What is Powdery Mildew?

Powdery mildew is a fungal disease that affects the foliage, stems and less often the flowers and fruits of plants, where a superficial fungal growth covers the surface of the plant. Powdery mildew diseases are caused by different species of fungi belonging to the order Erysiphales and these fungi are host specific, meaning a particular species will grow only on certain plants, and will not spread to others.

Powdery Mildew on a Tomato Leaf.

Powdery Mildew on a Tomato Leaf.

A wide variety of vegetable plants are affected by powdery mildews, including artichoke, beans, beets, carrot, cucumber, eggplant, endive, lettuce, melons, parsley, parsnips, peas, peppers, potatoes, pumpkins, radicchio, radishes, squash, tomatillo, tomatoes and turnips. It also commonly affects edible and ornamental garden plants, grasses (cereal crops), weeds, shrubs, fruit trees and forest trees.

Powdery mildews typically thrive in environments with high humidity and moderate temperatures. Crowded plantings, cool damp settings and poorly ventilated areas are highly susceptible to powdery mildew attacks.

Its symptoms are quite distinctive, making powdery mildews easy to spot. Infected plants display white or gray powdery splotches on the surfaces of the plant, especially the lower leaves and stem parts. The fungus if left unchecked grows and spreads to larger areas all over the plant. Powdery mildews are unattractive but rarely fatal. They do however, impair photosynthesis, affect flavor and reduce yields of fruits and vegetables. On some plants, powdery mildew may cause the leaves to eventually turn completely yellow and die off, twist, buckle or distort.

Powdery Mildew on Squash Plants.

Powdery Mildew on Squash Plants.

Control and Preventative Measures of Powdery Mildews

1. A key preventative step is planting mildew-resistant or mildew-tolerant varieties. Resistant varieties will get lesser mildew than susceptible ones and tolerant varieties may get attacked by mildews but their performance won’t be affected.

2. Another preventative measure that you should consider is growing plants where there is good air circulation and exposing as much of the leaf surface to direct sunlight, which inhibits the growth of fungus spores.

3. When transplanting plants outdoors or when buying seedlings, choose the healthiest plants as they will have a high tolerance to mildew damage.

4. It would be wise to not plant non-resistant varieties in the shade.

5. To avoid the spread of mildews to other similar variety plants, remove and destroy all infected plant parts. It is advisable to not compost infected parts, rather rid of them in the trash.

6. Some suggest watering plants at the base to avoid leaving the leaves and stems of the plant damp. Others recommend overhead watering because powdery mildew spores cannot germinate when foliage is wet, but when foliage dries off, the ensuing dampness could prove to be highly susceptible.

7. To control an infection, improve air circulation by thinning and pruning.

8. Once you have an infection, reduce the use of any nitrogen based fertilizer (organic or inorganic) to limit the production of succulent tissues as they are most susceptible to infections.

9. Mix a tablespoon of baking soda in a gallon of room temperature water. Add a drop or 2 of liquid soap to help the solution stick to the plant. Add a teaspoon of vegetable oil or horticultural oil and shake well. Spray the affected areas with this solution. This solution changes the pH balance on the plant’s surface, making for an unsuitable environment for powdery mildews to proliferate.

10. Neem oil has been found to be beneficial in the control of powdery mildews. Mint oil, rosemary oil and cinnamaldehyde (cinnamon tree extract) are also effective powdery mildew controls.

11. Garlic’s anti-fungal property is said to control powdery mildews. Prepare a garlic extract solution by blending 2 bulbs of fresh garlic in a quart of water with a few drops of liquid soap. This liquid should be strained to remove solids and then refrigerated. This concentrate should be diluted and then sprayed onto affected areas.

12. Biological fungicides (such as Serenade) that have beneficial organisms formulated into the product are available in the market and can help tackle powdery mildews and other diseases. Seek advice from a local expert before buying and using any product, though.

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