Organic Defense against Powdery Mildew: Protecting Our Plants Naturally

The Oidium, also known as powdery mildew, white mildew, albugo, and white mold, is one of the most dreaded diseases affecting horticultural crops and more. In this article, we aim to understand this disease better, its common targets, and how to defend organically without resorting to chemical pesticides.

By following appropriate agronomic practices and employing preventive remedies permitted in organic farming, it is possible to significantly reduce the damages caused by Oidium. We’ll now show you how to protect your crops while maintaining them fully organic.

What is Powdery mildew?

Powdery mildew spores under the microscope

The term powdery mildew refers to a nutritional disease affecting plants, caused by the withdrawal of nutrients from plant cells by pathogens. In the case of Oidium, the pathogen agents are microscopic fungi called Ascomycetes, belonging to the family Erysiphaceae. They are not visible to the human eye.
These fungi produce filaments that later develop into spores, called oidiospores.
Oidium pathogens establish a trophic relationship with the plant known as obligate parasitism. This means that without nutrients derived from the plants, the fungus would not survive, especially during winter latency. The new spores are usually released by the fungus at the beginning of spring. Wind action helps spread the infection from one plant to another.
Another characteristic of Oidium is that each species of pathogenic fungus attacks different parts of the plant, targeting specific families of plants.

Which Crops and Plants Does Oidium Affect?

Many plant species are attacked by this fungus, and as mentioned, each suffers from the attack of a specific pathogen agent. However, the symptoms and damages to the vegetation are quite similar.
Let’s see the main crops affected by powdery mildew and their respective pathogenic agents:

• Downy Mildew of Vine, caused by the pathogen agent Uncinula necator
• Cucurbitaceae zucchini, cucumber, melon etc), triggered by the agent Sphaerotheca fuliginea
• Peach and Rose Mildew (affects peach and rose), caused by Sphaerotheca pannosa
• Beet Mildew, affecting plants belonging to the Chenopodiaceae family (beet, spinach), whose culprit is the pathogenic agent Erysiphe betae
• Cruciferous Mildew (cauliflower, black broccoli, savoy cabbage), activated by the fungus Erysiphe cruciferarum
• Umbelliferae Mildew (fennel, carrot, parsley, celery, etc), whose pathogen agent is Erysiphe heraclei

There are many other plants and trees that can be affected by powdery mildew, including oak, laurel, peas, apple tree, and cereals.

Appearance and Damages Caused by Powdery mildew

First signs of powdery mildew

Oidium presents itself as a white/grayish mold patina that forms on plants, especially on leaves.
Initially, small roundish white spots appear on the lower side of the leaves. As these spots are beneath the visible surface, they are hard to detect.
These spots then multiply and, as they also attack the upper side of the leaves, become visible. As the attack progresses, the spots enlarge, merge, and form the annoying whitish haze. The texture of this patina is powdery, resembling talcum powder or flour.

Advanced powdery mildew attack on a zucchini plant

If the infestation is not stopped and contained, the white mold progresses. The leaves of the plants begin to turn yellow, curl, and necrotize.
Naturally, the vegetative activity of the plant significantly slows down as photosynthesis is compromised.
Moreover, it’s not only the leaves and aerial parts of the plants that are affected by oidium. The fungus also attacks other organs, albeit with less intensity, such as buds, shoots, flower buds, and flowers (as in the case of roses), not to mention young fruits.

Oidium on Vines

Cluster of grapes affected by powdery mildew

Vines are perhaps the most important cultivars subject to powdery mildew attacks. If the infestation is not immediately contained, it can cause serious losses, even the entire production. On grape bunches, damages are direct and vary depending on the time of infection. For instance, an infection during the pre-flowering phase causes the bunch to wither. If the attack occurs after flowering, a white powder forms on the grapes, and the development stops, leading to the grapes’ splitting and falling. Finally, when the attack occurs while the berries are fully formed, a compact and persistent white mold forms, rendering the grapes unusable.

Climatic Conditions Favoring Oidium Spread

Oidium develops through the release of new spores by the pathogenic agent. The fungus can grow between 6 to 32°C, with the best temperatures for disease proliferation being 20-27°C. Higher temperatures are limiting factors. At 35°C, conidial germination is inhibited, and conidia are killed at temperatures above 40°C. High temperatures also devitalize the mycelium.
Heavy rains discourage oidium attacks, as they wash away and dilute conidia and mycelium from the surface of infected organs. The presence of water on the plant impedes conidial germination. On the other hand, increased relative humidity following rain stimulates higher conidia production. Rarely does humidity become a limiting factor, as conidia can germinate with relative humidity values as low as 20-25%, while 40-60% values are sufficient for germination and infection.

Prevention and Organic Defense against Oidium – Agronomic Techniques

Rose affected by powdery mildew

By implementing appropriate agronomic practices, you can achieve biological defense preventively. First and foremost, pay special attention to irrigation. In hot periods, avoid watering the plants during the central hours of the day. The contrast between cold water and hot weather creates ideal humidity conditions for spore dispersion.
Therefore, irrigate either early in the morning or in the evening. This way, you will also achieve better water efficiency, preventing quick evaporation.
Another measure is to avoid directly wetting the leaves; it’s always better to use drip irrigation. Another agronomic technique useful for preventing oidium is to respect the appropriate distances for sowing and transplanting crops. Overcrowded plantings do not allow for proper air circulation, creating favorable conditions for the development of pathogenic agents. To improve aeration in the canopy of plants, it is essential to periodically remove dry or damaged parts, especially for horticultural crops like cucurbits.
For ornamental plants, such as roses, it is always recommended to perform periodic pruning to prevent oidium.

Sulfur Powder

Unfortunately, the techniques we have just described are not sufficient on their own. To prevent powdery mildew pathogens, it is necessary to resort to other products, always allowed in organic farming.
One such remedy is sulfur powder, which effectively prevents the manifestation of oidium. Sulfur is a natural element, and its use is permitted in organic farming. Naturally, it must be used following the correct dosages and the instructions on the label (here you can find some excellent products at an affordable price).
Of course, we recommend applying sulfur before oidium appears. For example, it is advisable to take action when the climatic conditions seem to be favorable for the fungus’s establishment, such as high humidity and temperature fluctuations. Moreover, it is best to intervene if you have experienced attacks in the past on the type of crop you want to sow. Remember, in organic farming, prevention is always better.
To apply sulfur to your vegetable crops, you will need to use a specific tool called a sulfur duster (you can find a manual one here). This tool can greatly facilitate your work. If your crops are not too extensive, we advise against spending money on electric sulfur dusters, which, although highly efficient, can be quite expensive.
Once the treatment is carried out, it is recommended to repeat it (at least once) after 10-15 days. This ensures that oidium will not jeopardize your vegetables.
Sulfur powder has a distinctive acrid odor, so when you blow it, do so with the wind in your favor. Moreover, it is essential to use a mask and protective gloves.

Use of Baking Soda (Sodium Bicarbonate)

Another highly effective remedy against powdery mildew is baking soda (sodium bicarbonate). It is a remedy that can be used alternatively to sulfur powder. In particular, given its immediate action but low persistence, it is best to treat with baking soda during the most critical phases of fungal infestation. For example, just before an expected storm.
Baking soda acts by temporarily modifying the pH of the leaf blades. Fungi need an acidic environment to proliferate, and therefore, they find it easy to thrive on plant leaves. Baking soda raises the pH, making the environment basic. Hence, the pathogenic fungus encounters an inhospitable environment for its proliferation.
Regarding usage precautions, both for sulfur powder and baking soda, it is recommended to apply them in the evening when temperatures are cooler. This is to avoid the risk of leaf burning.

Further Reading

• Oregon State University and USDA-ARS – Identification of Powdery Mildew Fungi – This article provides a comprehensive guide to identifying powdery mildew fungi, including insights into morphology, taxonomy, and disease cycle.
• Encyclopaedia Britannica – Powdery mildew | Description, Symptoms, & Control – The article discusses the appearance, causes, effects, and control methods of powdery mildew.
• University of California Statewide IPM Program – Powdery Mildew on Vegetables – This article provides detailed information on identification, damage, life cycle, and management of powdery mildew on vegetables.
• ScienceDirect – Powdery Mildew – Powdery mildew is a significant problem in various regions. This article discusses the biology of powdery mildew fungi and how they infect a wide range of plants.