Powdery Mildew

powdery mildew

Powdery Mildew Information, Treatment, and Control

We all do our best to keep fruit trees healthy and properly cared for to ensure robust fruit production and harvests. Yet, some diseases lay in wait for just the right time to attack our valuable specimens. Powdery mildew is such a fungal disease. Conditions in Los Angeles are often just right for this dormant fungus to overwinter, then come alive and attack.

Powdery mildew is composed of spores and affects many fruit trees, grapevines, berry bushes and blooming ornamentals. If allowed to progress, it drains nutrients from plants, causing severe damage to crops or allowing no crop at all. The best method of control, according to UC IPM Department, is prevention.

Powdery mildew appears as a white to gray powdery mycelium, on leaves, stems, flowers, fruits, and berries. It is most visible on new growth that may be covered with the mycelium and causes distortions and dwarfing.

Powdery mildew often attacks roses. Some orchards use these as Indicator plants to spot the fungi before it attacks fruit-bearing trees. Avoid growing roses near fruit trees in the home landscape, or keep a close eye on them for spread or development of powdery mildew.

The fungus spreads by spores and is usually moved by the wind until it contacts an appropriate host plant. There, it can survive, waiting for new growth to infect. Distorted leaves appear on new growth of the infected plant. The fungus is most comfortable in moderate temperatures protected from direct sun. Thus, we find thin white deposits of the mycelium on stems and the underside of new leaves on infected plants.

Treat During Dormant Period

If the fungus has progressed from the beginning stages, or if you are aware that some plants are already infected, treat them during the dormant season. Prune infected stems and remove them from the area. Bury them or get rid of them where they cannot affect other specimens in the garden. Don’t compost these or any other diseased trimmings. Clean pruners with alcohol between cuts, so that the fungus does not move to uninfected plants.

If the fungus has further developed, symptoms may appear on trees and on fruits. A weblike scar on fruit or a rough spot on bark results from infection on fruit trees. Strawberries exhibit leaf curl when infected and scars on berries. Grapes are particularly susceptible to powdery mildew, and those infected may split, crack or fail to grow. UC IPM recommends training grapes upward to allow sun to reach the bottom of the leaf and the fruit. They also advise the use of dormant season pruning for grapevines.

Horticultural oils such as Neem oil are great winter dormant sprays and sometimes an effective means of control. Spray infected areas with a Neem oil concentrate. Get rid of existing spores before the virus moves to other plants.

Avoiding The Disease

Plant fruit trees and bushes that are resistant or tolerant to the disease. Plant new specimens in a sunny spot. Provide good air circulation by allowing enough room between trees and bushes and by keeping