Cause Erysiphe ravenelii (formerly Microsphaera ravenelii), a fungus endemic to North America. Succulent new growth late in the season is particularly susceptible. Conidia are released in response to abrupt decreases in relative humidity. Wind blows conidia to healthy foliage where they start new mildew colonies. Too much water, such as flowing water or rain, destroys spores by causing them to burst. The fungus does not grow in a leaf but rather across the surface. Small anchor cells of the fungus, haustoria, remain in the leaf and take nourishment from the plant, but the main filament and the multitudes of spores it produces are outside the leaf. The disease can develop under relatively dry conditions in spring and summer so long as the air is humid. Multiple disease cycles occur during the growing season.
Symptoms Leaves and sometimes young shoots are covered with a powdery, gray-white growth. This most often appears in summer and gradually increases throughout the rest of the season. Late in the season, the powdery growth may be dotted with tiny black fruiting bodies of the fungus. Affected leaves may be distorted or stunted, or may drop prematurely.
Cultural control
- Clean up and destroy fallen leaves from beneath infected trees.
- Plant in sunny locations.
- Do not over-fertilize.
- Prune and destroy severely infected twigs and branches to reduce spread of disease.
- Space trees and prune to provide good air circulation.
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. Limit the use of any one group during crop production.
- Armada 50 WDG at 3 to 9 oz/100 gal water. Do not use a silicone-based surfactant. Not for nursery or greenhouse use. Group 3 + 11 fungicide. 12-hr reentry.
- Heritage, although not specifically registered for honeyust, are is safe on this crop, according to IRa. These This and similar products provide good control of powdery mildew on many other crops. Group 11 fungicide.
- Seido at 4 to 5 fl oz/100 gal water plus an adjuvant. Group 50 fungicide. 4-hr reentry.
- Tebuject 16 is registered for tree injections. The number of capsules used is based on tree size. Group 3 fungicide.
- Trinity at 4 to 12 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
Reference Braun, U. and Cook, R.T.A. 2012. Taxonomic Manual of the Erysiphales (Powdery Mildews). CBS-KNAW Fungal Biodiversity Center, The Netherlands.