Abstract
Areas under the disease progress curves were more reliable than the apparent infection rates or final disease ratings for the quantification of effects of fungicide and general resistance on infection by Phytophthora infestans in potatoes. General resistance and fungicide application each reduced disease increase and the effects of each were additive. This relationship between general resistance and fungicide was similar whether fungicide dosage was adjusted by altering the concentration of weekly applications or by varying the intervals between applications. Among 11 commercial cultivars and 1 breeding line, the difference in general resistance between the most resistant and most susceptible clones was estimated to be equivalent to weekly applications of about 0.7 kg mancozeb (a.i[active ingredient])/ha. Growers should be able to complement general resistance in potato cultivars by adjusting fungicide either by altering the dose per scheduled application or by altering the interval between applications.

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