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Daren Mueller, Plant Pathology, (515) 294-1741,
Del Marks, Continuing Education and Communication Services, (515) 294-9807,

Garden column for the week of Nov. 12, 2004

Powdery Mildew Fungi Widespread in 2004

By Daren Mueller
Plant Pathologist
Iowa State University Extension

Most people have seen powdery mildew, especially during a wet year like 2004. It is that white powdery stuff that gets on your roses, turf, phlox, lilacs, cherry trees, flowering crabapples and most of the other plants in your backyard or garden.

For those lucky folks who have never seen powdery mildew, it is a fungus causing a powdery growth on leaf, shoot or flower surfaces. The powder is millions of tiny spores, which the fungus uses to reproduce. In severe cases, it may cause leaf curling or twisting, or even premature defoliation. Thankfully, it usually is not a fatal disease, but infected plants can quickly become unsightly.

Most powdery mildew fungi are considered to be very host specific. In other words, they will attack only one type of plant. On the other hand, some powdery mildew fungi, such as Erysiphe polygoni, have been reported on over 350 host species. Typically, when you see powdery mildew in your garden or landscape, different plant species near the infected plant should not be at risk. For example, the fungus that causes powdery mildew on phlox cannot infect roses, and the fungus that causes powdery mildew on turf cannot infect lilac, and so on.

Here's a personal example. There was a patch of turf (and weeds) in a shady spot behind my house and along my fence line that I could not reach with the lawn mower and was too lazy to weed-whack. This patch of turf had powdery mildew almost all summer. Powdery mildew on turf is caused by the fungus Blumeria graminis. Fifteen feet away from this infected patch of turf are two of my wife's favorite Griffith Buck roses, which happen to be quite susceptible to powdery mildew. Thankfully, Blumeria graminis spores from the turf could land on these rose bushes and none of them will cause an infection. That is because powdery mildew on rose is caused by a different powdery mildew fungus, Sphaerotheca pannosa.

Want more good news? The powdery mildews can be controlled using similar strategies. Here are some suggestions for control of powdery mildew. First, purchase only mildew-free plants of resistant varieties and species, if available, from a reputable nursery, greenhouse or garden center. It's also helpful to plant in well-prepared and well-drained soil where plants will obtain adequate sunlight. Also, try to space plants for good air circulation. If you are planting highly susceptible plants such as phlox and rose, avoid damp and shady locations. Also, do not overhead water the foliage, especially in late afternoon or evening. Prune out diseased plant parts during the normal pruning period and remove and destroy all diseased material. If infection is too severe, fungicides may be used. Be sure to cover both surfaces of all leaves with the spray. For more information on resistance of certain plants and on proper use of fungicides consult your local extension office.


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