Powdery mildew is a fungal infection that commonly affects a wide range of plants, including vegetables, fruits, and trees. Recognizable by its distinctive white powdery appearance, it thrives in humid conditions and can spread quickly among similar plant species. Although often manageable, severe infestations can harm plant health and reduce crop yield. This blog explores powdery mildew’s life cycle, symptoms, and strategies for managing it, including how to prevent powdery mildew and biological control methods.
Overview:
- What is powdery mildew?
- What is the impact of powdery mildew?
- How do I know if I have a powdery mildew problem?
- How do I get rid of powdery mildew?
- Summary
What is powdery mildew?
Powdery mildew is caused by a group of similar fungi that can damage crops. It gets its name from its characteristic powdery appearance, which is usually white-grey. Powdery mildew is commonly found on many plant parts, including the leaves, stems, fruits, and flowers, and it can affect many common crops like cucumbers, squash, and beets. There are different species that cause powdery mildew, and each species has a different range of host plants and presents a different set of symptoms. This means that if you find powdery mildew on a particular crop, it doesn’t mean that different crops nearby are vulnerable to the fungus that caused it. However, powdery mildew spreads easily to host plants of the same species and other parts of the affected plant.
Life cycle
Humidity favours spore germination (similar to seed sprouting), but the fungi that cause powdery mildew do not need free water to start growing. In fact, excessive water can actually prevent the growth of these fungi in some cases. Powdery mildew can appear at any time of the year if the climate is favourable, and the fungi that produce it require a living plant host to grow. Fungi causing powdery mildew produce spores, which are tiny, seed-like particles that spread and grow. Spores are sometimes referred to as conidia and are produced in a larger fungal structure called pycnidia.
Spores from powdery mildew-producing fungi are resistant to harsh environments and can overwinter and be spread by the wind or rain splashing to other host plants. Some species are not able to overwinter in colder climates and instead may overwinter in greenhouses. The majority of the white powder seen on affected plants is made up of spores, which are vulnerable to high temperatures and direct sunlight.
What is the impact of powdery mildew?
Powdery mildew does not significantly affect plant health until the infection becomes more severe. The fungus causing the powdery mildew feeds on living plant tissue using structures that penetrate into the plant’s cells. It consumes the nutrients within the plant cells, which causes damage to plant tissue. Once it spreads sufficiently across host plants, it can cause leaf yellowing and dropoff, which can expose plants to sun damage. Depending on the plant type, leaves may also appear twisted, shrunken, or distorted if a large infestation is present. Ultimately, powdery mildew can reduce plant growth and lower crop yields. In some cases, it can cause plants to be less resilient to wintery conditions and may delay the emergence of leaves in the following growing season. Flowers may fall open early when infected with powdery mildew.
How do I know if I have a powdery mildew problem?
Powdery mildew initially appears as very small white powdery patches, usually on the upper sides of leaves but also stems and flower petals, depending on the plant species. Affected areas of leaves may appear as lighter shades of green before more obvious discolouration occurs. In some cases, distortion of leaf shape may occur before white powder is visible. The appearance of white powder and leaf discolouration differs depending on the powdery mildew species and the host plant.
When observed with a hand lens, the powder can look like chains or branching stalks if it is in its spore form. As the infestation grows, smaller spots of discolouration and white powder will gradually spread and join up. In severe cases, powdery mildew can cover entire plant surfaces and cause leaves to die. Depending on the species that is causing powdery mildew, dead leaves can remain attached to the stem or drop off.
How do I get rid of powdery mildew?
Powdery mildew fungi can be a significant cause for concern, particularly with large infestations. Fortunately, there are effective methods to prevent powdery mildew from appearing and to eliminate or lessen its harmful effects on crop health and yield.
Monitoring
It is important to monitor plants for signs of powdery mildew. This is particularly important if there was an infestation the previous year or nearby plants have been affected. If an affected leaf is spotted, it can simply be removed by clipping it off. When doing this, it is important not to allow the powder to spread to other parts of the plant or to disperse it when removing it from the growing area.
Cultural control
The most effective way to deal with powdery mildew infestation is to reduce the chances of it occurring in the first place. Several practices can help achieve this. Planting crops with enough space between them can help to avoid overcrowding, enable better air circulation, and increase plant exposure to sunlight, which can help prevent fungal growth. Avoid overhead irrigation methods (like sprinklers), which can increase relative humidity and favour fungal growth. However, it’s important to keep plants regularly watered as this can inhibit powdery mildew growth.
Using too much fertiliser can promote the early growth of young plant structures, which are more susceptible to powdery mildew infections, so avoid this where possible. Removing weeds and fallen material from the growing area can also be effective, as they can provide a host for fungi that cause powdery mildew to overwinter. Crop rotation practices may also help to avoid crop susceptibility to powdery mildew by disrupting the pathogen’s life cycle and reducing its chance to persist in the soil from season to season. Resistant varieties of plants that are resilient to powdery mildew infection are another option for avoiding infestations. Powdery mildew-resistant varieties also help reduce the need for harsh chemical treatments.
Biological control
Fortunately, there are several biological control methods available for dealing with powdery mildew that do not have the same hazards as conventional pesticides. These solutions derive from nature and include natural substances (compounds from plants and other natural sources) and microbials (microbes that kill pests).
Natural substances
Neem oil is an extract from the neem plant. Like other horticultural oils, it is effective against powdery mildew by preventing its spread and inhibiting fungal growth. Dipotassium phosphite is another effective substance to use against powdery mildew. It works by stimulating the plant’s natural defence systems and can also inhibit fungal growth. Sodium bicarbonate (baking soda) is often employed as part of a home remedy to tackle powdery mildew problems.
Microbials
Several species of bacteria from the genus Bacillus are effective at combatting powdery mildew. They do this by releasing different bioactive compounds that are toxic to the fungi. These species are available in liquid formulations, which can be sprayed directly onto infected plants. A yeast species called Saccharomyces cerevisiae is also effective at helping to control powdery mildew infections. Deactivated (dead) portions of the Saccharomyces cerevisiae can also stimulate the plant’s natural immune system against fungi, working similarly to a vaccine.
Pesticides
As a world leader in nature-based pest management knowledge implementation, CABI encourages Integrated Pest Management (IPM) as the preferred, ecologically-based approach to producing healthy crops, which allows for chemical pesticide use only as needed, and when adhering to measures that limit the exposure of people and the environment to them (see FAO, International Code of Conduct on Pesticide Management).
Before considering the use of chemical pesticides, farmers should explore all available non-chemical control solutions. These might include cultural practices like the ones mentioned earlier and consulting the CABI BioProtection Portal for identifying and applying suitable biological control products.
In case chemical pesticides are considered for use, farmers should look to select lower-risk chemical pesticides which, when used as part of an IPM strategy, help manage pest problems while minimizing harmful effects on human health and the environment. Agricultural advisory service providers can provide information on lower-risk chemical pesticides that are locally available and are compatible within an IPM strategy. These experts can also advise on the required personal protective equipment.
Summary
Powdery mildew is a persistent fungal threat that can compromise plant health and reduce crop yields, especially in fruits and vegetables. By understanding its lifecycle, identifying early powdery mildew symptoms, and employing effective control strategies, including cultural practices and biological treatments, farmers can protect their crops from this plant disease. Integrating monitoring, resistant plant varieties, and safe, natural solutions like neem oil and beneficial microbes provide a sustainable approach to managing and preventing powdery mildew.
Visit our resource page to learn about controlling other pest types, such as bean flies. To explore further options to tackle powdery mildew, consider visiting the CABI BioProtection Portal.
