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Powdery mildew is common

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By Jeneen Wiche

Powdery mildew is probably the most common garden fungus around. It is not too terribly picky about where it spreads. It likes humid and dry 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 with proper plant selection, spacing and treatment before it takes hold.

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.

Many common garden plants are susceptible to powdery mildew. Some of the most common herbaceous plants include phlox, beebalm, 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 to the problem. 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. Train vegetables such as peas, pole beans and vining cucumbers to grow up on trellises for better air circulation.

Spores can winter over on old plant debris, so clean up well in the fall, and mulch around plants in the spring to prevent soil from splashing up during heavy rains. Treat infected plants at the first sight of the disease. You can remove infected foliage, typically at the base of the plant and then apply a fungicide like neem oil on the healthy foliage. Be sure to wash your hands before handling the rest of the plant, because you can spread it this way, too.

Once powdery mildew has taken hold, you cannot get rid of what is there, but you can prevent it from spreading any further. If the disease goes unchecked this time of the year, the infected plant will look terrible by late summer as the spores spread. If an herbaceous perennial is too far gone, go ahead and cut the whole thing back and hope for new, clean growth.

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, rethink your fertilization schedule.