Department of Plant Pathology

Powdery mildew of cucurbits, caused by the pathogen Podosphaera xanthii or other Erysiphaceae, is an important foliar disease in California. Disease is often managed by application of synthetic fungicides. We tested 11 fungicide treatment regimes to assess control of powdery mildew on pumpkin in a field trial in northern California. Foliage was sprayed weekly or biweekly; two biocontrol treatments were also soil-drenched prior to foliar applications. Disease attained high levels of incidence (nearly 100% of leaves infected) and severity (from 0.97 to 2.2 colonies per cm2) on the upper surface of leaves in unsprayed/undrenched, unsprayed/water drenched and water-sprayed controls. Biological and organic treatments, including Actinovate (Streptomyces lydicus WYEC108) and Phyton 016-B (copper sulfate pentahydrate), did not effectively control the disease. Treatment with Rally alternated with Quintec (myclobutanil/quinoxyfen), LEM17 (penthiopyrad), LEM17 + other synthetics, or Inspire Super (difenoconazole + cyprodinil) however, lowered disease incidence on the upper surface of leaves and gave up to several fold reductions in colony density relative to plants sprayed with water.