Development of Drechslera sorokiniana on Sequentially Senescent Leaves of Poa pratensis Exposed to Postemergence Herbicide Combinations. Clinton F. Hodges, Department of Horticulture, Iowa State University, Ames 50011. Plant Dis. 68:213-215. Accepted for publication 6 September 1983. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/PD-68-213.

Combinations of 2,4-D, 2,4,5-TP, MCPP, and dicamba at concentrations of 10^-6 M were applied as soil drenches to Poa pratensis to determine their effect on development of Drechslera sorokiniana. Disease response to the herbicide combinations differed between the two youngest and two oldest leaves of the shoot. Disease severity on the youngest leaves decreased in response to combinations including 2,4,5-TP. Disease development was inhibited by 2,4-D + dicamba on the two youngest leaves. Disease response to the combination of 2,4-D + MCPP did not differ from that of the component herbicides. Only MCPP + dicamba showed an additive stimulatory effect on disease on the two youngest leaves. Most herbicide combinations induced an additive stimulation of leaf spot on the two oldest leaves. Only 2,4-D + 2,4,5-TP on leaf 3 and MCPP + dicamba on leaves 3 and 4 failed to stimulate disease additively.