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Diseases emerging in plants, grasses

Posted: June 2, 2012 - 11:08pm

We got some much-needed rain this past week and not much run-off. This slow, soaking rain will help our plants because most of the water soaked into the soil.

As much as the rain has helped, there are some plant diseases active now. One of the diseases affects on trees and shrubs. Another is active in lawns.

The disease that is attacking our trees and shrubs is powdery mildew. It can be active from May until October, but is most prevalent in May and June. Powdery mildew likes sunny, dry days followed by cool, moist nights.

The symptoms of powdery mildew are a whitish, powdery growth on the top of leaves, and on stems and flowers. This growth will attack the new leaves and shoots as they are emerging. It causes 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.

There are numerous plants that can be affected by powdery mildew. African violets and grape ivy are houseplants that can be affected by powdery mildew. One of the annual flowers that powdery mildew can be a problem on is zinnias. Perennials such as gerbera daisy, phlox and verbena can be affected.

To control powdery mildew, clean up the 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 fungicides.

These products need to be applied at the first sign of the disease, and then follow the instructions on the label on how often they need to be applied. 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 can be used as well, such as sulfur and neem oil.

The disease that I am seeing in turf is large patch, which is a disease that occurs in the spring and fall of the year. It likes to grow when the daytime temperature is higher than 80 degrees and the nighttime temperature is above 55 degrees.

At these times of the year, the grasses are not growing as actively as during the summer and are more stressed.

The best way to reduce the chance of a grass getting a disease is to properly manage the turf.

Too much water or improper irrigation are two of the causes of large patch.

The turf disease is caused by the fungus Rhizoctonia, which likes excess water. Our turf grasses need an inch of water per week to grow properly. To encourage a deep root system, I recommend that all of this water be applied at one time. In order for Rhizoctonia to grow, the fungus needs moisture for 16 hours.

This is why it is recommended that turf grass be watered early in the morning. When turf grass is watered late in the afternoon, the grass goes into the night wet. The grass will dry the next morning around 10. This is approaching the 16 hours needed for disease development.

Also, watering grass every other day doesn’t allow the grass to fully dry out before it is wet again. This light, frequent irrigation practice encourages a shallow root system which can stress the grass, making it more susceptible to diseases.

If grass has large patch, let the grass show signs of drought before watering again.

A fungicide treatment will be needed as well to control this disease. After treatment, the size of the patch should stop increasing. If the size of the patch continues to increase, another treatment is needed.

Charles Phillips is a retired Columbia County Extension Service agent and operates Hort Consulting. Reach him at cphillipshort@comcast.net or at (706) 836-2152.

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