Downy mildew is a very common mold, present around the world in greenhouses and grow rooms. It’s a humidity related fungal disease, caused by a family of different oomycetes. While symptoms are slightly different, it’s often misidentified as powdery mildew or gray mold.
Mildew affects a wide variety of crops – vegetables, like cucurbits (cucumbers), crucifers (cabbage, cauliflower, etc.) and tomatoes, as well as herbs such as basil, and flowers. It also commonly affects cannabis cultivation.
Downy Mildew’s Effects and Symptoms
You can identify downy mildew by pale yellow, green or brown patches appearing on the upper side of leaves. This usually occurs in older leaves. You may also see dark gray, fuzz-like spores, develop on the underside of the leaves, which often leads growers to mistake it for botrytis (gray mold) or powdery mildew.
Eventually, downy mildew causes leaves and branches to dry out, distort and fall off. While it mostly harms leaves and branches, it may also affect stems, flowers and roots.
Downy mildew refers to a family of fungi, many of which target specific crops, like basil downy mildew, for example. So, symptoms too can vary from host to host.
Conditions for downy mildew development
Downy mildew is extremely widespread, partly because it favors conditions common in greenhouses and grow rooms:
- Cool temperatures – 10-24°C (50-75°F)
- High humidity – over 85% relative humidity
After infection, downy mildew usually produces additional spores within 4 to 10 days.
Controlling & Preventing Downy Mildew
There are no non-chemical solutions to treat downy mildew, so prevention is the best way to deal with this disease. Prevention is also the first line of defense in integrated pest management (IPM) and is always better than post-infection treatment.
Downy mildew’s fungal spores travel most effectively in water droplets. So, avoiding the presence of water is critical in avoiding outbreaks.
There are 7 things you can do to prevent downy mildew outbreaks:
Avoid water presence
As mentioned, the best way to prevent downy mildew is by avoiding the presence of water.
Make sure to irrigate straight to the ground, in order to keep leaves dry. It’s also best to water your plants in the morning, when radiation can help evaporate the free water.
Another way to prevent the presence of water is by controlling humidity. High levels of humidity lead to dew point condensation, meaning water begins to appear on cooler surfaces, including leaves. This is the main way downy mildew spores develop and travel, so it’s important to control humidity buildup.
You can control humidity with a dehumidifier, or by heating and venting. Of course, ventilation isn’t always possible, as outdoor conditions may be unfitting for your greenhouse or grow room. For night time humidity, cold seasons and rainy or humid weather, dehumidification is the only way to achieve full humidity control.
Prune and don’t overcrowd plants
Humidity is risky, but it’s especially dangerous when plants are crowded or have dense foliage. Microclimates can form in these spaces, trapping the humidity inside. This can lead to condensation occurring, even if your overall humidity is kept at a low level.
Microclimates are an extremely common problem with cannabis, as the dense buds are perfect for trapping humidity.
To avoid humid microclimates developing, you should make sure to prune plants and maintain distance between them. Greater distances also make it harder for fungal spores to travel from plant to plant, reducing infection rates.
Introduce air circulation
Another way to reduce dew point condensation is by incorporating air circulation. One of the most common problems growers face, is uneven climate conditions in their growing facility. Even one plant surrounded by humid air can be enough for water to condense and downy mildew to develop.
Any air current is good for dispersing these humid microclimates. But proper 360° air circulation can provide much more. It helps homogenize the climate conditions in the entire space, avoiding different relative humidity levels in different areas.
Remove infected plants
Infected plants are a hotbed for fungal spore production. This includes dead plants and fallen leaves
As it’s nearly impossible to detect downy mildew before symptoms appear, it’s crucial to remove any plant that shows signs of infection. Make sure to remove plant debris as well, to avoid spore development in the soil.
Maintain sunlight or lighting
Proper lighting is important in the prevention of most plant diseases. Radiation is necessary for photosynthesis, helping the plant build the compounds it needs to grow and maintain itself. Well-lit plants tend to have stronger immune systems, which helps ward off fungal infections.
Lighting has another effect when it comes to humidity related diseases, such as downy mildew. Radiation evaporates water, so proper lighting can help keep your growing facility dry.
Choose downy mildew resistant crop varieties
The final way to prevent any disease is by growing resistant varieties of crops. This isn’t always possible, but some types of lettuce and onions, for example, have a higher resistance to downy mildew.
It’s important to note that fungi constantly change and adapt, so even resistant varieties can lose their advantage over time.