Powdery Mildew Management

Powdery Mildew Management - Solution Guide

Powdery Mildew on many plants is caused by a specific fungus called Erysiphe cichoracearum. This pathogen also afflicts summer squash, cucumbers, sunflowers, Phlox, and Monarda (Beebalm).

These fungi produce nearly microscopic mycelium (fungal threads) that grow on the surface of plant tissues like leaves, stems, and flower buds. These fungal spores appear first as blotches of white or grayish-white that everyone recognizes. As the infection spreads, it can kill plant tissues, causing yellow and browning necrotic areas. It is important to understand that this fungus does not grow in the soil at all. It is only found on above-ground plant tissues. The fungi feed by sending root-like structures (haustoria) into the top epidermal plant cells on leaves or herbaceous stem surfaces.

These fungi overwinter on plant debris in the yard or garden. In the spring, spores germinate upon the onset of warmer temperatures and spread to their specific host plants by splashing rain, by wind, and by insects, whose feet pick up microscopic mycelium threads, transferring them from one plant and depositing them onto another.

Management & Control of Powdery Mildew

  • Avoid fertilizing with high-nitrogen fertilizers, which make plants more susceptible.
  • Avoid overhead watering and water enough to keep plants healthy.
  • Do not work around plants while surfaces are wet from dew or rain.
  • As soon as symptomatic leaves appear on a host plant, pinch or prune off those leaves immediately and throw them directly into the trash. Then wash your hands before working with plants again, or change your gloves & wash the dirty pair.
  • When major infections occur, remove as much of the plant and its debris as possible.

    • TIP: Do not compost this infected debris; burn or toss it into the trash instead.

  • Do not overcrowd plants in the garden. Use proper spacing, especially for PM host plants. Maintain high levels of improved air circulation in the garden, especially for susceptible plant types.
  • Reducing relative humidity around susceptible plants is also beneficial for suppressing PM infections. Apply clean mulch around the base of plants to help prevent soil moisture evaporation.
  • Plant PM-resistant varieties whenever possible.
  • As for fungicides, applications should be made before an infection or outbreak. This may need to be started as soon as the end of May or beginning of June, weather-dependent, in northern regions.

    • ** Always read and follow all product label instructions and warnings.