Powdery mildew or white disease of zucchini

Powdery mildew or white disease of zucchini

L'powdery mildew it is a parasitic fungus that attacks various plants in the vegetable garden and orchard, very often attacking cucurbits (pumpkin, courgette, cucumber, ...). In particular, this cryptogamic disease is annoying on courgette and pumpkin plants, although we can find it on many other species, including perennials such as fruit and aromatic trees, such as sage.

The mushroom is also known as bad white because it manifests itself with a bleaching on the surface of the leaves, which makes it really characteristic and very easy to recognize.

Combating powdery mildew in organic farming is possible, first of all aiming to prevent it, secondly with interventions based on potassium bicarbonate, sodium or sulfur. Since it is one of the most common garden diseases, it is worth taking a look at it, so let's find out in detail how we can protect our zucchini from white sickness.

Characteristics and symptoms

The white mal is a fungus that develops thanks to spores, called oidiospores, originating from micro filaments. It is aAscomycete, of the family of Erysiphaceae. The damage it causes is mainly caused by the leaves.

As anticipated recognizing the symptoms of powdery mildew is not at all difficult: in particular on cucurbits such as courgettes and pumpkins, which have large leaves where evident stands outand the white patina of the disease. After the first stage in which the white spots are seen on the leaves, there will be yellowing and necrosis, up to decay or total curling. This damage obviously compromises photosynthesis, in general plants affected by powdery mildew have stunted growth

Like all fungal diseases too powdery mildew propagates through its spores, which can also be carried by the wind. The preferred conditions for the white sickness to manifest itself are a moderate temperature (between 20 and 25 degrees) and obviously humidity. Therefore, pay particular attention to the mid-seasons, a period in which treatment may be required.

How to prevent white sickness

In organic farming, prevention must always be a priority: it is better to avoid the problem from the outset than having to use products that, although natural, can still have minor consequences, altering the environment.

The first tip to prevent powdery mildew is to avoid planting our cucurbits too close. The courgettes must have at least one meter of distance between each plant, the pumpkins even more. If the plants are well spaced, there will be good air circulation, avoiding the onset of white sickness and if it occurs, however, making the spread of the disease more difficult. Also to maintain ventilation between the plants, situations that are too dense with shoots and leaves can be thinned with some pruning.

Another essential precaution to avoid powdery mildew is to rotate the crops, avoiding returning with pumpkins or courgettes to the same garden bed for at least 3 years. In general, for all diseases, alternating the different types of plants is essential.

You should then avoid water in the hours when the temperature is close to the most favorable to the fungus (22 degrees), if possible avoiding wetting on the leaf but spreading water on the ground directly.

Rock dust sprayed on the leaves are also useful in prevention: they can absorb excess moisture. For this purpose we can use kaolin or micronized zeolite.

How to intervene on diseased plants

Powdery mildew is a fungus very resistant and difficult to eradicate. In organic farming, there are no products that can cure powdery mildew by healing the affected part of the plant, but the infection can be stopped. It is very important to keep an eye on the plants constantly and intervene promptly before the fungus expands to excess, damaging the entire crop.

The intervention against this disease is not simply a treatment. To stop the fungus you need to first remove all diseased parts of the plant (taking care to eliminate them and not put them to compost and do not leave them on the ground) and then carry out a treatment to protect the healthy parts. If the plant is very sick we might as well uproot it entirely.

When the disease occurs it is important intervene with treatments, obviously using only products allowed by organic farming. The purpose of the treatments is to contain the disease, stopping its spread, so it is necessary not only to treat the plants that show symptoms of white sickness, but the entire cultivation of species potentially subject to the problem.

You can too decide on preventive treatments when conditions are favorable for powdery mildew, therefore mild temperatures and very humid.

Biological treatments against powdery mildew

In organic farming we have several possibilities to counteract white sickness on courgettes, pumpkins or other plants. We must not think that if a product is allowed in organic products then it is free from environmental effects: even a simple treatment based on sodium bicarbonate has a small effect in ruining the structure of the soil and in altering its pH. So before intervening it is good to take this into account and also to know the side effects, in order to dose the substance and not to abuse the quantity or frequency of treatments.

There are five most effective organic remedies for white sickness applicable in the garden, I list them in order of eco-sustainability, so it is better to start from the first, even if more bland. Then let's go to see in more detail the characteristics of each intervention.

  1. Horsetail decoction or macerate.
  2. Vinegar
  3. Potassium bicarbonate.
  4. Sodium bicarbonate.
  5. Sulfur.

Fighting powdery mildew with horsetail

Horsetail is a spontaneous plant that is easy to recognize and widespread throughout the Italian territory, due to its high silicon content it can be used to strengthen the defenses of plants against fungal diseases. It is necessary to make a decoction or a macerate, in order to obtain a liquid to be sprayed on our crops, these are really simple and self-produced preparations.

We must not think of horsetail as a remedy, but as a support for prevention, the treatments must therefore be done quite often in the seasons favorable to white sickness, before the onset of the problem. The beauty of this treatment is that it has no contraindications, so it does not pollute and can be used without precautions.

To learn more about how to prepare and use this natural remedy, you can read the article dedicated to horsetail decoction.


Vinegar is a very mild remedy for white sickness, but its acidity is unwelcome to the development of the spores of this disease. It comes with water in which we dilute a little vinegar, the right dose is one tablespoon per liter.

It is a homemade method suitable for small gardens and also for balcony crops, since we usually have vinegar available at home. But be careful not to overdo it because a good amount of vinegar in the soil can kill plants.

Sodium and potassium bicarbonate

Among the cheap and simple natural remedies we can also use sodium bicarbonate and potassium bicarbonate, which have similar action. It seems that in a similar way, hydrogen peroxide is also useful in combating the disease on zucchini plants.

The great thing about baking soda is that it comes with a cheap substance and often already present in the home, since it is used for many purposes. To carry out the treatment, a tablespoon of bicarbonate is used for each liter of water. However, be careful that use must be moderate in frequency and quantity: the presence of sodium bicarbonate in the soil is harmful to all plants, and also varies the pH, turning it to basic. If small doses of bicarbonate sprayed on plants do not do any harm, its abuse can create serious problems for the garden.

Potassium bicarbonate has an action very similar to that of sodium and is registered as a pesticide allowed in organic farming, his treatment has only one day left.

On Orto Da Coltivare you will find an in-depth study dedicated to sodium bicarbonate and potassium.

Sulfur: organic fungicide

The most important organic treatment against the white sickness of zucchini is certainly sulfur, considered the anti powdery mildew par excellence, allowed in organic farming. The first thing to know if you want to use sulfur in your garden is to pay attention to yours phytotoxicity, in fact, at some temperatures the sulfur becomes harmful to plants. In particular, the sulfur-based treatment is ineffective if the temperature is below 15 degrees and harmful to the plant above 30 degrees, so be careful to do it at the right time.

Then be careful to remember that they are to be kept 7 days shortage between processing and collection. Sulfur is allowed in organic farming.

To deal with this product using a spray pump is required buy wettable sulfur (for example this on Amazon has a good price ratio). It is used in doses of about 20 grams per 10 liters.

We can also simply use powdered sulfur, without dilution. It is better to spread it on the plants using a sulfurator, there are electric or manual models, always on Amazon you will find this manual which is quite cheap.

The treatment is best repeated after two weeks, even if an excellent result is found already after the first step, in order to definitively end the disease.

For those wishing to choose this remedy, you can learn more by reading Sara Petrucci's dedicated article sulfur.

Video: How to identify u0026 manage Powdery mildew