Fungicide resistance in cucurbit powdery mildew (Sphaerotheca fuliginea) and its effect on field control

Abstract

Decreased fungicidal control of powdery mildew (Sphaerotheca fuliginea (Schlecht: Fr.) Poll.) in commercial cucurbit crops led to an investigation to determine whether fungicide resistant strains were present. In field trials, fungicides such as bupirimate (100 mg a.i. L-I), dimethirimol (250 mg a.i. L-1), fenarimol (36 mg a.i. L-1), penconazole (40 mg a.i. L-l) and triadimefon (125 mg a.i. L-1) were less effective (P=0.01) in controlling the disease than oxythioquinox (100 mg a.i. L-1). This disagreed with earlier trial results and suggested that fungicide resistant strains may have developed. The sensitiyities of 6 isolates of S. fuliginea to 12 fungicides were determined using a leaf disc technique. Two isolates collected in fields where fungicides had been used intensively showed reduced sensitivity to fungicides from several chemical groups including the ergosterol biosynthesis inhibitors, hydroxypyrimidines, organophosphates and benzimidazoles. Several spraying strategies were compared. The S. fuliginea populations receiving spray schedules which included the protectant fungicide oxythioquinox alone, or in alternation with a systemic (triadimefon) developed a lower proportion (66-73%) of resistant strains than those treated with systemic fungicide alone (89%). These findings suggest that S. fuliginea can develop strains with low sensitivity to several fungicides against powdery mildew. To prolong their efficacy, systemic fungicides should not be used continuously but should be reserved for use during the latter part of crop growth when disease risk is highest. Alternating or tank mixing with an effective protectant fungicide is also recommended.