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Past Articles Library | Plant Diseases & Control | Powdery Mildew

POWDERY MILDEW
(Erysiphaceae)

There are numerous fungi that fall under the general description of a powdery mildew, but they tend to be treated the same, since most have similar habits and similar management practices. Powdery mildew affects a wide range of plants, and is caused by many different species of fungi in the order Erysiphales. It is one of the easier diseases to spot, as its symptoms are quite distinctive.

LIFE CYCLE

The fungi which cause powdery mildew are spread by spores produced in the white patches. These spores are blown in the wind to other parts of the plant or to other plants during the growing season. Generally each species of fungus will be limited in the number of plant species that can be attacked. For example the species of fungus infecting lilacs will not cause powdery mildew on apples.

During the winter the fungus survives on infected plant parts and in debris such as fallen leaves. It may produce resting structures known as cleistothecia, which resist harsh winter conditions. These will appear as small black dots within the white powdery patches. The next spring, spores (ascospores) are released from the cleistothecia, shot up into the air, and carried by air currents to leaves of plants where new infections will begin. During the growing season, the fungus produces asexual spores (conidia) that help the fungus to spread and infection to build. This is the general cycle for most powdery mildews of outdoor plants. With houseplants the overwintering stage is of little significance, because depending on the conditions indoors, the fungus could continue to grow and spread during the entire year.

PLANTS MOST AFFECTED

Affects a wide range of plants; particularly damaging to African violet, begonia, rose, lilac, zinnia, phlox, bean, cucumber, squash, grape, and various fruits

DAMAGE

Powdery white or gray spots appear on leaves or fruit, usually on the upper surfaces first. These spots can spread which eventually cover the entire surface of leaves, flowers, fruit, and stems. Most common in humid conditions, but powdery mildew can spread even in warm, dry climates because the spores do not require a layer of water on the leaf to germinate.

Injury due to powdery mildews includes stunting and distortion of leaves, buds, growing tips, and fruit. The fungus may cause death of invaded tissue (begonia, for example). Yellowing of leaves and death of tissue may result in premature leaf drop. Nutrients are removed from the plant by the fungus during infection and may result in a general decline in the growth and vigor of the plant. The seriousness of the disease will depend on the extent of the various types of injury.

MEANS OF CONTROL

Prevention:

• Plant resistant cultivars when available
  
  
• Ensure good air circulation around susceptible plants by spacing plants appropriately at planting time, and pruning out dense growth
  
  
• Clean up and remove plant debris at the end of the growing season to remove overwintering sites for fungi