University of Manitoba - Faculty of Agricultural & Food Sciences - Powdery Mildew
Powdery Mildew

Problem type: Disease

Name of problem:
 Powdery Mildew

Plant name(s):
 All plants; Roses, lilacs, alpine currents are more susceptible, especially if grown as a hedge.

Symptoms / Characteristics:
Dusty white or light coloured coating on leaves and other tender plant parts. Symptoms are worse in late summer and early fall because the disease is worsened by cool nights and warm days. Plant parts will slowly decline in health and may result in dwarfing, distortion, chlorosis or browning of leaves or premature leaf drop. Small black specks may also been seen late in the season on plant surfaces. Generally, powdery mildew is not harmful to a plant but can decrease vigor and aesthetics.

Powdery mildew is a fungus that spreads by wind, and prefers moderate temperatures and high humidity. Growing conditions such as close spacing, densely growing plants and shade can promote disease development. Powdery mildew can over winter in buds, other plant parts and in debris and can continually reproduce indoors.

Control / Preventions:
Cultural practices such as trimming, fertilizing and cultivating, which stimulate plant growth, encourage powdery mildew development. Avoid excessive watering which creates higher humidity around plant parts. Avoid shade and overcrowding plants and promote air circulation by pruning and thinning. Remove infected leaves if practical, prune out disease wood and rake up and destroy all dead, infected leaves. Do not compost infected plants.

Chemical applications can be avoided with proper cultural practices and planting techniques, but are available. Fungicides containing sulfur or triforine can be sprayed when disease is first noticed. Baking soda spray or a mixture of 4 tsp. baking soda and 1 tbsp. horticultural oil mixed in 1 gallon of water can also be used for control.