Failure of metalaxyl to eradicate late blight on potatoes (cv. Green Mountain)

Eradication of potato late blight with metalaxyl was examined in field studies from 1980 to 1983. In 1980 disease outbreak and subsequent progression were late and moderate. In 1981 and 1983, they were early and moderate to severe, while in 1982 the disease became established later in the season but developed rapidly and resulted in moderate to severe damage to foliage. There were no significant differences in foliar disease severity among metalaxyl treatments. While treated plots generally showed significantly less foliar damage throughout the season than untreated plots, except in 1983, the reduction in foliar damage in September was only 30–50% in 1980, 10–20% in 1981, 20–30% in 1982, and less than 10% in 1983. Sporulation of the pathogen occurred readily on infected leaves of both treated and untreated plants. There were no significant differences in yield, but application of metalaxyl to foliage significantly reduced the incidence of late blight tuber rot.