The Q&A Archives: white mildew

Question: How do I get stop this from happening?

Answer: Even though there are several types of powdery mildew fungi, they all produce similar symptoms on plant parts. Powdery mildews are characterized by spots or patches of white to grayish, talcum-powder-like growth. Tiny, pinhead-sized, spherical fruiting structures that are first white, later yellow-brown and finally black, may be present singly or in a group. These are the cleistothecia or overwintering bodies of the fungus.

The disease is most commonly observed on the upper sides of the leaves. It also affects the bottom sides of leaves, young stems, buds, flowers and young fruit. Infected leaves may become distorted, turn yellow with small patches of green, and fall prematurely. Infected buds may fail to open.

Conditions That Favor the Disease
The severity of the disease depends on many factors: variety of the host plant, age and condition of the plant, and weather conditions during the growing season.

Powdery mildews are severe in warm, dry climates. This is because the fungus does not need the presence of water on the leaf surface for infection to occur. However, the relative humidity of the air does need to be high for spore germination. Therefore, the disease is common in crowded plantings where air circulation is poor and in damp, shaded areas. Incidence of infection increases as relative humidity rises to 90 percent, but it does not occur when leaf surfaces are wet (e.g., in a rain shower). Young, succulent growth usually is more susceptible than older plant tissues.

Once the disease becomes a problem:

Avoid late-summer applications of nitrogen fertilizer to limit the production of succulent tissue, which is more susceptible to infection; Avoid overhead watering to help reduce the relative humidity; Remove and destroy all infected plant parts (leaves, etc.). For infected vegetables and other annuals, remove as much of the plant and its debris in the fall as possible. This decreases the ability of the fungus to survive the winter. Do not compost infected plant debris. Temperatures often are not hot enough to kill the fungus.

Selectively prune overcrowded plant material to help increase air circulation. This helps reduce relative humidity and infection.

If cultural controls fail to prevent disease buildup or if the disease pressure is too great, an application of a fungicide may be necessary. These include:

sulfur; neem oil (Rose Defense, Shield-All, Triact); triforine (Ortho Funginex);
potassium bicarbonate (Kaligreen, First Step)

Apply fungicides at seven- to 14-day intervals to provide continuous protection throughout the growing season. Follow the instructions on the fungicide label for use on specific plant species, varieties, rates to be used, timing of applications, and waiting periods before harvest.

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