When powdery mildew strikes, pay attention

When shoots and leaves are covered with a powdery white substance, suspect the disease "powdery mildew." This disease infects many edible and ornamental plants. Roses can be especially susceptible. This year, peaches seem to be particularly affected.

Many diseases require moist conditions to flourish, but not powdery mildew. It thrives in dry summer weather. Look for the fuzzy white growth that forms on the shoots and leaves, and sometimes on flowers and fruit. However, on tomatoes, peppers, eggplants and some other vegetables, no fuzz occurs. Instead, you see only yellow patches. On fruit trees, the disease infects new growth, making it distorted and dwarfed. Young fruit looks scarred and may have a corky skin.

Wind carries the spores from one host to the next. The disease thrives in moderate temperatures and shade, but does poorly where there is free moisture. It survives from one year to the next in infected buds and leaves or on the bark. Good cultural practices and spraying plants with a hose on a regular basis usually will deter powdery mildew infections. In landscapes, moderate levels of mildew can be tolerated.

To avoid powdery mildew problems, select resistant varieties when choosing new plants. Avoid over-fertilizing and over-watering, as both stimulate susceptible new growth. When the infection is limited, prune out the diseased plant parts and put them in the trash. Prune grapes and fruit trees to allow sun into the plants. Once a garden has been infected, when planting the next spring, rotate vegetable crops, never planting the same family of plants in the same spot. Each fall, remove all old strawberry and grape leaves and do not compost them.

Prune out infected buds on fruit trees, including peach trees. Infected buds look flattened or shriveled. Be sure to disinfect your pruning shears after pruning diseased tissue. Either spray them with isopropyl alcohol or dip them into a solution of one part bleach and nine parts water. You can also spray them with an antitranspirant early in the season and as new growth develops. This waxy substance is most commonly used on evergreens in the winter to prevent them from drying out. The wax prevents spores from growing into the plant tissue.

Fungicides are rarely needed in the home landscape if you plant resistant varieties, take good care of your plants and do a thorough fall cleanup. Sulfur or lime sulfur are traditional preventative methods, but can damage plants if used just prior to or on days with temperatures that reach 90 degrees or higher. Synthetic fungicides are available. Once a powdery mildew infection is severe, it is too late for a fungicide to do the trick, so be sure to monitor plants early and often.

For more information on gardening, contact me, 887-2252 or skellyj@unce.unr.edu, or your local University of Nevada Cooperative Extension office. Check out many useful horticulture publications at www.unce.unr.edu. "Ask a Master Gardener" by e-mailing mastergardeners@unce.unr.edu.

• JoAnne Skelly is the Carson City / Storey County Extension educator for University of Nevada Cooperative Extension.

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