Grow beautiful planters in the heat

Powdery mildews is a fungal disease that affects a wide range of plants. It is one of the easier diseases to spot, as the name implies, it often appears as superficial white or gray powdery spots on the leaves, stems, flowers, or fruit of affected plants. These patches may enlarge until they cover the entire leaf on one or both sides. The lower leaves are the most affected, but the mildew can appear on any part of the plant that shows above the ground. Young foliage and shoots may be particularly susceptible. Leaf curling and twisting may be noted before the fungus is noticed. Severe powdery mildew infection will result in yellowed leaves, dried and brown leaves, and disfigured shoots and flowers.

Although it usually is not a fatal disease, powdery mildew may hasten plant defoliation and fall dormancy, and the infected plant may become extremely unsightly. On roses, uncontrolled powdery mildew will prevent normal flowering on highly susceptible cultivars.

This disease occurs on many herbaceous perennials, shrubs and trees, but does not affect conifers. Some garden plants that commonly get powdery mildew include apple, ash, bee balm, chrysanthemum, cosmos, crabapple, dahlia, delphinium, honeylocust, honeysuckle, hydrangea, lilac, maple, phlox, roses, snapdragon, sweetpea and zinnias.

Powdery mildew is caused by many closely related species of fungi that belong to several genera in the family Erysiphaceae. They are fairly host specific, so the fungus species that infects lilac is not the same that infects phlox. In some cases the fungi are so particular that they only infect certain cultivars of plants! However a few plants are susceptible to more than one type of powdery mildew.

When weather conditions are favorable, the disease often develops on a wide range of plants. High humidity promotes disease development, especially when cool, damp evenings follow warm days. These fungi are unusual compared to most plant disease fungi because they do not cause infection when the leaves are wet; they just require high humidity during infection. However, they produce more spores when the humidity is low. Young growth is most susceptible on most plants. Many powdery mildew fungi only grow on the top side of the leaf, but some will grow on both sides. Fungal growth may completely cover the leaf, or may appear only in patches. The disease can spread rapidly (3 days) when conditions are optimal, but it usually takes 7-10 days from infection until symptoms develop and secondary spore production occurs.

Fortunately, powdery mildew is only a cosmetic problem on many plants. It will make the plant look a little ragged, but it doesn't kill the plant. On some plants, however, powdery mildew can cause severe leaf loss and yield reduction in vegetables. In this case fungicide applications may be necessary to reduce or prevent injury.

Control of Powdery Mildews
The following cultural practices should be beneficial for controlling powdery mildews.

Prune out diseased terminals of woody plants, such as rose and crabapple, during the normal pruning period. All dead wood should be removed and destroyed (do not compost).
Rake up and destroy all dead leaves that might harbor the fungus (do not compost).
Maintain plants in a high vigor.
Plant properly in well-prepared and well-drained soil where the plants will obtain all-day sun (or a minimum of 6 hours of sunlight daily).
Space plants for good air circulation. DO NOT plant highly susceptible plants--such as phlox, rose, and zinnia--in damp, shady locations.
Do not handle or work among the plants when the foliage is wet.
Water thoroughly at weekly intervals during periods of drought. The soil should be moist 8 to 12 inches deep. Avoid overhead watering and sprinkling the foliage, especially in late afternoon or evening. Use a soil soaker hose or root feeder so the foliage is not wetted.

Chemical Control of Powdery Mildews
In many cases, powdery mildew diseases do little damage to overall plant health, and yearly infections can be ignored if unsightliness is not a major concern. For example, lilacs can have powdery mildew each year, with little or no apparent effect on plant health.

On some plants, powdery mildews can result in significant damage. Thus, fungicides must be used to achieve acceptable control. For best results with fungicides, spray programs must begin as soon as mildews are detected. Spray on a regular schedule, more often during cool, damp weather. Use a good spreader-sticker with the fungicides. Be sure and cover both surfaces of all leaves with the spray.

Visite the Garden Solution Center at Pinehurst for all your gardening needs.