Brentwood lawn care: Lawn diseases and pests

Lawns in the Nashville Tennessee area are commonly infected by leaf diseases including dollar spot, rust, gray leaf spot, powdery mildew, anthracnose and Helmintho-sporium leaf spot. These problems often go unnoticed by the homeowner and do not always cause significant damage to the lawn. However, when conditions are favorable for disease development, serious damage can occur. Effective control programs for most diseases must include proper cultural care of the lawn, and a basic understanding of the factors affecting lawn disease development.

Dollar spot

Dollar spot is caused by the fungus Sclerotinia homoeocarpa. It causes straw-colored spots about the size of a silver dollar (2 to 6 inches diameter) to appear on closely mowed turf. Grass in affected areas may die and the spots may merge to form larger, irregular patches. In coarse textured grass that is cut high, the dead spots are larger and more diffuse. Leaf blades have light tan spots with reddish-brown margins that develop across the leaves. Early in the morning you may be able to see a cobweb-like growth of the fungus over the infected area.

Dollar spot most commonly occurs on bermudagrass, zoysia grass, bentgrass, fescue, and ryegrass. Dollar spot is most active from late spring through fall. The fungus develops during humid weather, when daytime temperatures are warm (59 to 86 °F) and nights are fairly cool. These conditions result in heavy dew forming on the grass.

Prevention and treatment: Adequate fertilization will help the lawn overcome this disease. Prevent thatch buildup and remove excess thatch. Avoid drought stress by watering the lawn deeply, and thoroughly when needed, timing irrigation for early morning. Remove morning dew if possible by mowing or irrigating the lawn. Fungicide applications may be needed during moist weather in the spring and fall, when day temperatures are between 70 to 80 °F. For the home lawn, fungicides can be applied that contain either thiophanate-methyl, propiconazole, fenarimol, thiram, mancozeb, triadimefon or myclobutanil. Always apply all chemicals according to directions on the label.

Rust

Rust fungi (Pucciniaspecies) can infect most types of grasses, but occur most commonly on bluegrass, fescue, ryegrass, and zoysiagrass in Nashville, Tennessee. Rust diseases are favored by warm, humid conditions and develop most 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.

Prevention and treatment: Rust is most often a problem on lawns with too much shade. Avoid stressing the lawn. Maintain adequate nitrogen levels and irrigate during drought conditions. Mow the grass regularly, and remove clippings, being sure not to cut the lawn too low. Do not overwater. Fungicides are usually not necessary in actively growing lawns.

Gray leaf spot

Gray leaf spot is caused by the fungus Pyricularia grisea, and causes severe damage primarily on St. Augustinegrass (aka "Charlestongrass") and ryegrass. Tall fescue may also be damaged by gray leaf spot. Leaf spots on grass blades are tan to gray with purple to brown margins. When the disease is severe, the entire planting may appear a brownish color or scorched, similar to damage caused by drought. This disease most commonly occurs during warm, rainy periods in the summer.

Prevention and treatment: Plant resistant cultivars. Avoid excessive applications of nitrogen, especially during warm, humid weather. Irrigate deeply only when needed and in the early morning. Avoid stresses induced by herbicides, drought, or compacted soil. Improve air movement and light intensity, by pruning trees and undergrowth. If chemical control is necessary, fungicides containing propiconazole or thiophanate-methyl are available for use in the home lawn. Always apply all chemicals according to directions on the label.

Powdery mildew

This disease is caused by the fungus, Erysiphe graminis, and appears as a grayish-white powdery growth on the surfaces of the grass blades. Leaves may turn yellow and gradually die. It is an important disease on bluegrass, fescue and bermuda, especially in areas of shade or little air movement.

Prevention and treatment: Increase sunlight penetration to densely shaded areas, or select a more shade tolerant cultivar. A balanced fertilization program is important for the lawn. Mow the lawn often and at the recommended height. Increasing air circulation will also help to control powdery mildew. Fungicides containing either triadimefon, propiconazole, fenarimol or myclobutanil can be used on the home lawn in areas where environmental conditions cannot be modified. Always apply all chemicals according to directions on the label.

Anthracnose

This leaf disease causes the most severe damage on bentgrass and annual bluegrass and is caused by the fungus, Colletotrichum graminicola. It occurs during the peak of hot weather when cool-season grasses are barely growing. It also infects centipedegrass during very rainy periods in the spring and summertime. In bentgrass, anthracnose causes a stem rot at the base of the plant, which can cause the lawn to turn
yellow and die.

Prevention and treatment: Maintain the lawn in as healthy a condition as possible with a balanced fertilization program. Reduce thatch and soil compaction. Chemical control is generally not needed on centipedegrass due to its quick recuperative potential. Fungicides containing either propiconazole, triadimefon or thiophanate-methyl are available for control, when other measures fail. Always apply all chemicals according to label directions.

Helminthosporium diseases

The diseases in this group are commonly referred to as melting out, leaf spot, net-blotch, and crown and root rot. These diseases are caused by the fungi Bipolaris, Drechsleraand Exserohilum, but were previously classified as Helminthosporium.

Bipolarisspecies cause leaf, crown and root diseases of fescue, bentgrass, ryegrass, bluegrass, bermudagrass and zoysiagrass during warm, wet weather in midsummer. The diseases generally start as leaf spots and may progress to crown and root rots. Exserohilumhas been reported to cause a leaf spot of St. Augustinegrass and bermudagrass. Centipede is rarely affected adversely by these fungi.

Prevention and treatment: Avoid high nitrogen fertilization and watering practices that provide long periods of wet or humid conditions. Frequent mowing at proper heights will provide better drying conditions in the turf and help to reduce the leaf spot phases of these diseases. Provide adequate water with infrequent but deep irrigation to help avoid crown and root rot phases. Fungicides for control of Helminthosporium diseases include: mancozeb, myclobutanil, propiconazole or thiophanate-methyl. Apply all chemicals according to directions on the label.

Source: Diseases of Turfgrasses in the Southeast, Martin, B., EB 146, 1994.