Investigation of Powdery Mildew on Kabocha Squash (Cucurbita maxima Duch.) in Tonga

Abstract

Disease management of cucurbit powdery mildew pathogens caused by Golovinomyces cichoracearum and Podosphaera xanthii is largely based on extensive use of systemic fungicides. P. xanthii is a highly variable pathogen, having developed resistance to the major agrichemical groups including the demethylation inhibitor (DMI) fungicides. The aim of this study was to investigate the powdery mildew disease and the causal pathogen occurring on Kabocha squash (Cucurbita maxima) in Tonga. The disease causes significant economic loss to production. A field disease survey conducted during the 2007 production season revealed widespread powdery mildew disease incidence >25%. There was no interaction between disease incidence and farm management practice. Powdery mildew disease severity was 5-25% in regions and 5->25% in districts. There was no correlation between powdery mildew disease severity and farm practice. High levels of disease severity caused significant reductions in yield with losses between >40% and 80%. The causal pathogen was identified as Podosphaera xanthii. The presence of characteristic fibrosin bodies in the conidia, conidia produced in chains on the conidiophore and conidiophore foot-cell size and shape were diagnostic for P. xanthii. Significant levels of fungicide resistance to the most frequently used flusilazole, myclobutanil, propiconazole and benomyl was detected in P. xanthii populations on Kabocha squash. EC50 values were >60 times the recommended rates of use. Resistant factors were 5 x 106 and 2 x 106 for flusilazole and myclobutanil, respectively. P. xanthii showed significant levels of cross-resistance to these fungicides. The results of this study indicate that current fungicide practice for powdery mildew on Kabocha squash is no longer effective and should be discontinued due to the significant levels of fungicide resistance in the P. xanthii population. The recommendation is for future use of environmentally safe and less harsh biocompatible products for disease management.