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White spots on leaves could be powdery mildew fungus

QUESTION: “I just planted some salvia in my garden and noticed some white spots on the leaves. Any suggestions?” – Mazzocco

ANSWER: The white spots could be from mineral deposits from watering, fertilizer residue or a fungus known as powdery mildew.

You will need to determine what is causing them before you can take any action. Fortunately, the first two wouldn’t require any action, but if they are caused by a fungus, you might want to apply a fungicide.

Powdery mildew is one of the most common landscape fungus problems. The first sign is often curled and twisted leaves before the white or grey powder is apparent. When the mildew appears, you might see the grey patches gradually enlarge and spread until they cover large areas on one or both sides of the leaf. If you are seeing that, then you have powdery mildew.

Severe infection can result in yellow or dry brown leaves and disfigured shoots. Although powdery mildew is not usually fatal, it can bring on early defoliation and unsightly plants.

Powdery mildew fungi are most likely to produce airborne spores and infect plants when temperatures are moderate and will not be present during hot summer days. When plants are overcrowded and shaded, creating a cool, humid environment, they are at greatest risk for powdery mildew infection.

Pick up any fallen infected leaves and pick off any severely damaged or dead leaves and destroy them, preferably by burning. If it’s any consolation, powdery mildew generally won’t adversely affect a plant’s overall health, so if it appears to be quite mild, or if you can live with unsightly plants, you could probably ignore it, other than destroying the infected leaves.

However, you can use a fungicide to treat the problem. For best results, begin to spray the plants as soon as the mildew is detected. Repeat the spraying as needed, which is usually during the cooler weather seasons. Not all fungicides are suitable for all plants, so be sure to check on the recommended usage before you spray. Read the package directions!

Here are two very useful sites with photos to help you identify powdery mildew and helpful instructions on how to treat the problem and create an environment that will discourage the fungus from returning. One article is posted by Cornell University’s Plant Disease Diagnostic Clinic and the other is from Ohio State University’s Extension Service.

http://plantclinic.cornell.edu/FactSheets/powdery/powdery.htm

http://ohioline.osu.edu/hyg-fact/3000/3047.html

You can click on direct links to those sites when you find this column at my Web site, www.landsteward.org

QUESTION: “I have a very steep slope in my back yard leading from a patio down to our lake front. There is a retaining wall at the top and a retaining wall at the bottom. I would guess that the incline is about 45 degrees in some areas. There are trees on one side leaving part of the slope in deep shade, part is dappled shade and a few areas get a good bit of sun, but the bulk of the area is pretty shady.

“Since it is so steep, we have concerns about erosion control. It's steep enough to be difficult to get out there and weed or do any other normal garden "tending", so it has to be very self sufficient. Can you give any advice on types of plants to use?” – Margaret Wilson

ANSWER: Anything you try to plant will most likely take several years to fill in at best. Vinca, pachysandra, purple wintercreeper and liriope will grow well on sloped shaded areas. Ivy might grow well, too, but may be a bit too invasive for your landscape.

If there is a problem with erosion now, I would recommend putting down erosion netting first and then planting. You can plant right through the netting, just cut an X out where you want to insert each plant. Erosion netting decomposes over 5 or so years allowing the ground cover time to take hold and spread. In the meantime, it does a good job at keeping weeds at bay. I hope this gives you some ideas to build on.

The Plant Man is here to help. Send your questions about trees, shrubs and landscaping to steve@landsteward.org and for resources and additional information, including archived columns, visit www.landsteward.org