Influence of preemergence herbicides on pathogenesis by Drechslera sorokiniana on sequentially senescent leaves of Poa pratensis

Abstract

Four preemergence herbicides (benefin, bensulide, DCPA, siduron) were evaluated for their effect on the severity of leaf spot induced by Drechslera sorokiniana (Cochliobolus sativus) on sequentially developing and senescing leaves of a single shoot of Poa pratensis. Preemergence herbicides stimulated, had no effect on, or inhibited leaf spot on shoot leaves of various ages. Stimulation of leaf spot primarily occurred on the two youngest visible leaves of the shoot and was induced by bensulide (all concentrations) and DCPA (10−9, 10−6 M). Benefin and siduron had no effect on leaf spot development on the two youngest leaves and none of the herbicides inhibited leaf spot on the two youngest leaves. The various herbicides were primarily inhibitory of leaf spot development on the two oldest leaves of the shoot. Leaf spot was inhibited on leaf 3 by benefin (10−12, 10−9 M), DCPA (10−12, 10−9 M), and siduron (10−9, 10−6 M); inhibition on leaf 4 was induced by benefin (all concentrations), DCPA (10−12 M), and siduron (10−9, 10−6 M). Only bensulide at 10−12 M stimulated leaf spot on the two oldest leaves and siduron at 10−12 M on the oldest leaf. The predominant stimulation and inhibition of leaf spot by the herbicides on the two youngest and the two oldest leaves of the shoot, respectively, is discussed relative to the usual increase in disease on progressively older leaves.