Avoid overhead watering. Powdery Mildew Faint yellow areas form on expanded leaves. White fungal growth forms on the yellow areas of some cultivars while only small dead spots with no fungal growth forms on other cultivars. Erysiphe polygoni or Microsphaera penicilata Apply Ampelomyces , azoxystrobin, paraffinic oil, myclobutanil, triadimefon, or triflumizole. Rhizoctonia Web Blight Small tan to black spots form on leaves.
Spots expand to entire leaf.
Webbing may develop. Small plants may die. Rhizoctonia solani Maintain good air circulation. Avoid late afternoon watering. Apply PCNB, fludioxonil, or chlorothalonil.