Hort Q&A: Fungal diseases on the rise

Carla Smith, horticulture educator
Pottawatomie County Extension Service
Powdery Mildew on Garden Phlox

I am seeing a white powdery looking film on my zinnias and cucumber plants. What is it and what should I do?

With the mild temperatures and plenty of moisture we have been experiencing this spring, conditions for fungal diseases are on the rise. Powdery mildew is a fungal disease we frequently battle that affects many host plants, including ornamentals, shrubs, and trees. On some ornamental plants, such as rose, lilac, crapemyrtle, oak and zinnia the disease can be very destructive. Severity of the disease depends upon many factors—variety of the host plant, age and condition of the plant, time of infection, and weather conditions during the growing season.

Although there are several different types of powdery mildew fungi, a lot of them produce similar symptoms on plant parts. Plant damage may range from an unsightly whitish powdery coverage of the foliage to drying out and browning of infected leaves. In some plants, buds may be infected, and production of flowers, fruits, or nuts may be affected. If diagnosed early, powdery mildew can be effectively controlled to prevent severe damage to plants.

Symptoms – The first sign of the disease is the development of a white to gray or slightly brownish growth of mycelium over the surface of leaves or other parts. Powdery mildew fungi may also attack stems, buds, and flower petals of various ornamental plants. Powdery mildew infection of broadleaf plants may cause distortion (curling and twisting) and a reduction in size of infected leaves. When older leaves become infected, they usually show only the white patches of the fungus. Infected leaves may become distorted, turn yellow, and fall prematurely. Infected buds may fail to open, and infections can spread to mature flowers, causing flower blight. Also, nuts of the pecan can be infected, causing a reduction in quality.

Powdery Mildew on Garden Phlox

Hosts (Susceptible Plant Species) – Powdery mildews are known to occur on almost all ornamental plants. Certain plant species and/or varieties are more susceptible to the disease than others. In Oklahoma, powdery mildews are common on ash, crapemyrtle, lilac, oak, photinia, roses, and zinnia.

Conditions Favoring Powdery Mildews – Humid conditions with widely fluctuating temperatures increase the occurrence of powdery mildew. The disease is common in crowded plantings where air circulation is poor and in damp, shaded areas. Young succulent growth is more severely affected than older tissues. Disease development is enhanced when cool, moist nights are followed by warm daytime temperatures.

Cultural Control of Powdery Mildews – Several practices will reduce or prevent development of powdery mildew. These practices involve reducing populations of the fungus in the vicinity of the host, changing environmental conditions around the host, and selecting more resistant varieties. Before plants are purchased, it may be to the homeowner’s advantage to inquire if the ornamental variety desired has any resistance to powdery mildew. If only susceptible varieties are available, avoid planting in low, shady locations. If powdery mildew becomes a problem, removal, and destruction of infected plant parts, should be practiced. Pruning of crowded plant material will also help increase air flow around leaves. This reduces humidity and thus helps prevent infection. Late summer application of nitrogen fertilizer should be avoided to limit production of succulent tissue, which may be susceptible to powdery mildew infection in the fall. Water only in the mornings so that the foliage will be dry by evening.

Chemical Control of Powdery Mildews – If cultural controls fail to prevent disease build-up or if the disease pressure is too great, fungicide spraying may be necessary. The best course of action is to combine both approaches, using cultural methods as well as following a good spray schedule.

When powdery mildew has been a problem in previous years, a recommended fungicide spray schedule should be started in the spring as new growth develops. The fungicide should also be applied during the flowering period to avoid blossom blight. For suggested fungicides you can contact our OSU Extension Office or nursery professional. Copper fungicides are labeled for many plants to control powdery mildew, but there are other effective fungicides also. Be sure to follow the instructions on the label for use on specific applications. If using on vegetable crops, make sure that the product is labeled for vegetable use and follow the days to harvest recommendations.

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