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Gray leaf spot, webworms emerge

Posted: July 22, 2012 - 12:13am

Area residents have finally gotten a little relief from the drought. The rains over the past few weeks have helped plants and turf recover, but the moisture and humidity also have caused disease problems.

One of these is gray leaf spot on St. Augustine grass. Webworms have also hit the low country of South Carolina and will soon be in our area.

Gray leaf spot first appears as small, brown spots on the leaves and stems. These spots quickly enlarge to about ¼-inch in length, become bluish-gray in color and oval or elongated in shape. The mature lesions, tan to gray in color, have depressed centers with irregular margins that are a darker color. The leaves then will turn yellow and brown as the lesions kill the leaf, which will drop off the plant, and the grass will thin out.

Gray leaf spot is most prevalent when the daytime temperatures are higher than 80 degrees and nighttime temperatures are higher than 65 degrees. It’s also found in lawns with high nitrogen levels that are stressed by factors such as drought and soil compaction. This disease is most severe during extended hot, rainy and humid periods. Also, this disease is more prevalent in areas that are shady or have reduced air movement.

The disease can be reduced by proper management of the turfgrass. High levels of nitrogen can increase the chance of this disease, especially in mid-summer and when more than one pound of nitrogen is applied to grass.

A fertilizer containing plenty of potassium will benefit the grass.

Deep, infrequent watering will allow the grass to dry out between irrigations. The best time to water is early in the morning.

Another factor that will aid the plant is to increase air movement and light intensity. This can be done by removing some limbs from trees.

There are few fungicides available to control this disease. Fungicides containing propiconazole or thiophanate-methyl are available for use in the home lawn. All chemicals must be applied according to directions on the label.

Fall webworms can have up to four generations in a year, with most of the activity occurring in late summer and early fall. The larvae or caterpillars are hairy with distinct paired bark spots on each segment of the back. They can be variable in color, but the primary color in this area will be whitish-yellow to pale green with white hairs.

The caterpillar, which is about 1-inch long when mature, will twitch and jump about when disturbed to deter potential predators. The caterpillars turn into a satiny white moth, with a wingspan of 1½ inches.

This type of webworm will spend the winter as a pupa in a flimsy, light-colored cocoon. These cocoons can be found in protected areas such as bark furrows, crevices along the sides of buildings, under mulch and debris in the area. The adult moths will emerge in June or July and begin laying eggs. The eggs are laid on the underside of leaves. When they hatch, the larvae feed together, skeletonize the leaf and then start to form their web.

The larvae do all of their feeding inside of a tent. When they run out of food, they make the tent larger. If you look into the webworm’s tent, you will find leaf fragments, droppings and cast skins. When full grown, the caterpillars leave the tent to find a protected area to spend winter.

The best way to control webworms is to remove the web from the tree, which can be done while the webs are forming. A pole pruner can be used to reach and remove the portion of the limb containing the web.

Because fall webworms don’t feed outside of the web, all of the caterpillars will be removed when the web is removed. There are insecticides that can be used but can be hard to apply, because most of the webs are high in trees.

Webs shouldn’t be burned. This will cause more damage to the tree than to the caterpillars.

Both of these problems can be controlled easier if they are treated early.

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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