What is powdery mildew?
Powdery mildew is caused by ascomycete fungus called “Podosphaera leucotricha” primarily in apples and pear and is found in all apple producing regions worldwide. Powdery mildew can be found on buds, blossoms, leaves, twigs, and fruit. It is an obligate biotroph and will not survive off of its host. It can result in defoliation, and reduces both apple yield and quality due to stunted growth or die back of twigs and leaves and reduced fruit set. The optimum weather for infection is relative humidity over 90% and temperature range 66-72 °F.
What does powdery mildew look like?
Leaf and shoot
- First symptoms generally appear on the lower leaf surface as isolated colonies of white-silver gray fungal mycelium growth.
- Dormant shoots that were heavily infected in the previous growing season are covered with dense white mycelium, and the terminal bud is pinched and shriveled.
Flower buds
- Silver-gray appearance of infected flower buds.
- Infected petals are distorted and show pale yellow or light green appearance.
- 5-8 days delayed opening and inhibition of flower bud formation.
Fruit
- Infections on the blossom receptacle or of young fruit will cause net-like russet and discoloration as the fruit matures.
- Infected fruit may also become distorted and/or dwarfed
Where does powdery mildew come from?
Powdery mildew fungus overwinters in dormant flower and shoot buds infected in the previous year. High disease levels at the end of a season may increase the percentage of infected buds, leading to high levels of primary inoculum the next spring. The spores will not germinate if immersed in water, so leaf wetting is not conducive to powdery mildew development. Fungal spores are spread by the wind. Secondary infections occur when wind-borne spores land on young leaves. There are two phases in the fungal lifecycle, sexual and asexual, and of these, the asexual conidial spores have the major role in infection. Amelanchier, crabapple, dogwood, euonymus, hydrangea, lilac, magnolia, oak, phlox, rhododendron, rose, and, zinnia could be the alternative hosts.
How do I prevent powdery mildew?
- Plant powdery mildew resistant varieties
- Remove sources of the primary inoculum (i.e., flowers, twigs, and shoot buds that were infected in the previous year).
- Plant orchard/trees in sunny locations with good air circulation to reduce humidity around trees.
For more information on Powdery Mildew:
American Phytopathological Society (APS) (Marine et al. 2010)