Zucchini and squash are particularly susceptible to powdery and downy mildew. Here is how you can recognize an infestation and how you can prevent it.
Unfortunately, those who grow zucchini and pumpkin often have problems with powdery mildew. Both plants can be attacked by the same powdery mildew fungi, both powdery mildew and downy mildew. This is not surprising, since both belong to the cucurbit family (Cucurbitaceae) and are closely related. Thus, zucchini (Cucurbita pepo var. giromontiina) form their own subspecies of the garden squash.
How can you recognize powdery mildew on zucchini and squash?
If white spots appear on the tops of the leaves of your zucchini or pumpkin, it is probably powdery mildew. This fair-weather fungus is particularly common in the summer months and in hot, dry areas. You can recognize it by a white to gray, wipeable coating on the leaves. The spores are usually spread by the wind or by splashing water. Initially, the flour-like fungal lawn spreads only on the tops of leaves, but later it can also appear on the undersides of leaves and stems. The fruits are usually not affected. However, yield losses must be expected, because the fruits of diseased plants often cannot be adequately supplied and therefore grow poorly.
Note: There are some varieties of zucchini that naturally have a white leaf marking – this should not be confused with powdery mildew.
Downy mildew spreads primarily in humid weather, readily in the fall when temperatures drop and humidity rises. On the upper leaf surfaces of zucchini and pumpkin, pale yellow spots appear at first, later intensely yellow, which are bordered by leaf veins in an angular shape. A reddish-brown fungal lawn develops on the underside of the leaf. With increasing infestation, the leaves turn brown from the edges and eventually die.
What can you do against powdery mildew on zucchini and squash?
Unfortunately, the pathogens of the two types of powdery mildew are everywhere, so you should already take preventive action. Especially in the greenhouse, it is advisable to maintain a sufficient planting distance between the pumpkins and zucchini and to ventilate extensively. In addition, you should choose varieties that are as robust as possible. For example, resistant to powdery mildew are zucchini varieties ‘Soleil’, ‘Mastil’ or ‘Diamond’. Squash varieties that are resistant to downy mildew include ‘Merlin’ or ‘Neon’. Also, be careful not to over-fertilize your vegetable plants with nitrogen, the tissue will become soft and susceptible to fungal diseases.
To increase the resistance of cucurbits to powdery mildew, treatments with plant tonics have proven effective. For both squash and zucchini, apply a horsetail liquid manure as a preventative. Since it contains a lot of silica, it strengthens the tissues of the plants and makes the leaves more resistant to fungal diseases. To make such a horsetail liquid manure yourself, soak about one kilogram of fresh or 150 grams of dried field horsetail in ten liters of water for 24 hours. Then boil the liquid manure for half an hour, strain it and dilute it with water in a ratio of 1:5. Apply the horsetail liquid manure about every two to three weeks in the morning.
To specifically avoid downy mildew infestation, keep the above-ground plant parts of zucchini and squash dry. Water only in the morning hours and never over the leaves, but only in the root area. As soon as the first symptoms appear, you can carry out spraying measures with special fungicide spray. For very severe powdery mildew infestations, you can also use relatively environmentally safe wetting sulfur preparations. However, always study the instructions for use before applying the plant protection products.
Regardless of whether it is powdery mildew or downy mildew: Diseased plant parts should be removed early and disposed of with household or organic waste. The fruits of infested plants can in principle be eaten, but you should wash them thoroughly beforehand. In case of very severe infestation, the beds must be completely cleared.