The epidemiology of <i>Phytophthora infestans</i>. IV. Spraying trials, 1959 to 1963, and the infection of tubers

Abstract

SUMMARY The methods needed to decrease losses from potato blight in the variety Majestic are different from those required for King Edward. Few Majestic tubers become infected and control depends chiefly on prolonging the life of the haulm to ensure the maximum yield of tubers. With King Edward, infected tubers often cause more loss than premature defoliation. Tuber infection often occurs very early in the attacks, when profuse sporulation coincides with sufficient rain to penetrate through soil to the tubers. Spraying with fungicide delayed initial tuber infection of King Edward but did not necessarily decrease the proportion of tubers infected at harvest. The largest yield of sound tubers from fungicide‐sprayed plots in the 1960 epidemic was obtained after killing the haulm when only 5% had been destroyed by blight. Allowing the foliage to survive later than this stage did not increase the yield of sound tubers even in years when blight attacks were late and slow. Protection with fungicide and haulm destruction are complementary treatments, both of which require careful timing to obtain the best result with tuber‐susceptible varieties.