Oregon Grape (Mahonia aquifolium)-Powdery Mildew

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Sometimes there are individual colonies, but a heavy, white growth develops on leaves by late in the season.
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Many leaves can become infected causing them to decline.

Cause The fungus Erysiphe berberidis (formerly Microsphaera berberidis) has been identified. This disease was observed widespread during the summer of 2002 on landscape plants in western Oregon and Washington. Only conidia were observed during the growing season. No chasmothecia have been observed. OSU Plant Clinic records indicate another sample of powdery mildew was observed in 1990 from Oregon. These fungi are obligate parasites that generally overwinter as chasmothecia. The fungus is favored in shady, humid environments.

Symptoms The most common signs of this disease are white, powdery fungal colonies up to 1 cm in diameter on the upper surfaces of leaves. This fungal growth can easily be removed by wiping the leaf surface. Colonies frequently grew together and covered the entire leaf surface with a dense white powdery growth. There may be some yellowing of leaf tissue under the colonies. Heavily infected plants had reduced terminal growth and some deformation of new leaves.

Cultural control

  • Plant in locations with moderate to low shade, remembering that the plant is sun-tolerant, but prefers shade.
  • Plant with adequate space between plants to allow for good air circulation.
  • If feasible, wipe leaves with wet cloth to remove fungus.
  • Wash leaves frequently with water to remove fungal conidia.
  • Remove and destroy infected leaves on the plant and fallen leaves.

Chemical control Fungicides will do best when used before symptoms develop. Few materials have good eradicant activity. Use at 7- to 14-day intervals; using shorter intervals when environmental conditions favor disease development. Alternate or tank-mix products from different groups that have different modes of action.

  • Heritage at 2 to 4 oz/100 gal water plus a non-silicone-based wetter-sticker. Group 11 fungicide. 4-hr reentry.
  • Terraguard SC at 2 to 8 fl oz/100 gal water. Although a 16 oz/100 gal water rate can be used for initial applications to existing infections, use only in conjunction with a regular scouting program that detects initial symptom development. Group 3 fungicide. 12-hr reentry.

Reference Glawe, D,A. 2003. First report of powdery mildew of Mahonia aquifolium caused by Microsphaera berberidis (Erysiphe berberidis) in north America. Plant Health Progress doi:10.1094/PHP-2003-0206-01-HN.