Overview: Blight occurs worldwide and is the most common bacterial disease of soybean especially during cool, wet weather. It occurs throughout the United States, where many cultivars are susceptible. Estimates of yield losses of up to 40% have been reported. The disease is most conspicuous in fields during mid-season under moist conditions when the bacterium remains active until checked by hot, dry weather.
Symptoms: Blight lesions are most conspicuous on leaves but also occur on stems, petioles, and pods. Small, angular, translucent, water-soaked, yellow to light brown spots appear on leaves. The centers soon dry out, turn reddish brown to black, and are surrounded by a water-soaked margin bordered by a yellowish green halo. Young leaves are most susceptible. Infected young leaves are distorted, stunted, and chlorotic. The angular lesions enlarge in cool, rainy weather and merge to produce large, irregular dead areas.
Causal Organisms: Pseudomonas savastonoi pv. glycinea, the causal bacterium. Colonies on nutrient agar are circular, smooth, and glistening, with an entire margin; they are white and raised but not viscid. The optimal temperature for growth is 24° to 26°C. At least nine races of the pathogen have been identified. Strains of the bacterium infect bean, cowpea, lima bean, and tepary bean. Strains of the pathogen carrying the avrB gene elicit a hypersensitive reaction in cultivars carrying the resistant gene Rpg1.
Disease Cycle: P. savastonoi pv. glycinea overseasons in surface crop residue and in seeds. Seeds can be infected through the pods during the growing season, or they may be invaded during harvesting. Primary infections on cotyledons often result in secondary lesions on seedlings. The bacterium is spread during windy rainstorms and during cultivation while the foliage is wet.
Management: Highly susceptible cultivars should not be planted in areas where the disease is a potential problem. Four resistance genes, Rpg1, Rpg2, Rpg3, and Rpg4, have been identified, using a cross between cvs. Flambeau x Merit. Line LL 489-605 also is resistant.
