Submitted to: Bioscience Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/26/2013
Publication Date: 5/3/2013
Publication URL: http://handle.nal.usda.gov/10113/59346
Citation: Cortina, J.V., Theodoro, G., Walker, D.R. 2013. Identification of fungi on diseased soybean seeds harvested during a high rainfall period in Mato Grosso do Sul, Brazil. Bioscience Journal. 29:386-391. Interpretive Summary: Excessive rainfall during the harvest period can increase the incidence of diseases on soybean seeds by prolonging the period of time that the seeds are exposed to fungal pathogens and by promoting the growth, dispersal and reproduction of the fungi. An abundance of rain during the 2010/2011 soybean harvest season in the state of Mato Grosso do Sul (west-central Brazil) resulted in an unusually high incidence of seed disease and presented an opportunity to survey seeds for pathogens. To determine which fungal genera were present in or on diseased seeds, a sample of 800 seeds harvested from 110 plants was incubated for 7 days before being inspected with microscopes to identify the fungi to the genus level. Half of the 800 seeds were superficially disinfected prior to incubation to allow determination of which fungi were actually established in the seed tissue, and not only on the surface of the seed coat. On average, 32% of the seeds inspected had visible disease symptoms. The most frequently observed fungal genera were Fusarium (on 80-90% of the visibly diseased seeds), Phomopsis (39-45%), and Cercospora (22-30%), while Colletotrichum, Rhizoctonia and Penicillium spp. occurred at frequencies of 10% or less. Since seed fungal diseases cannot be controlled by fungicide applications, the development of cultivars with higher resistance to the most common seed pathogens found would provide soybean growers with an effective way to reduce losses in seed yield and quality resulting from excessive rainfall during the harvest season. This information will be useful to soybean pathologists and breeders.
Technical Abstract: 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.