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First Report of Sudden Death Syndrome of Soybean in Delaware and Eastern Shore of Maryland

In August and September of 2000, soybean (Glycine max (L.) Merr.) plants from two fields in Sussex County, Delaware, and one field from Somerset County on the eastern shore of Maryland exhibited typical symptoms of sudden death syndrome. The season had been wetter and cooler than normal. Leaf symptoms ranged from small chlorotic spots to elongated regions of interveinal necrosis. Leaflets dropped leaving attached petioles in the upper canopy. Severely infected plants were easily pulled from the soil and had taproots with blue sporodochia, necrotic cortical tissue, and necrosis of secondary roots (2). Initial isolations from the infected plants were made from the basal stems, discolored taproots, vascular tissue, and directly from blue sporodochia. Sections were plated on water agar (WA) amended with neomycin and streptomycin, WA with antibiotics and chloramphenicol, and acidified potato dextrose agar (PDA). The isolates were slow growing on PDA, often staining agar dark maroon, produced little aerial mycelium, and formed macroconidia in blue sporodochia. The fungus was identified as Fusarium solani (Mart.) Sacc. based on spore morphology. Plugs (5 mm) of the fungus from 14-day-old cultures were placed next to the stem just below the soil line of 14-day-old plants of soybean cvs. Essex and Lee 74. Eighteen plants of each cultivar (three per pot) were inoculated and placed on a greenhouse bench for 43 days at 21°C (±2°C). Six noninoculated control plants were also included. Plants were rated for the presence of stem lesions and foliar symptoms. Of the inoculated plants, 70% had mottling, rugosity, and leaf cupping, 6% had severe interveinal leaf necrosis, and 52% had distinct stem lesions at the soil line. Control plants were symptomless. F. solani was recovered from all symptomatic plants and presumed to be F. solani f. sp. glycines based on spore morphology, color, lack of microconidia, and symptoms (1). A more extensive test was conducted to confirm Koch's postulates. Eleven isolates of F. solani f. sp. glycines were grown as before and used to inoculate Essex soybeans as previously described. Inoculated and control plants were randomized on the greenhouse bench and watered using an individual pot irrigation system. Fifty-six days after inoculation plant height was reduced 12% compared with the noninoculated controls. Lesions produced on the lower stem and taproot of the inoculated plants averaged 4.5 cm long. Most plants had mild foliar symptoms that included mottling, rugosity, and leaf cupping. Only three plants had severe foliage symptoms. F. solani f. sp. glycines was recovered from 56% of inoculated plants, completing Koch's postulates for all 11 isolates. Noninoculated controls were symptomless. Sudden death syndrome was not observed in 2001. Soybean is an important crop in the region; 250,000 ha were harvested in 2000 on the Delmarva Peninsula, which includes the three counties of Delaware, nine eastern shore counties of Maryland, and two counties of Virginia. Sudden death syndrome could be a serious threat to profitable soybean production. To our knowledge, this is the first report of sudden death syndrome from this area and represents the most eastern occurrence of this disease reported in the United States.

References: (1) K. W. Roy. Plant Dis. 81:259, 1997. (2) K. W. Roy et al. Plant Dis. 81:1100, 1997.