Powdery mildew and its prevention

Powdery mildew is a fungal disease caused by various types of fungi, such as Erysiphe, Podosphaera, Oidium, Leveillula.

Life cycle and emergence of powdery mildew

Powdery mildew fungi (Erysiphe, Podosphaera, Oïdium, Leveillula) are biotrophic fungi that feed on living plant cells and rarely survive without a living culture. They survive as ascospores or perithecia, structures containing ascospores.

Fungal spores germinate on the leaf surface, where germ tubes also grow and branch. Small structures, haustoria, are formed, from which the fungus penetrates into the plant cell and absorbs nutrients from the epidermal layer of plant cells. Most of the fungus remains on the outer surface of the plant. New conidiophores, structures that contain new spores, conidia are formed on the mycelia on the surface of the plant. These conidiophores are fluffy fungal formations typical of powdery mildew. Only on pepper, infection with Leveillula taurica occurs through the stomata, and the fungus grows inside the leaf. In this case, the conidiophores later also protrude from the stomata.

Conidia are the main means of dispersal. They are dispersed by wind, like ascospores. In general, powdery mildew fungi are not very resistant to water, so rain often limits outbreaks, and dispersal of spores by rain is negligible, as spores burst after a few hours of contact with water.

Powdery mildew spores - ascospores or conidia - do not require nutrients or water to germinate. In general, the epidemic develops fastest at a normal temperature of 18-25 °C. High relative humidity promotes germination but inhibits spore formation, so the net effect of relative humidity is very small. Wind and drafts in greenhouses contribute to the disease. In greenhouses, dispersal of spores over short distances (within a short time) occurs mainly through work clothes.

Symptoms of damage

The symptoms of powdery mildew are similar to those of any plant. These are white fluffy colonies, primarily on the upper side of the leaf. In severe epidemics, ears of corn may be infected.

On pepper, fluffy colonies are found mainly on the underside of the leaf, because the fungus infects the plant by entering through stomata, and there are more stomata on the underside of the leaf. Yellow spots are visible on the upper side. In the case of a strong epidemic, fluffy colonies also appear on the upper side of the leaves. Many sweet pepper varieties respond to powdery mildew infection by shedding leaves that are not yet infected. How quickly this happens depends on the variety.

On apples, pears, roses and other Rosaceae, the fungus that causes powdery mildew is extremely sensitive to water and "hides" on the underside of the leaves. Therefore, on such cultures, the colonies are located at the bottom of the leaf.

In most cultures, the white colonies later turn brown or gray. Severely affected leaves may turn yellow, curl or curl.

Depending on the culture, crop loss from powdery mildew infection can reach up to 80%. This disease especially affects vineyards.

Prevention of powdery mildew

The main method of preventing the development of powdery mildew is the systematic treatment of plantations with fungicides. If the disease is not yet present, we recommend treating with biological fungicides Sporomax™ B and Sporomax™ T at 10 g/10 l of water. If necessary, the rate of drugs can be increased to 20 g/10 l of water. The combination of these products allows you to significantly suppress pathogens and prevent their development. To maintain the phytosanitary condition, treatments are recommended at intervals of 10-14 days. If it has rained or there has been heavy dew, it is recommended to repeat the treatment.
If the plants are affected by the disease, it is necessary to add Microstim™ PS 50-75 ml/10 l of water to the Sporomax™ B and Sporomax™ T biofungicides.
Treatments can be started when the minimum daily temperature is +12C, which is significantly lower than the temperature of powdery mildew spores awakening.

To increase the effectiveness of vegetation treatments, we recommend adding Izomax™ adjuvant - 2 ml / 10 liters of water to the tank solution. Thanks to this:

• the coverage of plants with the working solution improves
• the penetration of the working solution into the plant improves
• resistance to being washed away by precipitation increases
• the volume of consumption of the working solution decreases due to better plant coverage
• the quality of processing on plants with a wax coating and a hairy vegetative part increases

It is also recommended for prevention:

• If possible, use disease-resistant varieties
• Additional applications of silicon or calcium fertilizers strengthen the cell walls and make it difficult for the fungus to penetrate the leaves.
• To reduce the likelihood of powdery mildew in greenhouses, it is recommended to ventilate initially only from the leeward side. Note that this does not apply to other diseases that are inhibited by reducing relative humidity.
• Fruit trees must be pruned at the beginning of the season.
• Do not apply excess nitrogen, as this promotes the spread of the disease