What’s that white powdery substance on my plants leaves?
I had a query recently asking what a white powdery substance was on a grape leaf? They actually said “it looks like powdery mildew, but it’s not on zucchini”! It looked like powdery mildew because it was powdery mildew.
It got me wondering “how many other gardeners consider powdery mildew a disease restricted to plants of the cucurbit family?”
Most gardeners will have struck powdery mildew on their cucurbits (i.e. cucumbers, cantaloupes, pumpkins etc). It shows up as a white or grey powder on the leaves. While it generally doesn’t kill your plant, it can significantly impact your crop as the plant can’t produce lots of fruit while its energy is being drained off and the plant is trying to fight the disease.
Powdery mildew actually affects a wide range of plants, not just zucchini, pumpkins and squash. It can affect both edible and ornamental plants. Edibles that can be affected include carrots, apples, grapes, tomatoes, eggplants, peas and beans and capsicum to name a few. Ornamentals can include chrysanthemums, gerberas, dahlias and roses. In fact it’s probably quicker to write a list of plants that aren’t affected!
So what is powdery mildew? Powdery mildew is a fungal infection. The fungal spores attach to the leaves of the plant where they drive filaments or threads into the leaf structure to draw the nutrients away from the plant. The fungi then develop fruiting spores which can be transmitted by wind or water splashes. The fungus thrives in warm, dry conditions and warm, humid conditions.
Powdery mildew is actually caused by a range of different fungi, some of which are specific to a species of plant while others can affect a whole host of plant species.
There are several things the organic gardener can do to prevent powdery mildew in their garden:
- Grow the plants in the conditions they need to thrive. With veggies this means plenty of sunlight. Struggling and weak plants are more susceptible to attack.
- Don’t crowd your plants. Allow plenty of room for air circulation.
- Encourage beneficial insects. The orange/yellow lady beetle actually eats the fungus!
- Good hygiene is essential, as it is with all gardening tasks.
- Clean tools with a weak bleach or hydrogen peroxide solution between tasks,
- Remove infected plant material and put it in the bin.
- Do not compost as the spores will hibernate and can then infect other plants.
- Crop rotation assists by removing susceptible plants from the soil for several seasons.
- Don’t over-fertilise as lush new growth is more susceptible to the fungi
- Morning watering is best. Some say to not wet the leaves, although scientific evidence shows that water can interrupt the sporing cycle.
If you find that you continually battle with powdery mildew then you may need to investigate the range of crops that have been developed to be resistant to the fungus.
Lady beetle feasting on powdery mildew
If you find that you have an attack of powdery mildew, there are several organic treatments. These include:
- Spraying with milk. This is the number one organic solution and, surprisingly, it works better than many commercial treatments. The standard spray is 1 part milk full-cream milk to 10 parts water, but I find a 25 percent solution works better. For severe infections you may need a 50 percent solution. Milk can also help to prevent infections if you spray your plants weekly;
- Spray with a sodium or potassium bicarbonate solution. I find you need to add a soap to help the bicarb adhere to the leaves;
- Spray with Neem oil. Again a small amount of soap or white oil will assist it to stick to the leaves;
- Dust with wettable sulphur or spray with lime sulphur;
- Use a commercial biofungicide.