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.

Efficacy of fungicides and organic oils to control powdery mildew disease of jujube (Ziziphus mauritiana Lam.)

“…From Bangladesh, Bakr (2007) reported that Tilt 250 EC (0.05%), Thiovit (0.2%), and Bordeaux mixture (1%) were effective to control powdery mildew disease of jujube. Similar findings have also been reported by other investigators (Bent, 1978;O'Brien et al, 1988;O'Brien, 1994;Singh et al, 1995;Cheah and Page 1997). Yadav et al (1980) and Singh and Sidhu (1985) found that Carbendazim (0.2%) can control powdery mildew disease.…”

Management of Powdery Mildew in Squash by Plant and Alga Extract Biopesticides

“…Plots were regularly monitored for occurrence of powdery mildew, and the disease was rated at 60, 74, 80, and 88 DAP. Disease severity of powdery mildew on both sides of leaves was evaluated on five randomly selected plants in each plot by visual assessments of the entire canopy using a rating scale of 0–5, where 0 = no symptoms; 1 = no dead leaves, powdery mildew present as scattered lesions; 2 = less than 1/3 of leaves dead and scattered lesions on remainder; 3 = less than 1/3 of leaves dead and concentrated lesion development on remainder leaves; 4 = 1/3 to 2/3 of leaves dead; and 5 = more than 2/3 of leaves dead (O’Brien et al, 1988). …”

Fungicide resistance in populations of cucurbit powdery mildew (Sphaerotheca fuliginea)

“…First, there was no observed hydroxypyrimidine resistance in Queensland-north compared with a high incidence in the Riverland-Swan Hill zone. Strains resistant to hydroxypyrimidines were known to be present in Queensland-north in the mid 1980s (O'Brien et al 1988) but their frequency has apparently declined. The other notable difference was the generally lower incidence of DMI resistance in New South Wales.…”

“…Previous work (O'Brien et al 1988) showed the growth and sporulation of sensitive isolates was prevented or inhibited by fungicide concentrations of 10 ug/ml. In the assay, sporulation (mean rating > 1) on leaf discs treated with the higher rate of each fungicide was used as the criterion to identify fungicide resistance.…”