Biocontrol of powdery mildew

Symptoms of powdery mildew on apple leaves.

Biocontrol offers many solutions to fight powdery mildew in a variety of crops. However, it can be challenging to get the most out of these soultions, and even choosing the most adequate tool for every case presents its own difficulty. In this article we present an overview of the currently available strategies and lay out some points to facilitate an informed decision making.

Powdery mildew

Powdery mildew is a common fungal disease that affects a wide range of plants, including crops, fruits, vegetables, and ornamentals. The disease gets its name from the white or grayish powdery coating that appears on the surface of infected plant parts. This powdery coating is made up of fungal spores, which are the primary means of dispersal for the disease (ideally indicate link to articles on triple performance).

Powdery mildew is caused by many different species of fungi, but the most common genera include Erysiphe, Leveillula, Podosphaera, Sphaerotheca, and Uncinula. These fungi are obligate parasites, which means they need a living host plant to survive. This is why powdery mildew rarely kills plants, but it can still weaken them. It also diminishes fruit quality, and often degrades the agricultural products too (such as in wine production).

Unlike many other fungal diseases, powdery mildew doesn't need free moisture to germinate and infect plants. The spores of powdery mildew fungi can germinate and infect plants in dry conditions, as long as the relative humidity is high enough. This makes powdery mildew a particular problem in warm, dry climates.

Farmers can take various steps to prevent or control powdery mildew in their crops. This includes: management practices (such as using resistant varieties, spacing and pruning plants effectively, removing infected material) as well as application of fungicides (conventional / biocontrol).

List of biocontrol products available against powdery mildew

Biostimulants

Priming the defences of the plant before powdery mildew onset can reduce the severity of the disease. Strategies based on this principle are preventive, efficient only if applied before the first symptoms manifest.

Messager/ Mesalia/ Mestar: This product, sold by Cérience, is a Natural Defence Stimulator that triggers the plant's own defenses against powdery mildew. It uses a combination of chitosan (derived from crustacean exoskeletons) and pectin (obtained from citrus and apple skins). The Messager is specifically mentioned to be effective against powdery mildew in vineyards (spraying aerial parts).
Romeo: This biocontrol product from Lesaffre utilizes LAS117 cell walls, derived from Saccharomyces cerevisiae (a yeast species), to protect grapevines against both downy and powdery mildew. It works by stimulating the plant's natural defense mechanisms.
Vacciplant by Goemar; (Laminarin). A natural polysaccharide extracted from seaweed that stimulates plant defenses against fungal pathogens.

Fungicides

Aiming for a direct effect on the fungi causing powdery mildew, biocontrol products with a fungicidal activity can be both preventive and curative, although it must be taken into account that even curative methods should be used promptly at the appearence of first symptoms.

Fungicides based on microorganisms

AQ-10: This biofungicide by Biocontrol technologies, developed in Israel, is based on the mycoparasitic fungus Ampelomyces quisqualis and is registered in France for use against powdery mildew on various crops, including tomatoes. It works by directly attacking and destroying powdery mildew colonies.
Serenade ASO: This product from Bayer AG contains an antagonist Bacillus subtilis strain. This product is spray-dried, requiring the addition of water before application. Serenade Max is a commercially available product composed of beneficial microorganisms formulated to destroy fungal pathogens. It colonizes plant surfaces providing a protective barrier and outcompeting phytopathogens, while also producing antimicrobial compounds and inducing plant defense. Recommended use as preventive, it can also be used at onset of infection.
Amylo-X: This product from CERTIS AG uses cells and metabolites of Bacillus amyloliquefaciens. Like Serenade ASO, Amylo-X is a spray-dried product that needs water added before use, in accordance with the manufacturer's instructions. Preventative treatment.
Blindar/Escalator/ Tusal: They all contain Trichoderma asperellum as their active ingredient. These beneficial fungi help suppress powdery mildew by competing for nutrients and space, and producing enzymes that degrade the fungal cell walls. Preventative in nature.

Other fungicides

Some other solutions are well known, such as those based on sulfur or copper. These products have long been used in organic farming and are widely approved for combating powdery mildew in vineyards, fruit trees, and vegetables in France.

Besides these classical solutions, some biofungicides based on natural substances are also available, like Regalia, by Marrone Bio Innovations. It uses an extract from Reynoutria sachalinensis (giant knotweed), and is authorized in France manage powdery mildew on various crops.

Home remedies

In the management of powdery mildew, several home remedies are commonly used. These are predominantly used by small scale farmers, as they may not prove as effective as other solutions for a larger scale farm. Their popularity can be attributed to the fact that they are cost efficient, easily accessible and considerably eco friendly. However, it is important to note that athough these options can be used, their results may often not be reproducible. Notably, it involves uncertainty and risk.

Though these solutions are applicable for organic farming, it is important to note that they are often most effective when used in early stages of disease development - especially when used in a preventative manner. These solutions also require frequent applications because they are not as persistent as commercial formulations.

Oils: Horticultural oils, neem oil, orange oil and jojoba oil can be effective in smothering powdery mildew, particularly early in the growing season owing to their antifungal properties. These oils create a protective barrier on the surface of the plant preventing the germination of fungal spores. Typically, a mixture containing approximately two tablespoons of oil, a tablespoon of liquid soap and about 3.8 litres of water is made, and sprayed foliarly, on either a weekly or biweekly basis depending on the disease severity. However, these oils can also suppress photosynthesis and potentially burn the vines, especially in hot or cold weather or if applied too soon after sulfur treatments.
Potassium bicarbonate: This works by increasing the pH of the plant surface, and thus making it inhospitable for fungal growth. Its use involves mixing approximately a tablespoon of potassium bicarbonate, tablespoon of liquid soap into about 3.8 litres of water, followed by foliar application on both top and lower plant surfaces on a weekly basis.
Milk: Milk contains compounds which when exposed to sunlight, produce free radicals that are toxic to fungi. It can be used by diluting whole milk in water (ratio of 1:10), followed by foliar application on affected plants every 7-10 days. It is recommended to apply in the morning to allow for sufficient drying on the plant surface.
Soapy water: Soapy water requires direct contact with the fungal spores (affected tissue) for it to wash away or reduce spread of fungal spores. However, it does not directly kill fungal spores,therefore it does not provide long lasting protection against fungal pathogens. This involves mixing two teaspoons of liquid soap in approximately 3.8 litres of water and spraying both upper and lower leaf surfaces every 7-10 days or after heavy rains. However, take note to avoid spraying early morning or afternoon to avoid the occurrence of leaf burning in the presence of direct sunlight. It is recommended to use this in combination with potassium bicarbonate or oils for the best results. However, take note to avoid excessive use or strong concentrations because this can damage plant tissues and cause discoloration.
Apple cider vinegar: Its acidic nature destroys powdery mildew spores. A mixture of approximately 3 tablespoons of apple cider vinegar in 3.8 litres of water is sprayed onto affected plant surfaces every 10-14 days. However, it is important to consider that this acidity may damage the plant and thus this solution should be used with care.
Garlic: Garlic contains sulfur compounds which have an antifungal effect, and so can disrupt fungal growth. Its use involved straining a dilution of blended garlic cloves in water (ratio of garlic cloves to water should be approximately 1:10), and spraying affected plants weekly.

Why use biocontrol to manage powdery mildew?

Ecological sustainability: Biocontrol products, like those based on Bacillus subtilis and Bacillus thuringiensis strains, offer an environmentally friendlier alternative to synthetic fungicides. They are less likely to directly harm beneficial insects, pollute the environment, or contribute to fungicide resistance in powdery mildew populations.

Effectiveness: Research shows that biocontrol agents can effectively reduce the severity of powdery mildew infections. For instance, Bacillus species have demonstrated efficacy in controlling powdery mildew in zucchini, improving plant growth and yield. Similarly, products based on Saccharomyces cerevisiae LAS117 cell walls, can activate a plant’s natural defense mechanisms to fight powdery and downy mildew in grapes.

Integrated Pest Management (IPM): Biocontrol products can be seamlessly integrated with other disease management techniques, such as choosing resistant cultivars, improving air circulation, and employing proper cultural practices. This promotes more robust and sustainable disease control.

Reduced chemical use: Biocontrol agents can help reduce reliance on chemical fungicides, which benefits the environment and human health. This is particularly important given the increasing concerns about fungicide residues in food and the development of fungicide resistance in powdery mildew populations.

Addressing specific needs / Precision agriculture: Biocontrol solutions can be tailored for specific crops and regions. For example, the BCA_GRAPE project focuses on developing new biofungicide prototypes based on Ampelomyces quisqualis strains for controlling powdery mildew in European vineyards. This targeted approach can lead to more effective and sustainable powdery mildew management solutions.

Points for concern with biocontrol

Research and Development: While promising, the field of biocontrol is continually evolving. More research is needed to understand the effectiveness, optimal application methods, and long-term impacts of various biocontrol agents against powdery mildew in diverse crops and environments.
Availability and Cost: The availability and cost of biocontrol products can vary depending on the region and specific product. They are often more costly than conventional pesticides.

How to choose

Although there is a large variety of products available to control powdery mildew in agroecological farming, the choice to use one or another is a very complex task that will depend on the reality of each farm

Impacts of weather

The first thing to consider for any particular crop system is the risk of infection, which will be greatly determined by the climatological conditions of the current season. Keeping track of the evolution of the disease at regional scale is vital to treat effectively, since the efficiency of any treatment will be much diminished by a late response. For outdoor treatment using solutions that depend on foliar adherence to work, weather forecast is also essential to avoid the product being washed by rainfall.

Preventive vs curative

The development of powdery mildew after the first symptoms appear may be too quick to respond in time, especially in larger farms. It is for this reason that we emphasize the use of preventive methods when the risk is high. Preventive methods can be divided between biostimulants and systemic fungicides, and the choice between one type or the other will be determined mainly by the crop system to protect and the time-scale at which they are intended to work. While preventive methods might require an upfront investment, they can reduce the potential for severe outbreaks that may result in significant crop loss. On the other hand, systemic fungicides may be more expensive than biostimulants but could provide longer-term protection under high-risk conditions.

Combination of multiple methods

While the compatibility of biostimulants will rarely pose a problem, solutions with fungicidal activity should be used with caution, as they pose the risk of affecting other actors in the agroecosystem. For this kind of products it should be checked if they will have an impact on other agroecological practices being used simultaneously.

Resistance to the treatment

Using the same strategy season after season can result in diminishing effects over the years. Being familiar with multiple options can greatly increase the chances of reliably controlling the advance of this disease and can offer more resources to exploit in a difficult year.