of three anastomosis groups of Rhizoctonia solani placed either on the soil surface or buried 15 cm deep in the field in organic soil Disease rating~ Anastomosis group AG-I-IB AG-I-IC AG-4

Surface

15 cm d e p t h

2.6 3.0 3.0 1.s.d.(o.os) 0.5

2.9 2.9 2.1

aDisease rating (DR) scale: 1, healthy; 2, diseased; 3, dead; mean DR based on four replications of five romaine lettuce plants each, infested with ten recovered barley grain inocola per pot (all plants were rated healthy in uninfested control pots)

At Hartville, 48% of all R. solani isolates were A.G1-1B (Table 2): 26% were obtained from romaine lettuce (highest CVR), whereas 20% were from endive (second highest CVR), 2% from escarole and 0% from Boston and bibb lettuces (the latter three had similar low CVRs). Thus, at Hartville, as at Celeryville, the AG-I-IB isolates were unequally distributed among the hosts. Further, for both romaine and escarole from the two locations, the percentages of total isolates and the CVRs varied independently [i.e. isolates for romaine accounted for 8% of total isolates at Celeryville (CVR 1.5), and 30% of total isolates at Hartville (CVR 2.6), whereas 20% of total isolates were from escarole at Celeryville (CVR 2.3) and only 11% of total isolates at Hartville (CV R 1.5)]. A differential seasonal effect of environm-ent on the susceptibilities of romaine and escarole wou!d have to be postulated to account for the results. Even more noteworthy were the high populations of AG-2-1 isolates obtained from both Boston and bibb lettuces at Hartville, especially the high numbers of non-pathogenic AG-2-1 isolates (for Boston, six of nine isolates and for bibb, five of eight isolates were non-pathogenic). The occurrence of these non-pathogenic (avirulent) AG-2-1 isolates on Boston and bibb lettuces was undoubtedly attributable to factors other than their 'high' virulence capabilities. Possession of a potentially high capacity for virulence thus does not necessarily result in high densities of given AG isolates on given lettuce hosts. The hypothetical positive relationship of isolate virulence to isolation frequencies of given AGs from hosts is not substantiated by these results. A more probable (albeit incompletely described) hypothesis of the lettuce type-R, solani AG isolation frequency observed could be associated with the existence (however brought about) of variable inoculum densities of AGs among fields before cropping to lettuce. Presumably, R. solani AGs in 'relatively' high inoculum densities, specifically those that lie within zones influenced by plant-microbial interaction within soils, could rapidly exploit the substrates available and thus could predominate on plant surfaces and conse-

quently in isolations from plants. Virulent isolates would destructively parasitize the lettuce hosts, whereas non-pathogenic isolates could possibly sustain their populations by symptomless parasitism of hosts, as reported by Daniels (1963) for (Corticium solani) Rhizoctonia species. Moreover, these non-pathogenic or low-virulence isolates might compete with more virulent AG isolates and suppress their occurrence on lettuce hosts. In this context, Ischielevich-Auster et al. (1985) reported biocontrol of cotton damping-off by a non-pathogenic (hypovirulent) isolate of AG-4, which protected seeds and seedlings from invasion by virulent AG-4 isolates. Previous cropping effects are the most probable cause of variable AG inoculum density distribution patterns among fields. Many vegetable crops have short growth cycles; consequently, vegetable fields are often intensively cropped to several short cycle vegetable crops per season. After harvest and incorporation of crop residue in soil, new crops are planted quickly. Because little or no decomposition of previous crop residues occurs between successive crops, the following crop is exposed to those R. solani AGs (and other plant pathogens) present in the residues incorporated. Assuming that specific crops may have differential effects on inoculum densities of given AGs of R. solani, these effects could then carry over to the following crop. Additionally, other unknown saprophyticparasitic fitness capabilities may be involved in determining the prevalence of various AGs on lettuce. Differential survival over winter could constitute one of the saprophytic fitness capabilities affecting inoculum densities in soil. An R. solani AG-4 isolate survived less well in barley grain inoculum overwintered in the field than did an isolate of AG-I-IB and AG-I-IC. Survival of all AGs tested was better in inocula on the soil surface than in buried inocula. The latter results are in accordance with those reported by Townsend (1934) for lettuce and those of Herr (1973) for R. solani survival in sugar beet. Further, the reduction in disease incidence found with the buried, overwintered AG-4 inoculum (Table 5) probably was directly related to the reduced survival of that inoculum (decreased inoculum potential).

Table 5. Comparative virulence of a Rhizoctonia solani A G - I - I C isolate (from escarole in the field in 1989) and an A G - I - I B isolate (from green leaf lettuce in 1989 at Celeryville) on escarole in greenhouse tests Test

Anastomosis group

D i s e a s e r a t i n g a _+ s.d. b

ic

AG-I-IB AG-I-IC

2.02 + 0.18 1.24 _+ 0.11

2d

AG-t-IB AG-I-1C

2.24 _+ 0.17 1.20 _+ 0.16

aDisease rating: 1, healthy, 2, diseased; 3, dead plant; mean of four replicates of ten plants each per AG isolate (noninfested control disease rating 1.00, both tests); bs.d., standard deviation of the mean; Ctest I isolate difference p = 0.0001; dtest 2 isolate difference p = 0.0001

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M o s t isolates of R . s o l a n i w e r e r e c o v e r e d f r o m leaves ( T a b l e s 1 and 2 ) , fewer f r o m crowns and n o n e f r o m

References

roots. T h e s e results are o f interest b e c a u s e lettuce heads with diseased leaves often can be t r i m m e d to saleable heads, b u t c r o w n infections result in loss of heads. A c c o r d i n g to T o w n s e n d (1934) and K o o i s t r a (1983), o u t e r leaves o f lettuce b e c o m e diseased first and, o n c e initiated, d e c a y spreads u p w a r d s and inwards f r o m leaf to leaf. If, as h y p o t h e s i z e d , the particular A G - I S G isolates occurring on lettuce are d e t e r m i n e d m o r e b y those occurring on previous crops t h a n by the virulence of given A G s to given lettuce hosts, it m a y be possible t h r o u g h crop r o t a t i o n to shift the 'mix' o f A G s t o w a r d less virulent g r o u p s and thus to r e d u c e disease incidence and yield losses. Cultural practices, such as ploughingu n d e r crop residues in the a u t u m n , m a y also contribute to disease control.

Anonymous (1975) Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys. USDA Soil Conservation Service Agric. Handbook 436. US Government Printing Office,

Washington, DC Danieis, J. (1963) Saprophytic and parasitic activities of some isolates of Corticium solani. Trans. Br. Mycol. Soc. 46, 485-502 Gaskill, J. O. (1968) Breeding for rhizoctonia resistance in sugarbeet. J. Am. Soc. Sugar Beet Technol. 15, 107-119 Herr, L. J. (1973) In field survival of Rhizoctonia solani in soil and in diseased sugarbeets. Can. J. Microbiol. 22, 983-988

Herr, L. J. (1988) Biocontrol of Rhizoctonia crown and root rot of sugar beet by binucleate Rhizoctonia spp. and Laetisaria arvalis. Ann. Appl. Biol. 113, 107-118 Herr, L. J. (1992) Characteristics of Rhizoctonia isolates associated with bottom rot of lettuce in organic soils in Ohio. Phytopathology 82, 1046-1050 Ischielevich-Auster, M., Sneh, B., Koltin, Y. and Barash, I. (1985) Suppression of damping-off caused by Rhizoctonia species by a nonpathogenic isolate of R. solani. Phytopathology 75, 1080-1084 Kooistra, T. (1983) Rhizoctonia solani as a Component in the Bottom Rot Complex of Glasshouse Lettuce. Meded. Plantenziektenkundige Dienst. No. 160, 144 pp

Notes and acknowledgements Salaries and research s u p p o r t p r o v i d e d by State and F e d e r a l funds a p p r o p r i a t e d to the O h i o Agricultural R e s e a r c h and D e v e l o p m e n t Center, T h e O h i o State University and grants f r o m the O h i o Fresh M a r k e t and Processing V e g e t a b l e R e s e a r c h F o u n d a t i o n , O h i o State R e s e a r c h Challenge F u n d and K. W. Zellers & Son, Inc. J o u r n a l Article 220-92.

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Ogoshi, A. Anastomosis and intraspecific groups of Rhizoctonia solani and binucleate Rhizoctonia. Fitopatol. Brasil. 10, 371-390 Shaner, G., Stromberg, E. L., Lacy, H., Barker, K. R. and Pirone,

T.P. (1992) Nomenclature and concepts of pathogenicity and virulence. A. Rev. Phytopathol. 30, 47-66 Townsend, G.R. (1934) Bottom Rot of Lettuce. N.Y. (Cornell) Agric. Exp. Stn Mere. 158, 46 pp

Received 4 January 1993 Revised 4 March 1993 Accepted 4 March 1993