Effect of host plant resistance and reduced rates and frequencies of fungicide application to control potato late blight

Field experiments were conducted during 1998 to 2000 to determine the respo nse of commercial potato cultivars and advanced breeding lines (ABL) differ ing in susceptibility to foliar late blight (caused by Phytophthora infesta ns) to reduced rates and frequencies of residual, contact fungicide applica tions. When environmental conditions were most favorable for the developmen t of late blight, the lowest application rate of the fungicides chlorothalo nil or fluazinam (33% of the manufacturers' recommended application rate [M RAR]) gave unsatisfactory control of potato late blight. Under conditions m oderately conducive for late blight development, effective control was achi eved with 33 to 66% MRAR with either fungicide. The Michigan State Universi ty advanced selection, MSG274-3, was the least susceptible ABL tested and, during 1998 to 2000, late blight was effectively managed using reduced rate s of fungicides. Application rates of chlorothalonil (33 to 100% MBAR) sign ificantly reduced late blight in the cultivar Snowden (5-day application in terval) compared with the nontreated control; whereas, late blight was not effectively controlled in Snowden even at 100% MRAR of chlorothalonil at ei ther 10- or 15-day application intervals in 1999 or 2000. The ABL MSG274-3 was the least susceptible of all cultivars and ABL used in this study, and required minimal chemical protection against late blight. The study demonst rates that ABL with reduced susceptibility to late blight can be managed wi th reduced fungicide rates and longer application intervals, thus offering more economical control of this disease.