Mildew is a common fungus that affects a wide range of plants. There are two main types of mildew, namely powdery and downy. They are easily identified and typically appear as light grey or white powdery spots usually found on infected leaves. During the summer months, powdery mildew is most prevalent and gardeners should be on the lookout for signs of the fungus in humid conditions.
Fungal diseases such as powdery mildew and downy mildew affect cucurbits such as cucumbers, pumpkins, squashes and melons, as well as peas, onions, radishes and brassicas such as cabbage, cauliflower, kale, Brussels sprouts, broccoli. Nightshades, including tomato, eggplant and peppers, can also be affected. And, powdery mildew can affect flowers including chrysanthemum, begonia, dahlia, phlox, sunflower and zinnia.
Although rarely fatal, if left unchecked it can eventually cause serious harm to plants by robbing them of water and nutrients. Most infections cause minor damage such as leaves turning yellow or becoming withered or distorted, but plants can also become weak, bloom less, and grow slower. Both powdery and downy mildew produce lightcoloured masses of spores on foliage.
Downy mildew develops spores only on the undersides of leaves, whereas powdery mildew will appear on both sides of leaves as well as shoots, buds and sometimes flowers. Downy mildew is a disease that appears in cool, wet conditions and is generally stopped by warm, dry, windy weather.
Powdery mildew thrives when foliage is dry and the weather is warm; wind spreads the spores to other plants. There are some ways to prevent powdery mildew from damaging crops in summer.
- Thin out existing susceptible plants to improve airflow within the plant.
- Maintain adequate spacing between plants and keep them far enough away from walls and fences to ensure good air circulation and help reduce relative humidity.
- Remove dead or diseased foliage.
- Disinfect pruning shears between uses, with a solution of diluted household bleach.
Powdery mildew spores can’t germinate or grow when foliage is wet, so overhead watering is sometimes recommended as a preventative on highly susceptible crops.
If preventative measures have failed and you have noticed signs of powdery mildew on your crops, there are a number of methods you can try to eradicate the issue:
Baking soda solution: Mix one tablespoon baking soda and ½ teaspoon dishwashing liquid in three litres of water. Spray liberally, getting to both the top and bottom leaf surfaces and any affected areas. This will also work with sodium bicarbonate (bicarb).
Milk: This method originated in Brazil in the late 1990s. Mix one part milk to two to three parts water and spray liberally. While the science behind this solution isn’t fully understood, it seems to work rather well, especially on marrow, melons and cucumbers. It is believed that naturally-occurring compounds in the milk not only combat the disease, but also boost the plant’s immune system.
Garlic: Blend two bulbs (a whole head, not two cloves) of fresh garlic in a litre of water with a few drops of liquid dishwashing soap. The liquid should be strained through cheesecloth to remove solids and then refrigerated. Then, dilute the concentrate 1:10 with water before spraying. The active compound allicin, will help prevent germination of powdery mildew spores. Once the spores are active, though, a higher concentration is needed to cure powdery mildew.
Neem and other oils: Most commonly used to treat houseplant pests, neem oil can be added to your baking powder or bicarb mixture. You can also use canola oil at a rate of two to three teaspoons per one litre. Spray or wipe down infected areas.
Pruning: Trim or prune all affected leaves, buds, branches and flowers and dispose of adequately. Do not compost any damaged foliage as the spores can spread and persist in your composted material.
Vinegar: Some organic experts recommend frugal use of apple cider vinegar for mildew, among other things. Mix three tablespoons of cider vinegar (5% acidity) with three litres water and spray in the morning on infested plants.
Pesticides: If you are using store-bought fungicides, whether chemical or organic, make sure to check the ingredients for sulphur as this is the compound that will directly treat mildew.