POWDERY MILDEW DISEASE

Introduction

Powdery mildew can affect numerous plants including fruit, vegetable, and agronomic crops as well as herbaceous ornamentals. This common and widespread disease can occur in commercial and residential plantings. Susceptible plants that are infected early in the season can be seriously damaged, while plant species affected later in the season may not be significantly harmed. Powdery mildew may adversely affect flowering, fruit development, plant vigor and yield. Plants that become stressed by powdery mildew infections may succumb to other abiotic or infectious agents.

Signs and Symptoms

Powdery mildew is easily identified by the presence of white, tan, or gray powdery fungal growth (mycelium and spores) that is present primarily on surfaces of infected plant parts. Immature plant parts are often most susceptible. Affected leaves, stems, buds, flowers and fruit appear as if they have been dusted with white powder. Later in the growing season, tiny, round, black fungal fruiting structures of the overwintering stage may appear.

Powdery mildew may occur as isolated spots or cover entire plant surfaces. When young expanding leaves are infected, they may become distorted and stunted. Leaves may curl at the margins. Discolored, irregular spots and blotches may develop on foliage severely infected leaves drop prematurely. Fruit may be misshapen, fail to color properly, become russetted or split open. Fruit of many vegetables do not become infected but powdery mildews can still reduce yield and quality.

Crops/Plants that are generally affected by powdery mildew in Zimbabwe include: Wheat, Tomatoes, Butternuts, Cucumbers, Peas, Water melons, Roses, Fruit trees etc.

N.B The danger with Powdery Mildew is that it can become problematic even when it is too dry for other diseases to develop.

Disease management

Management options are often determined by the production system e.g. Commercial field, residential landscape and greenhouses as well as by the specific crop. Follow as many of the management options listed below as possible whenever they are applicable.

Cultural Practices

Variety selection

Plant cultivars vary in their susceptibility to powdery mildew. Resistant/tolerant cultivars of apples, grapes, some cucurbits, pepper, rose, wheat and many other crops are available. Whenever possible, select cultivars with known resistance or tolerance to this disease.

Planting site

Do not plant susceptible crops in low areas or sites that are shaded; these are often high humidity areas. Rotate annual crops to reduce pressure from overwintering powdery mildew fungi.

Fertility

Avoid excessive applications of nitrogen fertilizer, which stimulate succulent growth that is most susceptible to infection.

Sanitation

Remove and destroy infected plant tissues and debris that may serve as overwintering sources of these fungi. Crop residues can also be incorporated into the soil to reduce sources of overwintering fungi.

Air circulation

Keep plants well-spaced and properly thinned to promote air movement, light penetration, and rapid leaf drying.

Fungicides

Fungicides may be warranted for early season management of powdery mildew on susceptible crops, especially in commercial plantings or where highly valuable plants are grown. Fungicides are most effective when applied as preventatives, before primary infections can occur. Some crops, such as grape, remain susceptible all season and require consistent and prolonged sprays. Fungicide use in residential plantings is generally not recommended, especially when disease occurs late in the season. Late season infections cause minimal damage, even when plants appear severely affected.

The following products are recommended for the prevention and control of this economically important disease:

Always consult Agricura Agronomists before using any of the stated products as there are a whole lot of unique technical aspects involved of which failure to adhere to these instructions may result in permanent crop or tree damage and significant reduction in yield and quality!

Cossan Wettable Sulphur, Agridust/Vegidust, Carbendazim, Propiconazole/Tilt, Orius/ Tebuconazole, Shavit/Triadimenol, Lime Sulphur and Funginex.

Additional Resources

Extension Plant Pathology publications (plant disease fact sheets and spray guides)

http://plantpathology.ca.uky.edu/extension/publications