Department of Crop and Soil Sciences - Cooperative Extension

Revised by Peter Landschoot,
associate professor of turfgrass science,
from Extension Circular 339,
Managing Turfgrass Diseases
by Patricia L. Sanders

Contents

Introduction

Disease in turfgrasses, as in other plants, develops from an interaction among a susceptible plant, a disease-producing organism (pathogen), and an environment favorable for disease development. Susceptible grasses and pathogens (usually fungi) are present in all lawns. In most cases, the pathogens exist in a dormant or saprophytic (feeding on dead or decaying substances) state and do not attack living plants. Diseases occur when environmental conditions (weather, management, and / or site conditions) become favorable for the build up of pathogen populations and / or cause an increase in the susceptibility of the plant. When this happens, turfgrass loss can occur.

There are a group of turfgrass diseases for nearly every environmental condition that may arise. Some diseases develop during the frigid months of winter under a blanket of snow, whereas others occur only during the hottest and most humid conditions of summer. Some diseases appear more frequently in wet soils and following applications of high rates of fertilizer, while others are triggered by drought and low fertility.

Disease Diagnosis

The first step in turfgrass disease management is identifying the true nature of the problem. Diseases are only one cause of turf loss, and disease control measures will do nothing to alleviate damage from other causes such as insects or drought stress. It is therefore essential to determine whether the problem is disease, and if so, which disease. Disease management strategies that are effective against one disease may have no effect on, or may even worsen, another disease.

The three disease factors (susceptible grass, pathogen, and environoment) provide the sources of information for diagnoses. Symptoms are the expression of the susceptible grass to the disease and can take on a variety of sizes, shapes, and colors. Two kinds of symtoms should be examined in diseased turfgrass areas: symptoms on individual plants and symptoms on the turf stand. On individual plants, symtoms may appear as small, circular, tan-colored lesions surrounded by brown or purple borders (leaf spotting); as yellow, red, or tan blotches over most or all of the leaf blade (blighting); stunting; wilting; or as a brown or black rot on the crowns and roots. On stands, symptoms may take the form of circular patches, rings, spots, or irregular areas.

The first step in turfgrass disease management is to identify the true nature of the problem.

In some cases, the pathogen that causes a disease can be observed. Although most turfgrass pathogens are only visible when observed through a microscope, on occasion, pathogens produce structures such as spores, mushrooms, or massive amounts of mycelium (small, thread-like filaments produced by fungi) that can be seen without the aid of a microscope. These fungal structures are the signs of the pathogen and are useful, when combined with symptoms, for identifying some diseases.

The environment during the onset of the disease is another source of information used in diseases diagnosis. Factors such as temperature, light intensity, as well as humidity and precipitation just prior to and during disease development can give you an indication of which disease is present. The site characteristics also are important in disease diagnosis. Air movement, drainage, soil conditions, the amount of sun or shade, slope, and nearness of other plantings or buildings all may be important in the development of turf diseases. Poor fertilization and mowing practices may stress the turf and trigger or amplify certain turfgrass diseases, providing additional clues to disease identity.

Disease Control

Successful disease control practices involve manipulation of the environment, the grass, and / or the pathogen, to favor the health of the grass and inhibit the casual fungus. The environment can be altered in many ways, depending on the disease to be managed. For example, some diseases require free water for development. Effective stratagies to reduce free water include removing dew and reducing the amount and / or frequency of irrigation. Improved air movement, drainage, thatch reduction, reduced shade, proper regulation of fertilizer applications, and good mowing practices may be appropriate methods for reducing damage from particular diseases and ensuring vigorous turf for recovery from disease damage.

When establishing new turf areas or when renovating disease-damaged turf, it is important to select grasses that are resistant to diseases known to be common in your area or that have damaged your turf in the past. The seeding of disease-resistant grass varieties is an excellent way to minimize turf loss from diseases. For example, certain varieties of Kentucky bluegrass are resistant to leaf spot, a devastating disease on many Kentucky bluegrass turfs. Varietal resistance to other common diseases is available in all cool-season grasses. Another strategy is to seed a grass-species that is resistant to a prevalent disease problem. For example, ryegrass may replace bluegrass in an area damaged by summer patch disease, or bluegrass might replace ryegrass in an area where Pythium blight is a problem.

In selecting grasses for turf establishment or renovation, it is always preferable to use mixtures of different grass species or blends of different varieties, rather than seeding a single species or variety. Seeding mixtures and / or blends produces a diverse population of grass plants. Such turf is usually more successful in surviving stress and attack by disease. It doesn't take much imagination to envision what would happen in a planting of one variety of a single species if a disease occurred that was able to cause severe damage on that variety. Diversity in a planting almost always increases odds of survival.

Diversity in a planting almost always increases the odds of survival.

The third method of disease control is reduction of the pathogen population by applying fungicides that will either kill the fungus or keep it from growing. Again, it is important to have identified the disease correctly, so that an appropriate fungicide can be selected. Arbitary selection and application of fungicides without knowledge of the disease cause can do as much harm as good. Using the wrong fungicide wastes money and may involve the risk of exacerbating the disease, as well as causing other unwanted side effects.

Turfgrass fungicides can be divided into two broad categories: contact fungicides and systemic fungicides. The contact fungicides generally are applied to the leaf and stem surfaces of turfgrasses and do not move appreciably within the plants. Hence, these materials may be washed or mowed off the leaf and stem surfaces. Consequently, they are only effective for short durations (usually 7 to 14 days) and do not protect the new foilage. These fungicides are usually used for the control of foliar diseases and not diseases of the roots and crowns. In general, contact fungicides have a broad spectrum of control and have been used extensively in the turf industry for a number of years.

The systemic fungicides are a newer group of chemicals that are absorbed and translocated within the plant. Thus, they are not as likely to be removed from the plant by rainfall and mowing. (There are several fungicides used on turf called localized systemics, meaning that they are translocated to a lesser extent in the plant than the other systemic fungicides.) The systemic fungicides may protect plants for a period of 2 to 4 weeks and will protect new growth. Most systemic fungicides can control both foliar and root / crown diseases. Systemic fungicides tend to have a rather narrow mode of action, thus, they are somewhat prone to a phenomenon called resistance.

Resistance in fungi to systemic fungicides occurs because these fungicides generally poison fungi at only a single location in their growth and development cycles. In such cases, it is possible that a small portion of the fungus population has the ability to short-circuit or get around the poisoned site and “resist” the poisoning effects of the fungicide. As these individuals reproduce, a large percentage of their offspring will be resistant to the fungicide as well. If this population grows large enough, the fungicide is no longer effective in controlling the disease and the population is said to be resistant. While resistance does not occur often, it has developed where many fungicide applications have taken place, often with repeated applications of the same or similar fungicides. To reduce the occurance of resistance, only apply fungicides when absolutely necessary, alternate and mix fungicides with different modes of action, and use broad spectrum contact fungicides in place of systemic fungicides whenever possible.