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Title: RATE-REDUCING RESISTANCE TO FUSARIUM SOLANI F. SP. PHASEOLI UNDERLIES FIELD RESISTANCE TO SOYBEAN SUDDEN DEATH SYNDROME (SDS)

Author
item NJITI, V - SOUTHERN ILLINOIS UNIV
item SUTTNER, R - SOUTHERN ILLINOIS UNIV
item Gray, Lynn
item GIBSON, P - SOUTHERN ILLINOIS UNIV
item LIGHTFOOT, D - SOUTHERN ILLINOIS UNIV

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Soybean sudden death syndrome (SDS) is a serious soybean disease caused by the fungal pathogen Fusarium solani. Under favorable environmental conditions the disease can limit soybean productivity. There is little information available concerning when the fungus first invades soybean roots in the field. To answer this question, field resistant and susceptible soybean plants were planted in two field locations in Southern Illinois in 1995. At specific time periods, plants of each susceptible and resistant soybean variety were sampled for the presence of the SDS Fusarium fungus. Small root pieces from each plant were placed on culture medium and the number of root pieces containing the fungus were determined for each sample. The fungus was recovered from the roots of both susceptible and resistant soybean plants soon after planting in both field plots. However, when the plants started to flower, the Fusarium fungus was recovered less frequently from the roots of resistant soybean plants than from susceptible plants. These results document that the Fusarium fungus does not develop as fast on resistant soybean plant roots as on susceptible plant roots during the critical seed development period of the soybean plant. These results have provided new information about an important soybean disease that will be useful to soybean plant breeders and soybean plant pathologists that are working on this disease.

Technical Abstract: This study aimed to separate field resistance into partial resistance and/or tolerance to the fungus Fusarium solani (Mart.) Sacc. f. sp. phaseoli (Burk.) Snyd. & Hans., type A. We planted seven field resistant cultivars (including 'Forrest', 'Ripley', 'Jack' and PI 520.733) and five field susceptible cultivars (including 'Essex', 'Spencer' and Asgrow 5403) in two locations that were naturally and uniformly infested with F. solani. Twenty taproot samples per cultivar were taken at 1-3 week intervals after plant emergence through the R8 growth stage. f. solani isolation was attempted from six taproot sections per plant, on selective media. Infection frequency (IF) was determined directly as the percentage of infected plants. Latent period (LP) was determined indirectly from the infection severity (IS) measured as the percentage of root segments with F. solani. Disease incidence (DI) and disease severity (DS) were measured in the field from R5 to R7, standardized to the R6 stage. Disease index was calculated as (DI*DS)/9. Infection of the soybean plants was detected within 15 days after planting (DAP) (IF=5-10%). Over the whole season all susceptible and resistant cultivars tested were infected by F. solani. However, both the IF and IS were significantly (P< 0.05) higher (IF-35.2-54.7%, IS=10.0.0-20) from susceptible cultivars than from resistant cultivars (IF=25.4-42.3%, IS=4.6-14.6). We concluded that SDS resistance in Forrest, Ripley, Jack, ExF 44, and PI 520.733 is a late season rate-reducing resistance analogous to partial resistance to phytophthor root rot. Resistance reduces both IF and IS, probably by extending the LP. In contrast, susceptible cultivars Essex and Asgrow 5403 showed reduced leaf symptoms but high IF and IS.