POTATO FUNGICIDES INTERFERE WITH ENTOMOPATHOGENIC FUNGI IMPACTING POPULATION-DYNAMICS OF GREEN PEACH APHID

Fungicides applied to potato can enhance green peach aphid, Myzus pers icae (Sulzer), outbreaks by interference with entomopathogenic fungi. (Order Entomophthorales). Late season aphid numbers were highest in po tatoes sprayed with metalaxyl + mancozeb, captafol, or mancozeb, and l owest in potatoes sprayed with benomyl, triphenyltin hydroxide, chloro thalonil, or copper hydroxide. In field-collected aphids, Pandora (= E rnyia) neoaphidis (Remaudiere et Hennebert) and Entomophthora planchon iana Cornu (F. Entomophthtoraceae) were the predominant cause of mycos es, 66.7% and 22.3%, respectively. Conidiobolus obscurus (Hall and Dun n) Remaudiee and Keller (F. Ancylistaceae) accounted for 8.5% of mycos es. In the laboratory, fungicides were shown to have direct effects on these entomopathogens. Metalaxyl + mancozeb, mancozeb and captafol we re strongly inhibitory of germination of conidia, copper hydroxide was intermediate, and chlorothalonil had little effect. Triphenyltin hydr oxide, benomyl, metalaxyl + mancozeb, and mancozeb were strongly inhib itory of growth of mycelia, copper hydroxide was intermediate, and chl orothalonil and copper hydroxide had least effect. Benomyl was highly toxic to green peach aphid, copper hydroxide and chlorothalonil interm ediate, and captafol, mancozeb, and metalaxyl + mancozeb least toxic. Possible interference of potato fungicides with aphid pathogens is now an important consideration because of the intensity of spraying requi red to protect the crop from infection by metalaxyl-resistant strains of the late blight pathogen, Phytophthora infestans (Mont.) de Bary. M innesota potato growers reported high green peach pressure in both 199 5 and 1996, years of intensive fungicide spraying. Concomitantly, ther e was a marked increased in the incidence of PLRV in seed lots entered for winter testing.