Numerous disease problems occur on turfgrass in Kentucky and these frequently cause extensive damage. In many cases, a disease is blamed for poor quality turf when, in reality, it may be only a contributing factor or not involved at all. Frequently, dead and dying grass is caused by improper fertilization, chemical burn, mower problems, dog or insect injury, dry or wet spots, thatch, competition from other plants, or from any other improper management. Accurate diagnosis of the problem is essential for proper control.
Two types of pathogens (fungi and nematodes) are found in turf in Kentucky. Observation of symptoms is an important aid in determining which pathogen is causing a disease. The following information will explain the identification and biology of some common turf diseases. Helminthosporium Disease
Helminthosporium leaf spot (ex. Helminthosporium Leaf Spot ) is a common disease problem of Kentucky bluegrass and is often referred to as “melting-out.” (Another example)Other Helminthosporium leaf spots are important on fescues and bermudagrass. From a distance, leaf spot-affected areas appear chlorotic or yellowed. Individual spots on the leaves have dark margins with tan centers. The spotting is most noticeable in spring and early summer. Infection in the crown of the plant during the summer can lead to the death of plants (thus “melting-out”). Cool, wet weather during spring followed by drought during summer accentuates the damage from this disease.
Dollar spot affects a wide variety of grasses, including Kentucky bluegrass, bermuda, perennial ryegrass, zoysia, tall fescue, and bentgrasses. The fungus is active throughout the growing season, especially when there is low soil moisture and an excess of dew or fog. It is most prevalent in the spring. The disease is characterized by small white patches, one to three inches in diameter (ex. Dollar Spot). A large number of spots can come together and form larger dead areas. Leaf spots are usually found along the edges of the grass blade and may come together across the blade, causing the tip to die. Individual leaf spots are tan with reddish margins.
Pythium blight is caused by a number of species of the fungus Pythium. The fungus primarily attacks perennial ryegrass and bentgrass although other grasses can be affected. Conditions that favor Pythium blight include abundant moisture and poor air circulation. The disease is most active in hot, humid weather when the night temperature does not go below 70°F. The blight appears first as small spots a few inches in diameter. Diseased leaves are at first water-soaked, soft, and slimy, and may mat together (ex. Pythium Blight). Dense, cottony fungal growths often are apparent in affected areas during a heavy dew. The leaves soon shrivel and the color of the patch soon fades to light brown as dew dries. The shape of the diseased area may be streaked following the drainage flow of water over the turf.
Brown patch is a common fungal disease of fescues, perennial ryegrass and bentgrass. The disease develops most readily when daytime highs exceed 80°F and nighttime lows are in the mid- 60’s°F or higher. Brown patch is one of the more common turf diseases, especially in tall fescue (ex. Brown Patch). In addition to ideal temperatures and humid weather, heavy applications of nitrogen fertilizer favor disease development. Brown patch is characterized by nearly circular areas of dead leaves that may be a few inches to several feet in diameter. On closely mown turf, the edges of the dead area may have a gray, smoky color, particularly in early morning. Affected areas are generally tan or brownish in bent and ryegrass. Affected fescues usually have straw-colored leaves.
This disease affects Kentucky bluegrass and annual bluegrass. Circular to irregular patches of dead turf up to 1-2 feet in diameter develop during hot weather in mid- to late-summer (Figure 46). Below ground, roots and crowns of affected plants are brown and decayed, a result of fungal colonization. A tuft of healthy, green grass is sometimes evident in the center of affected patches, giving them a characteristic “donut” appearance.
Necrotic ring spot is another disease of bluegrasses with symptoms similar to summer patch. In contrast to summer patch, symptoms of necrotic ring spot can develop following cool, wet weather in late spring or mid-autumn. Necrotic ring spot is less common in Kentucky than summer patch.
Figure 46. Summer Patch
Rust is sometimes a problem on Kentucky bluegrass, fescue, zoysia, perennial ryegrass, and bermuda grasses. Rust infection results from rust spores which are blown to the plant from distant areas or from nearby alternate hosts. Large numbers of spores are produced in the leaf spot (pustule). These spores are then the source of new infections. The disease is most frequently found during cool, humid weather during autumn. Grass varieties differ in susceptibility to rust.
Red thread is seen as irregularly shaped patches of blighted turfgrass, ranging from a few inches to a few feet in diameter. Often, as diseased leaves turn brown, pink or reddish fungal growth can be observed on the leaf surface or emerging from the cut ends of leaves. This disease affects most of the common grasses grown in Kentucky and is often found during spring and early summer. The disease is favored by conditions of low nitrogen fertility.
Fairy ring can occur in any turf. The ring appears as a circular discoloration of grass from several inches to many yards in diameter. Mushrooms (toadstools) may appear at the edge of the ring during warm, moist periods (ex. Mushroom Fairy Ring). The ring of grass is generally a darker green than the grass inside and outside the ring. During periods of moisture stress, the grass inside the ring may die. This general decline of grass inside the ring adds to the unsightliness of the fairy ring problem. Fairy rings gradually increase in size.
Slime molds are commonly found on lawns in warm, moist weather (ex. Slime Mold). This fungal growth on grass leaves may be either a small, crust-like, light to dark mass with a sooty appearance, or a tan to orange shapeless mass. The fungus causing this unsightly problem does not infect the grass blade; it simply uses it for support. The only effect it has on the plant is to temporarily reduce food production by the grass leaf as a result of shading.
Nematodes weaken and reduce the vigor of turfgrass by restricting the development of the root system. The symptoms of nematode injury may be confused with nutritional problems, insufficient water, hardpan, or any factor that restricts root development. Symptoms commonly associated with nematode injury include thinned or completely killed areas, pale green to chlorotic color, excessive wilting during drought stress, poor response to fertilization, and a greater weed problem due to sparse grass. The intensity of the symptoms will vary with the grass variety, the kinds of nematodes present, the nematode population level, and the fertilization-watering program being practiced. The most reliable method for determining whether a nematode problem exists is by a soil assay. Nematode damage to turfgrass is uncommon in Kentucky.
Lawn ecosystems often include a variety of insects, some of which are direct pests of grass, or nuisances and pests to humans and pets. Some may be predators or parasites of other insects, or harmless scavengers. Through complex interactions between the insects and other factors, the lawn ecosystem becomes more or less balanced. If we are not satisfied with the balance, we may use maintenance practices to improve our lawns. However, the solution may trade one problem for another. For instance, fertilization to increase grass lushness may favor the development of certain insect and disease problems. Insecticidal control for one kind of insect may kill predators or alter competition, allowing a different insect pest to flourish. Often, the side effects of management practices cannot be precisely predicted, so lawn situations need to be monitored over time and maintenance practices modified, if necessary. Some of these interactions and problems are demonstrated in the case of white grubs as lawn pests in Kentucky.
White grubs are the larval stages of scarab beetles such as masked chafers, rose chafer, May beetles, green June beetle, and Japanese beetle. White grubs look more or less alike (ex. White Grub). They have brown distinct heads and thoracic legs, and the body is whitish, fat, and usually curled into a C-shape. Size varies from 1/8 to 1-1/2 inches long depending on the age and species. The grubs occur in large patches of sod an inch or so below the soil line where they consume the anchoring roots of grass. During dry weather, the infested sod may die for lack of water.
Soil insecticides for white grub control should be applied in August before the grubs cause serious damage. Most instances of control failures are a result of poor timing or techniques of insecticide applications.
Although masked chafer and Japanese beetle white grubs are the most widespread and prevalent lawn problem in Kentucky, other insects occasionally cause serious damage.
Small scarab beetles, Ataenius spp., that normally develop in cow dung have recently become problems in certain kinds of sod. Bluegrass and bentgrass turf with thick thatch are the most frequently attacked. The damage is like that of typical white grubs, except Ataenius white grubs are only about 1/5-inch long when full grown, and there may be as many as 500 grubs per square foot of sod. There are two generations per year, with the first generation grubs being most numerous in June, and the second generation in August.
Both the larvae and adults of billbugs attack grasses. The adult is a snout beetle that eats grass blades and burrows into stems near the crown. The larvae are about 2/5-inch long, resemble legless white grubs and feed on the crown roots. The pattern of billbug damage resembles that of sod webworm damage, but the dead grass is easily pulled loose from the soil.
Sod webworms graze baseball-sized patches of grass that turn brown and die. Patches of grass that are clipped off at the soil surface may be numerous and run together to form large dead areas. Dirty silk tubes containing the inch-long caterpillar or pupa can usually be found in the thatch of killed spots. The adult stage of the pest is a small buff moth that is often seen fluttering over lawns at dusk and at night around lighted doorways about two weeks before larvae become numerous. There are up to three generations per year (Figure 50).
Figure 50.-Sod Webworms
Armyworms, including the true armyworm and the fall armyworm, are characteristic caterpillars about 1-1/2 inches long when full grown. They vary in intensity from year to year, but during outbreaks they may move across an area in army fashion completely stripping grasses in their path. Fescue is more often attacked than bluegrass. These insects are also important pests of grain crops.
Various species of cutworms occur in turf and some are hard to distinguish from armyworms based on body characteristics. However, they never occur in large numbers as do armyworms.
Greenbug, a small species of dark green aphid, is a relatively recent bluegrass pest that is not as yet widely distributed in Kentucky. However, this may change if it becomes transported on infested nursery sod. Infestations often begin next to trees and structures that partially shade the grass. Infested grass turns yellow and dies, and large numbers of aphids can be seen in the affected area. The aphids remain active into fall when most other foliar pest insects have abated.
Insects and their relatives that are annoyances to humans may also require control. These pests include ticks, chiggers, clover mites, millipedes, sowbugs, ants, ground nesting wasps, fleas, mosquitoes, and gnats.
Skunks damage turf when they discover abundant white grub populations. Skunks dig through the sod and feed on the white grubs, thereby uprooting the sod and aggravating the damage already begun by the grubs. Skunks also may spray a disagreeable smelling substance on unwary people or pets who disturb them.
Birds, especially crows, starlings, and grackles, commonly tear up infested turf in search of grubs. Flocks of blackbirds frequenting a turf site, or holes left in the turf by their beaks, may indicate a grub problem.
Moles feed primarily on earthworms, but they may also feed on white grubs, wireworms, beetles, and many other invertebrates. They do not feed on plant roots or other underground plant growth. However, as they tunnel along in their surface runs, moles damage turf roots and may destroy newly seeded lawns. In established turf, the mower may skin the tops of the runs and dull the mower blade as well as create gaps in the sod. Moles also tunnel deep, throwing the excavated soil out of surface openings, thus forming mole hills.