In a recent column, I discussed the effects of powdery mildew on your garden plants. Today, I want to share with you about downy mildew. The two are often confused and while they have similar names, they are two very different diseases.
While powdery mildew is caused by a true fungus, downy mildew is caused by parasitic organisms that are more closely related to algae. Because it is closely related to algae, downy mildew needs water to survive and spread. It also needs cooler temperatures. You are most likely to see downy mildew in your plants in the spring, where rainfall is frequent and temperatures stay cool.
One of the tricky things about downy mildew is that it can appear in different ways, depending on what kinds of plants it is infecting. Most often, an infection of downy mildew will also include a fuzzy, soft looking growth that can be white, grey, brown or purple. This growth is most commonly seen on the lower leaves of the plant. This growth is where this disease gets its name from, due to its downy appearance. Other common symptoms for downy mildew include mottling or spots on the leaves. The spotting will be yellow, light green, brown, black or purple.
The best control of downy mildew is prevention. Because downy mildew needs water to survive, the very best thing you can do to prevent it is to water your plants from below. Water that sits on the leaves of the plant gives the downy mildew a way to infect and spread on the plant. The spore of downy mildew spreads by literally swimming through water until they come across live plant material to infect. If there is no water on your plant leaves, the downy mildew cannot travel to or infect your plants. This disease overwinters on dead plant material, so removing dead plant material from your garden in the fall will help prevent the disease in the following spring.
There are some sprays that are put there that can help in some cases. Once your plants have downy mildew, the best thing you can do is to try to eliminate moisture and humidity around the plants. As mentioned, make sure you are watering from below. If possible, try to improve air circulation through selective pruning. Normally the disease will dry itself up in the outdoor garden once the weather warms up, as it does not survive well in warm temperatures. If your plants only have a mild case of downy mildew, you can wait for warmer weather.
The reason I am sharing this now is that if you are starting plants indoors, they can be susceptible to downy mildew. I have had downy mildew in the spring when I started my plants indoors. I seem to have more problems when I use peat pots for starting seeds. It is my opinion that the consistent moisture in the peat pots caused it to thrive. When you see a plant infected in the garden, you can pick off affected leaves and remove them from the garden. My experience is that it will help to control the disease.
Downy mildew is a very common, but often, under-diagnosed problem in the spring garden. This disease can damage or stunt plants and is difficult to diagnose. But, if you are familiar with the different ways it presents itself and with the conditions that help it thrive, you will be better able to take steps to control downy mildew in your garden. For further information about downy mildew contact Nikki Bell at the University of Kentucky Cooperative Extension Office. Happy Gardening!!