Evaluation of soybean cultivars for their reaction to sudden death syndrome, caused by Fusarium solani f. sp. glycines, in Tucumán, Argentina

ZAMORANO, M A; PLOPER, L.D.; GONZÁLEZ, V; GÁLVEZ, M R; JALDO, H; DÍAZ, C

Foz do Iguassú, Paraná, Brasi

Conferencia; VII World Soybean Research Conference (VII WSRC).; 2004

Embrapa SOJA y WSRC Continuing Committee

Sudden death syndrome (SDS), caused by Fusarium solani f. sp. glycines, is a disease of increasing importance in soybean crops of northwestern Argentina (NWA).  One of the most effective disease management strategies is the use of resistant cultivars.  The objective of this study was to evaluate the reaction to SDS of commercial cultivars available in NWA.  A test was conducted during the 2002/03 growing season in La Invernada, Tucumán, in a field where high levels of the disease had been observed in previous years.  Twenty-four commercial cultivars, from Maturity Group (MG) IV to IX, were evaluated using a randomized complete block design with 3 replications.  Plots consisted of four 6-m rows spaced 0.5 m apart.  Evaluations were carried out, starting at growth stage V1, every 15 days until the appearance of SDS foliar symptoms, and weekly thereafter. To analyze the data a Disease Index (DI) was calculated, based on the Maximum Incidence (MI) and its corresponding Severity (S), according to the following formula: DI= (MI x S) / 5.  Incidence was determined as the percentage of diseased plants in the plot, and the Maximum Incidence was the highest incidence observed in each plot during the season. Severity was estimated using a 0 to 5 scale, where 0= no foliar symptoms, and 5= >70% of the foliar area with chlorosis or necrosis. In most cultivars first foliar symptoms appeared between R1 and R3, and only two of them showed symptoms before flowering.  All cultivars showed the MI between R5 and R6 with an average severity of 3.5.  Cultivars were ranked according to a Disease Grade (DG) scale, from 1 to 5, which was established by grouping the DIs.  In this DG scale, 1= no disease; 2= DI from 0.1 to 9; 3= DI from 9.1 to 15; 4= DI from 15.1 to 30; and 5= DI greater than 30.  Three cultivars were classified as DG 4, nine cultivars as DG 3, and eleven as DG 2. None of the cultivars showed DIs corresponding to DG 1 or 5.  These results confirm the differential response of soybean cultivars to SDS.  The data also show that there are cultivars available in the region with acceptable levels of resistance to the disease.  The use of a DG scale, which uses a disease index based on maximum incidence, was considered to be useful for evaluating cultivar reaction to SDS under natural conditions.