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Powdery mildew is probably the most common garden fungus around. It is not too terribly picky about where it spreads, it likes humid weather, thrives in the heat of the summer and is hard to control once it has started. The trick here is to prevent it from happening by proper plant selection, spacing, pruning and treatment before it spreads.
Powdery mildew is caused by several different fungi, I won’t bore you with their names because they all act the same way.
The spores of the fungus are carried by the wind and by splashing water. When a plant is infected we see a gray, powdery coating on the foliage that saps nutrients from the plant causing the foliage to yellow. In severe cases it will cause the foliage to die. In some cases it simply causes the plant to be less vigorous and mars the appearance; in other cases it may cause the plant to die.
Many common garden plants are susceptible to powdery mildew. Some of the most common herbaceous plants include phlox, bee balm, asters and hollyhocks. Common annuals that are prone to infection are zinnias, snapdragons, bachelor buttons, cosmos, sunflowers and dahlias. Trees and shrubs to keep an eye on include dogwoods, lilacs, crape myrtles, azaleas and rhododendrons. And vegetables are not immune; watch your beans, cucumbers, squash and peas.
The best way to combat powdery mildew is to take a couple of different precautions. First, select plant material that is resistant, if you can. There are varieties of mildew-prone plants that show tolerance to the problem and they are typically labeled as such. Second, create a growing environment that will help deter the onset of the problem. Provide good air circulation and light penetration by spacing plants according to their cultural requirements. Provide vining vegetables like peas, pole beans and cucumbers with adequate trellising to maximize air circulation.
Spores can winter over on old plant debris so clean up the garden in fall; and mulch plants in spring to prevent soil from splashing up during heavy rains. Monitor plants, too, so you can treat infected plants at the first sight of the disease. You can remove infected foliage, typically at the base of the plant (wash hands before handling the rest of the plant because you can spread it this way, too) and then apply a fungicide to the healthy foliage.
Once powdery mildew has taken hold you cannot get rid of what is there, rather the goal is to prevent it from spreading any further. Use a fungicide containing benomyl to prevent the spores from germinating on nearby foliage. If the disease goes unchecked this time of the year, the infected plant will look terrible by late summer as the spores spread. Reapply the fungicide every 7 to 10 days to protect new growth and reapply after rain. I shy away from synthetic chemical use so if I am dealing with herbaceous perennials I often just cut them back to the ground if they are infected and let new growth emerge (and keep my fingers crossed.) Of course this is not an option if you are experiencing problems with a woody plant or annual vegetable!
Powdery mildew even infects our lawns during rainy weather in the summer. Night-time temperatures between 65 and 70 degrees coupled with hot and humid days create the right environment for the disease. If lawns are infected you will see a fine white powder on the blades of grass. The blades will begin to yellow and the stems and roots will gradually decline.
You know that I am not a proponent of spring fertilization of lawns, this is one reason why: nitrogen fertilizer causes lush, rapid growth that is more susceptible to powdery mildew.
If you do see powdery mildew developing in your lawn apply the same fungicide you would use for your garden plants. This type of stress on the lawn opens it up to other problems throughout the summer so reconsider your lawn care plan accordingly!