Downy mildew is sometimes confused with another fungal disease called powdery mildew. The main difference in the two is the location of the symptoms.

Powdery mildew appears as a grayish, fine wood ash colored covering on the TOP of the leaf.

Downy mildew appears on the BOTTOM of the leaf where it forms irregular yellow to dark brown lesions. In the presence of high humidity, it will produce spores of white, tan, gray or purple which gives it a "downy" appearance.

There are several species of fungi that cause downy mildew and there appears to be a different species for each host plant species. Also, some species of this organism can persist in the soil for long periods of time.

In recent years, outbreaks of downy mildew have been found in impatiens, coleus, basil and several other ornamental plants not previously known to be bothered.

The symptoms may not show up even though the plant is infected. Once the environmental conditions are correct i.e. moisture and heat, the disease can take off suddenly and spread rapidly.

Perhaps the key to preventing this disease is to be sure to purchase your plants from reputable sources. In recent years, some greenhouse operations have been caught off guard since downy mildew has not traditionally been a serious problem.

Over head irrigation and the splashing water it creates is one way the disease may be spread.

Remove nearby weeds that may harbor downy mildew spores.

Since this is a relatively new disease problem for ornamental landscape plants (it has been around in vegetable crops for a long time), most of the treatment options are relatively new too. Most of the work by plant pathology departments at universities is being directed at preventing and treating downy mildew at the greenhouse before the plants are sold to gardeners.

Check with your local Cooperative Extension Service or land grant university if you suspect that your ornamental plants are infected with downy mildew. They should be able to tell you of the current recommendations for your area.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

 
 

 

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