In many hot and humid environments, powdery and downy mildew can wreak havoc on your cucumber, zucchini and even tomato crop. This is especially true for gardeners in Toronto and the GTA where powdery mildew is a common occurrence on both plants and trees. The good news is that mildew is not usually fatal, and can be controlled if detected early enough. Here’s what you can do to identify, prevent and control it if you see it coming.

Powdery Mildew
Powdery mildew generally looks like white or grey powdery splotches on the leaves (and stems) of plants. Leaves can also develop yellow splotches if left untreated. Powdery mildew is a fungus that spreads readily in high humidity and moderate temperature environments. In addition, dampness, crowded planting, and poor air circulation between plants is also a leading cause of powdery mildew. Powdery mildew impairs photosynthesis and can leaves to fall off prematurely. Different types of powdery mildew fungi affect different kinds of plants. For example, the type of powdery mildew found on grapes will be different than the one on your tomatoes.

Powdery mildew can affect both sides of the leaves (and stems, flowers, and fruit), though is most commonly found on the upper part of the leaves. Spores are produced in a tree-like formation, typically with circular spots.

Powdery mildew spores are most commonly spread by the wind. Practice preventative measures by thinning and pruning plants to improve air circulation.

Downy Mildew
Downy mildew is often confused with powdery mildew, and refers specificaly to different types of parasite microbes. While similar, downy mildew only affects the underside of the leaves and produces spores in a single chain-like formation. Common symptoms include a gray or white fuzz on the underside of leaves and yellow blotches on the upper surface of the leave. Both powdery and downy mildew can occur at the same time on the same plant.

Downy mildew spores are most commonly spread as a result of splashing water on dead leaves on the ground. The spores are then transmitting up and attach to the underside of plant leaves during rainfall or if watering from above. A drip irrigation line can help minimize downy mildew transmission. Also, avoid fertilizing as mildew prefers young, succulent growth on plants.

If you start to see symptoms, which include yellow spots on the leaves, here is a simple home remedy you can make.

Baking Soda, Dish Soap and Water

  • 1 tablespoon baking soda
  • few drops of dish soap
  • 1 gallon of water

Mix all of the above ingredients into a spray bottle, shake to combine and then spray onto plants. The best time to spray is in the morning or evening and not during the heat of mid-day.

Other Remedies
Neem oil is another effective natural remedy, especially for downy mildew. You can also choose disease resistant varieties of plants. Finally, practicing good farming practices such as improving your soil through compost and organic materials will help to ensure strong plants that can naturally resist the mildew and survive despite its presence.

Cover Image by nociveglia, used under its Creative Commons license.

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