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Identification of fungi on diseased soybean seeds harvested during a high rainfall period in Mato Grosso Do Sul, Brazil

Cortina, Josian Vogel, Theodoro, Gustavo de Faria, Walker, David Russell
ARS USDA Submissions 2013 v.29 no.2 pp. 386
Cercospora, Colletotrichum, Fusarium, Penicillium, Phomopsis, Rhizoctonia, agronomic traits, breeding, developmental stages, discoloration, fungal diseases of plants, fungi, genotype, harvest date, microbial growth, microscopes, pesticide application, pods, rain, seed germination, seeds, sodium chloride, sodium hypochlorite, soybeans, Brazil
The aim of this work was to evaluate the incidence of several genera of fungi on soybean seeds harvested during a period of high rainfall in Mato Grosso do Sul, Brazil. These plants had received four fungicide applications between the R1 and R6 developmental stages, but disease symptom were nevertheless observed on many seeds. Agronomic trait means from 110 plants were determined from data obtained at the time of harvest. From the seeds obtained, 800 were selected that showed discoloration of the tegument, with or without visible fungal colonies. Half of the seeds were superficially disinfected by immersion in a 1% sodium hypochlorite solution for 3 minutes, and all 800 seeds were then incubated to stimulate fungal growth. A modified blotter-test method was used in which 25 seeds were deposited on filter paper placed in a germination box, and a saturated NaCl solution (-1,0 MPa) was used to inhibit germination of the seeds. After incubation for 7 days at 25ºC, fungal growth was inspected using optical and stereoscopic microscopes to identify the genera of the fungi present on the basis of their morphologies. On average, there were 50.3 pods per plant, 2.0 seeds per pod, and 31.7 visibly diseased seeds per soybean plant. The mean weight of 100 seeds was 14.72 g and there were 15.30 g of seed per plant, of which 4.58 g were visibly diseased on average. Among the fungi observed were Fusarium spp. (80-90%), Phomopsis spp. (39-45%), Cercospora spp. (22-30%), Colletotrichum spp. (5-10%), Rhizoctonia spp (<2%). and Penicillium spp. These results showed that there is a need to breed new genotypes with resistance to the most common seed diseases.