Conditions make for mold, mildew

The extended spring, with the cool temperatures and excess moisture, has been great. The lake is full, and we have good soil moisture.

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These conditions caused a number of problems in our landscape, however.

In previous columns, I covered problems such as fire blight and insect galls. There are other problems that recently started showing up.

One of these is found on grass, and the others are found on trees and shrubs. The good news is that these problems are not serious. The problems that I am talking about are slime mold on grass, oak leaf blister and powdery mildew.

Slime mold is a black, grayish substance that grows on grass and has tiny bumps in the powder. This is a fungus that likes cool, wet conditions. Slime mold can be caused by fungi in the genus Physarum and Fuligo. There are a number of species that will cause these problems. These patches can be a few inches wide to as much as 4 feet wide.

These fungi do not cause any harm to the grass. They will grow on the grass and produce spores for the next generation. Slime mold does tend to occur in the same areas every year. It will spread to new areas, and it likes grass that has some thatch buildup, because it will feed on the thatch.

There are a couple of ways to get rid of slime mold. The first is to wash it off with a water hose. The other option is to mow your grass. This will remove the fungus.

The second problem that I have seen many times this spring is oak leaf blister. This is caused by a fungus. Again, this fungus likes cool, wet conditions. Leaves become infected when buds are beginning to open in the spring. The pathogen can survive the winter on plant twigs and bud scales.

Spores causing oak leaf blister are spread by wind and rain. The symptoms of this disease are very easy to recognize. The leaves begin to show light green/yellow, blisterlike areas on the upper surface that can be as large as a half-inch in diameter. The lower surface has gray depressions that correspond to the raised blisters. The depressions look like a bowl.

As the disease progresses, the blisters turn brown and the leaf will curl as the blisters get larger and grow together. Oak leaf blister can cause the leaves to drop prematurely.

Trees that are infected with oak leaf blister are not severely damaged, but the appearance of the tree may be unsightly.

All of our oak species are susceptible to this disease. Fungicide treatments don't help. You would have to spray the trees as the leaves emerge. You can help reduce the incidence of this problem by raking up infected leaves and discarding them.

The newest problem I am seeing in landscapes is powdery mildew. Powdery mildew can be active from May until October, but is most prevalent in May and June.

Normally when we hear the word "mildew," we think of high humidity and wet conditions. Powdery mildew prefers the opposite conditions. It likes sunny, dry days followed by cool, moist nights. This past week, we started having cool nights followed by hot days and drier conditions.

The symptoms of powdery mildew are a whitish, powdery growth on the top of leaves, on stems and flowers. This growth will attack the new leaves and shoots as they are emerging. The disease will cause these leaves and shoots to be distorted.

Once a leaf or shoot has been infected, the fungus penetrates the outermost layer of plant cells to obtain its nutrients. This feeding is the cause of the leaf distortion. The white powdery substance is the mycelium, or reproductive part of the fungus. The mycelium can be dispersed by wind or water to plants of the same species.

To control powdery mildew, you can clean up leaves and plant parts that have been infected with this fungus. Another option is to plant varieties of plants that are resistant to powdery mildew. The last option is chemical control, using a fungicide to help control powdery mildew. Apply the fungicide at the first sign of the disease, and then follow the instructions on the label on how often you need to apply. The fungicide will protect the foliage that hasn't been infected by the fungus.

Some of the fungicides that will control powdery mildew are chlorothalonil (daconil), myclobutanil, and triforine. There are some organic type products that you can use as well. These are sulfur, neem oil and jojoba oil.

When you use any of these products, you should read the labels and follow the directions.

Columbia County Extension Agent Charles Phillips can be reached at (706) 868-3413 or by e-mail at charlesp@uga.edu. The Extension Web address is www.ugaextension.com/columbia.