Bacterial Tan Spot, A New Foliar Disease of Soybeans
1983
Dunleavy, J. M.
A new soybean [Glycine max (L.) Merr.] disease that causes large tan necrotic spots on soybean leaves was first observed in Iowa in 1975. Necrotic areas usually fell to the ground during high winds, giving a ragged appearance to leaves. The disease was confined to leaves and was never observed on stems or pods; wilting did not occur. The causal bacterium was gram positive, motile, formed a medium-yellow endopigment, grew well at 37 C, hydrolyzed esculin, did not produce urease, and was pathogenic to soybeans and beans (Phaseolus vulgaris L.). The bacterium was identified as Corynebacterium flaccumfaciens (Hedges) Dowson, and the name bacterial tan spot was proposed for this disease (which had been observed in 16 counties in Iowa through 1981). One isolate of C. flaccumfaciens from soybeans, two isolates from beans, and one isolate each of C. flaccumfaciens subsp. aurantiacum Schuster and Christiansen and C. flaccumfaciens subsp violaceum Schuster, Vidaver, and Mandel from beans were used to inoculate leaves of soybeans and beans. All isolates caused a leaf spot and wilt of beans, but only the soybean isolate caused a leafspot on soybeans. Of 20 northern-grown soybean cultivars tested, three were resistant to the disease, seven were slightly susceptible, three were susceptible, and seven were very susceptible. Of 20 southern-grown cultivars, eight were resistant, and 12 were slightly susceptible.
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