Evaluation of the sensitivity of Podosphaera xanthii to several fungicides for management of powdery mildew on squash in Florida

Abstract

Cucurbit powdery mildew caused by Podosphaera xanthii is a major challenge to cucurbit production worldwide. In the United States, the fungus has become resistant to fungicides in FRAC Groups 1, 3, U6, 7, 11, and 13, however the extent of this resistance is not known in the Florida population. Commercially labeled fungicides, thiophanate-methyl (Topsin 70WDG), cyflufenamid (Torino), flutriafol (Rhyme), quinoxyfen (Quintec), and myclobutanil (Rally 40 WSP), were selected to screen for sensitivity against locally obtained isolates under laboratory, greenhouse, and field conditions. Laboratory and greenhouse assessments of disease severity and control were made after a single application of each fungicide, while field trial assessments were made after weekly applications. Quinoxyfen, myclobutanil, cyflufenamid, and flutriafol gave adequate control of powdery mildew, while thiophanate-methyl failed to control P. xanthii. No formulation was successful in preventing the establishment and sporulation of P. xanthii at the maximum or near-maximum labeled field rate under laboratory, greenhouse, and field conditions. Further research is required to monitor the Florida population for early detection of fungicide resistance, to establish new effective fungicidal concentrations, and to optimize fungicide application regimes.