Powdery mildew common in summer

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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 by proper plant selection and placement and adopting good cultural practices. Most powdery mildew problems won’t do too much harm but some plants suffer decline if it is a repeat problem. 

Powdery mildew is caused by several different fungi, I won’t bore you with their names, because they all act the same way. 

Different species infect different plants so the fungi are basically plant specific essentially the powdery mildew on your dogwood cannot spread to your phlox or vice versa.

The spores of the fungi that cause powdery mildew are transferred to our plants by wind and splashing water.  When a plant is infected we see a gray, powdery coating on the foliage that disrupts efficient photosynthesis and 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; serious and repeated incidence of powdery mildew (along with other stress factor) could case a plant to die. 

Some of the most susceptible herbaceous plants include phlox, beebalm, asters and hollyhocks.

I mostly ignore mildew on perennials and if it is really bad I just cut them back and let new foliage emerge all together. 

Common annuals that are prone to infection are zinnias, snapdragons, bachelor buttons, cosmos, sunflowers and dahlias.

Here I may just throw in the towel if it’s really bad.  Vegetables are not immune to the problem and your harvest will be adversely impacted; watch your beans, cucumbers, squash and peas. 

Trees and shrubs are a bit more valuable, however. Watch dogwoods, lilacs, crape myrtles, azaleas and rhododendrons. The last two have a greater payoff and greater investment so I may resort to using neem oil to slow the spread.

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 like peas, pole beans and vine 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; mulch around plants in the spring to prevent soil from splashing up during heavy rains.  And, 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 on the healthy foliage in order to prevent spores from germinating on healthy foliage.  I prefer organic controls so when push comes to shove I will break out the neem oil for its fungicidal properties; if you are chemically inclined look for a fungicide containing benomyl as the active ingredient.  Reapply application about every 10 days or after rain. 

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 and you have an automatic sprinkler you may want to rethink your fertilization and watering schedule…its been a wet, hot and humid summer so far, your lawn will suffer the more you feed it and water!