Zucchini diseases: prevention and treatment

Zucchinis are undoubtedly among the easiest to grow: if planted at the right time, with the right techniques and the right distances between plants, they tend to take root quickly and offer generous yields for a long time.

The plants respond well to organic cultivation, such as that usually practiced in private gardens and which relies on organic fertilization and care with low environmental impact due to adversity. While it is true that cultivation is simple, this does not mean that the zucchini plant is immune to possible diseases and pests, since it grows organically, it is necessary to recognize the symptoms early and to intervene with appropriate solutions.

So let’s try to identify and analyze in detail what are the main diseases that can affect zucchini plants and partly also other cucurbits (cucumbers, watermelons, melons and pumpkins), but first let’s recall some prevention strategies basic.

Contents [Ocultar]

  • How to prevent diseases

  • Powdery mildew or bad powdery mildew

  • Cladosporiosis

  • zucchini virus

  • sclerotinia

  • botrytis

  • Erwinia carotovora

  • insufficient items


How to prevent diseases

In the garden there are some basic rules that must always be respected for a good prevention of adversities, these are valid for practically all crops, whatever disease you want to avoid. So let’s keep these points simple as rules, which are good practice and help reduce problems considerably.

  • Never water the aerial part plants: humidity promotes fungal diseases and it is therefore good to water the plants only at the base.
  • Soak macerated plants with a strengthening action (and these are the only cases where the vegetable surface part can be moist) such as those made with horsetail, garlic and onion.
  • Do not over fertilize , so do not overdo it even with doses of manure pellets, for which it can be easy to be “heavy-handed”. Too much nitrogen in green tissue makes it more susceptible to disease.
  • Keep aphids away potential vectors of certain viral diseases.
  • Always respect the rotations , changing the place of the courgettes each year and possibly avoiding putting them where the previous year there were other cucurbits. Although in a small orchard it is difficult to respect this precept to the letter.
  • Remove old plants : Although the zucchini, after the peak of production of two months, continue to produce fruits, after this time it is better to plant them, because the old plants are more prone to diseases. The ideal is that while waiting for others to be planted to replace them, so as not to run out of courgettes as long as the season allows it;
  • Carefully remove diseased leaves to limit the spread of pathogenic fungi.

Even if we pay attention to these indications, it is likely that some plants will be affected by certain adversities, of which we see the main ones.

Powdery mildew or white sore

Powdery mildew, photos by Sara Petrucci

Powdery mildew, also called “mal bianco”, is a fungal disease that causes grayish-white powdery blooms on the leaves and stems of zucchini, also affecting the fruit. In summer, especially in the hottest and poorly ventilated areas, this disease is very common in orchards and also affects pumpkins and other cucurbits. Some varieties of zucchini naturally show extensive silvery-white streaks on the foliage, but these should not be confused with powdery mildew, which has a bumpy appearance and a powdery consistency.

To eradicate this zucchini disease with the methods allowed by organic farming, it is necessary to repeat the treatment with Sodium bicarbonate dissolved in water, and in the most serious cases use products based on sulfur . The important thing for the latter is to always follow the instructions on the packaging of the purchased product and also to respect the “shortage time” which are the days that must elapse between the treatment and the harvest. Given an almost daily harvest of zucchini, this need could lead to loss of fruit for just under a week, which does not happen with baking soda, although its action may be milder than that of sulphur. In addition, sulfur should not be used if the ambient temperature exceeds 30-32°C, as it could be phytotoxic.

As an alternative to these treatments, you can use the entomoparasitic fungus Ampelomyces quisqualis by making two treatments 7-10 days apart.


The fungus strikes all the pods of cucurbits and manifests itself in the aerial part. In the presence of cladosporosis, spots appear on the leaves of the zucchini plant that become necrotic, tear in the center, while on the fruits there are round, sunken indentations, with gummy lumps and mold. The underlying tissues usually rot. Against this pathology, we can intervene with copper-based products, but we must also eradicate the infected plants.

zucchini virus

Virus diseases are particular diseases that manifest themselves in deformations, dwarfism of plants and unusual discolorations. Among the most common are the » zucchini yellow mosaic virus “, which can also be seen in the fruit, which becomes bent and lumpy and cracked, and the ” cucumber common mosaic virus which, despite its name, is most pronounced in zucchini and pumpkins, bearing small, misshapen, mosaic fruits.

There are no solutions against the virus other than the certainty that the propagation material (seeds and plants) is healthy, the uprooting of infected plants and the care to disinfect the knife used to harvest the fruits of the affected plants. The virus is transmitted by aphids and also by the aleirodide Bemisia tabaci (the white butterfly), so the prevention and control of these parasites, even with ecological means, is important.


Plant tissues affected by sclerotia are covered with white cotton mold and round black corpuscles (sclerotia). In soils that have harbored crops affected by this disease, it is advisable to spray a suspension of the good fungus Thricoderma asperellum, a natural antagonist of sclerotinia.


Symptoms of botrytis are similar to those of the previous disease, but instead of sclera in this case, typical gray mold is noted. In organic cultivation, botrytis is fought with copper-based products, even if prevention strongly limits the appearance of this disease in the garden.

Erwinia carotovora

Erwinia carotovora is a soft rot of a bacterial nature that usually begins to attack the zucchini fruit. This problem is prevented by avoiding the stagnation of humidity and by limiting the contact of the fruits with the ground thanks to mulching.

tier bags

Sometimes certain deficiencies in mineral elements such as magnesium or potassium can be interpreted as pathologies, since the symptoms are usually yellowing or darkening of the edges of the leaves. If these alterations of the leaves are due to a deficiency, we are not in the presence of a disease but of a simple physiopathy: restoring the adequate conditions solves the problem. Therefore, no treatment is necessary, only proper fertilization.

Fertilizing each year with plenty of compost and manure pellets (without overdoing it), possibly adding wood ash and rock meal, should naturally avoid these imbalances.

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