Interference of Fungicides
Effects on Entomophthoran
Pathogens of Green Peach Aphid
Abdelaziz Lagnaoui and Edward B. Radcliffe
Potato is sprayed routinely with fungicide for control of foliar pathogens, especially early blight, Alternaria solani (Ell. and G. Martin) Sor., and late blight, Phytophthora infestans (Mont.) de Bary. In Minnesota, Nanne and Radcliffe ( 1971) showed that three fungicides then commonly used on potato--captafol, mancozeb, and Bordeaux mixture--all favored green peach aphid, Myzus persicae (Sulzer), by suppressing entomophthoran (Order: Entomophthorales) fungi. In that experiment, fungicide-treated potatoes had late-season green peach aphid populations 1.7 to 2.6 times greater than did nonfungicidal controls. Aphids in nonfungicidal controls were 22.4 percent infected by entomophthoran pathogens, whereas those in fungicidal treatments were only 4.0 to 5.2 percent infected. Enhancement of green peach aphid on potato is of concern because this aphid species is the most efficient vector of both potato leafroll virus and potato virus Y ( Radcliffe et al. 1991). Annually, these two viruses cost the U.S. potato industry tens of millions of dollars in losses and control costs.
The contribution of naturally occurring entomopathogenic fungi to regulation of aphid pests on agricultural crops has excited little enthusiasm on the part of crop protection specialists. Most who have written on the subject appear to have concluded that entomopathogenic fungi are usually of minor importance in the population dynamics of aphids. Reasons suggested for this presumed general ineffectiveness are inadequate inoculum levels, infection being too dependent upon specific environmental conditions, and dissemination too dependent upon uniformly distributed and abundant hosts ( Hall and Bell 1960, 1961; Shands et al. 1958, 1962,