High humidity from recent rains can lead to problems in the lawn

This past week, there was a little break in the extremely hot temperatures that we have had. The rains that came cooled the temperature, but the humidity levels were still high.

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These high humidity levels can lead to some disease problems.

Home lawns are commonly infected by leaf diseases, including gray leaf spot, dollar spot and rust. Most of the time, these problems go unnoticed by the homeowner and do not cause significant damage to the lawn. However, when conditions are favorable for disease development, serious damage can occur.

To control these leaf-spot diseases, a basic understanding of the factors affecting disease development is needed, as is an understanding of proper lawn care.

Gray leaf spot mainly occurs on St. Augustine grass. It is caused by the fungus Pyricularia grisea . Gray leaf spot first appears as small, brown spots on the leaves and stems. These spots quickly enlarge to about 1/4 inch in length and become bluish-gray in color and oval or elongated in shape. The mature lesions are tan to gray in color and have depressed centers with irregular margins that are purple to brown. A yellow border on the lesions can also occur. The leaves will then turn yellow and brown because the lesions will kill the leaves, 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 above 65. It is also found in lawns with high nitrogen levels that are stressed by various factors, including drought and soil compaction. This disease is most severe during extended hot, rainy and humid periods and more prevalent in areas that are shady or have little air movement.

This problem can be reduced by proper management of the turfgrass. The weather can't be controlled, but the way the grass is managed can reduce the severity of the disease.

The first step is to control the amount of nitrogen applied to the grass. High levels of nitrogen can increase the chance of this disease, especially in midsummer. When more than one pound of actual nitrogen is applied at one time to the grass, the chance of disease problems goes up.

Also, a fertilizer that contains plenty of potassium will benefit the grass. Potassium is the last number on the fertilizer bag.

Proper irrigation will help. Deep, infrequent watering will reduce the amount of moisture on the leaf and will allow the grass to dry out between irrigations. The best time to water is early morning. Increasing air movement and light intensity by removing some tree limbs will also help.

If this disease requires treatment, there are few fungicides available to homeowners. Fungicides containing propiconazole or thiophanate-methyl are available for use in the home lawn. Always apply all chemicals according to directions on the label.

Dollar spot is caused by the fungus Sclerotinia homoeocarpa . It causes straw-colored spots about the size of a silver dollar to appear on closely mowed turf. The grass in infected areas might die, and the spots can merge to form larger, irregular patches. Leaf blades have light tan spots with reddish-brown margins that develop across the leaves. Early in the morning with dew on the ground, a cobweb-like growth of the fungus can be seen.

Dollar spot most commonly occurs on Bermuda and zoysia grasses. Dollar spot is most active from late spring through fall. The fungus develops during humid weather, when daytime temperatures are warm and nights are fairly cool. However, this disease can be seen in the middle of the summer when the grass is drought stressed and has low nitrogen levels.

Rust fungi (Puccinia species) can infect most types of grasses but occur most commonly in zoysia grass. Rust diseases develop more frequently on lawns that are stressed by drought, low nitrogen and shade. Disease first appears on leaves as tiny orange to reddish-brown flecks that enlarge to form raised pustules. Lawns that are heavily infected become thin and weak with an orange or reddish color. This disease can be a problem in the spring but is most active in this area in the fall.

Both dollar spot and rust can be controlled by meeting the water needs of the grass with deep irrigations. Also, a light application of nitrogen can help the grass overcome the disease. Most of the time these diseases don't need to be treated with a fungicide.

Charles Phillips is a retired Columbia County Extension Service agent and operates Hort Consulting. He can be reached at cphillipshort@comcast.net.