An Epidemic Record of Endemic Alternaria Blight of Spurred Anoda. X. B. Yang, Department of Plant Pathology, 217 Plant Science Building, University of Arkansas, Fayetteville 72701. D. O. TeBeest, Department of Plant Pathology, 217 Plant Science Building, University of Arkansas, Fayetteville 72701. Plant Dis. 77:1164-1167. Accepted for publication 9 July 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-1164.

Alternaria blight (Alternaria macrospora) of spurred anoda (Anoda cristata) is endemic in the Mississippi Delta area. An epidemic of the disease was observed in a 33-ha field of soybean (Glycine max) in Arkansas in 1992. The soybean field was heavily infested with spurred anoda with densities varying from less than five to 27 stems per square meter. In 2 wk, the disease developed from a few foci observed in early August throughout the field. Field maps of soybean stand, weed density, and disease levels were made. Defoliation varied from 30 to 100%, depending on distance and direction from disease foci in middle August. Disease gradients were determined by examining infections on leaves or stems at 3-m intervals from a focus to 180 m away. The infection gradient was shallow for leaf but steep for stem infections. A high incidence of rust (Puccinia heterospora) on spurred anoda was also observed in the field. Spurred anoda plants over the entire field were killed by the end of August (soybean growth stage R4). The control of this weed by natural infection of an endemic disease over such a large area was associated with cool and rainy weather.

Keyword(s): biological control.