Powdery mildew on zucchini

A reader has asked about powdery mildew on zucchini plants and fungus-eating ladybirds:
Hi. Wonder if you can sort this.
1. Most fungi need moisture and organic material. This seems to be supported by my zucchinis which seem to get worse powdery mildew when I get water on the leaves. I have read that they like dry weather. Is there evidence for either opinion?
2. Some people say that the ladybirds that feed on this mildew spread it by carrying spores, others reckon they are a controller, eating the fungus down. What is the evidence please for either of these?  Many thanks, Barb

Powdery mildew is likely to occur on stressed plants in humid weather when temperatures are between 11-28° C. and, once established will continue to affect the plants even if weather becomes dry. Avoid wetting leaves whenever possible Barb. However, because they like low-humidity weather, it doesn’t mean that they are drought tolerant. Zucchini and some other members of the cucurbit family (melons and squash) produce a lot of foliage and need plenty of water and fertiliser. An efficient way to water this group of plants without wetting the leaves is to put a large drink container (with the base and cap removed) neck downwards near the roots so that all the water goes directly to the root area where it is needed, (see photo). Keep topping up the container until it empties slowly.
The yellowish ladybirds with 26 or 28 spots are the only pests of the ladybird family. They eat the leaves of stressed plants of cucurbits. The beneficial fungus-eating ladybird and larvae can be clearly distinguished from the pest in the photos below. From the far left is the ‘Fungus-eating ladybird’, then the leaf-eating ’26 spot Ladybird’ that damages plants. Next is the larva of the ‘Fungus-eating Ladybird’, which also eats fungus and, last of all is the prickly larva of the ’26 spot Ladybird.

Rather than blame the fungus-eating ladybird for spreading the disease, gardeners should check that their plants have sufficient water and nutrients to avoid stress, and the soil pH is suitable for them to absorb what they need for healthy, disease-resistant growth. Also see Powdery Mildew for treatment of this disease.

5 thoughts on “Powdery mildew on zucchini

  1. Just seen some grey powder on my zuccini leaves in the hydroponic garden. I am guilty of watering the leaves on occasion – but won’t any more.

    Thanks for the tip!

  2. Hi Lyn and Adam, when I was researching for my book, I found that researchers in Brazil discovered that weekly sprays of milk controlled powdery mildew in zucchini just as effectively as synthetic fungicides such as fenarimol or benomyl. Just like Lyn said, the concentration needed to be at least 1.5 cups per US gallon of water (about 1 part in 10), although subsequent tests have successfully used as little as 1/2 cup per gallon of water.

    That’s very interesting, because powdery mildews are caused by a different group of fungi (Oidium spp.) from those that usually respond to milk solutions, and the recommended chemical treatments for downy and powdery mildew are different – with wettable sulphur being the usual treatment for powdery mildew but not downy mildew. But, if research shows milk to be effective – try it, and let us all know the results. – Lyn

  3. Hi
    I’ve heard that spraying milk diluted in water 1:5 can be effective in controlling powdery mildew. I have a lot of yellow lady beetles feeding on powdery mildew on my zucchini. Will spraying the milk/water harm the lady beetles?

    Milk spray is more effective on downy mildew Adam, and the correct dilution is one part milk to 9 parts water – any stronger and you can cause other problems, and it won’t get rid of the ladybird pests. Full cream milk is best as the fat helps it stick to leaves. You can use chamomile tea for powdery mildew. Make a cup of chamomile tea and let it steep for 15 minutes before removing the teabag,then dilute it with another cup of cold water to make each 500 ml of spray. You would have to spray both sides of the leaves (preferably in the morning) several times a week, but the tea won’t get rid of the ladybird pests either. – Lyn

  4. I find if I focus on improving my soil in a new garden bed for a few years, I eventually get almost no disease at all. Like Lyn said, it’s all about providing a healthy environment for the plants.

  5. Oh, thank-you! I have these little ladybirds on my (powdery) zucchini leaves and I was wondering. Now I know they are doing a good job for me.

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