Take a CLOSER LOOK
Some turf diseases, like summer patch, are difficult to diagnose and control without elaborate investigative methods and an expensive fungicide program. But for some of the most common turf diseases, such as necrotic ring spot, rust, red thread and powdery mildew, you need only rudimentary diagnostic tools and a comprehensive cultural control program to address the problem.
SYMPTOMS VS. SIGNS
When diagnosing turfgrass diseases, it is important to know what you are looking at to make your diagnosis. You can recognize diseases by their symptoms and signs. Symptoms are defined as the visible effects of the disruption of normal functioning by the host. Examples of symptoms could be bleaching of leaves, chlorosis, loss of rooting or wilt stress. Signs are defined as visible realizations of the infecting pathogen. Some examples of signs could be mushrooms growing in a turf, white powder on leaf blades or orange pustules erupting on leaf surfaces. Paying attention to the symptoms and signs that the common turfgrass diseases cause will go a long way toward making your diagnosis accurate.
EQUIPMENT YOU NEED
All you need to diagnose most common diseases is optical equipment (your eyes and a 10X hand lens). This equipment, coupled with a good reference source with high-quality photos should be enough to properly diagnose the most frequently encountered turfgrass diseases.
THE DIAGNOSTIC PROCESS
Diagnosing turfgrass diseases is a skill that you can successfully acquire with practice. The process of diagnosing a problem is rather straightforward, but oftentimes it is not followed. The questions below will help you zero-in on the cause of turfgrass disease and lead you to a correct diagnosis.
#1. Evaluate the overall location where diseased turf occurs. Some examples of these types of questions could be:
What is the environment like in this area?
Is the site steeply sloped?
How does water move over the area?
Who/What uses the area regularly?
When did the problem begin?
What were the weather conditions like when the problem began?
#2. Examine the area where disease is occurring and describe any visible symptoms and signs on the turf. Possible questions might include:
Are the plants different in appearance than normal?
What symptoms is the plant displaying?
Are there any clues as to what might be occurring below ground (wilting, poor rooting, etc.)?
Is the pathogen itself visible on the plant (mushrooms, pustules, etc.)?
Does a closer examination (with a hand lens) reveal any further detail about the symptoms or signs that are visible on the plant?
Once you address these questions, you should have a very good picture about what is occurring to the turfgrass and the conditions under which it is occurring. Armed with this information, you should be able to identify the disease or, if not, consider taking a turf sample to a disease diagnostic clinic.
FOUR COMMON DISEASES
Now that you know there is a problem, the trick is to use the information that you generated to correctly identify what is damaging the turf. Here is a description of the four most common turfgrass diseases encountered by lawn-care operators in cool-season turf communities and how to identify them.
NECROTIC RING SPOT
Necrotic ring spot (NRS) is probably one of the most frequently encountered disease problems on Kentucky bluegrass. The disease is caused by a root-infecting fungus that destroys the water-gathering capabilities of the plant, causing it to wilt and die more rapidly than healthy plants. NRS also is one of the diseases that has a comprehensive pesticide-free management plan that has been used successfully in the field.
The symptoms of NRS begin initially as wilting circles of turf (as a result of the diseased root system) and progress to circles of turf that have died from the repeated moisture stress. These circles of diseased turf often have centers that are filled-in with immune grasses or weeds. This symptom is often called a “frogeye” patch and is very characteristic of patch diseases like NRS.
The environmental conditions that occur around an outbreak are hot, humid temperatures and moisture stress in the area where symptoms occur. This disease most often occurs in sodded areas of lawns, so asking whether the area was established using seed or sod can often help in the diagnosis of this disease.
Once you have decided that the problem is NRS, the next challenge is managing the disease. Researchers have developed cultural controls for managing this disease. You can manage this disease through an irrigation and fertilization program. We've long heard the recommendation of deep, infrequent irrigation as a part of good turf management. But in this case, for NRS control, you'll want to irrigate lightly and frequently (0.1 inch per day), because the infected turf will have a shallow root system and will be easily stressed. You should also irrigate in the most stressful period of the day (early afternoon), when turf needs it the most. By irrigating frequently, you'll also keep thatch moist. A moist thatch layer increases the microbial activity that may help to attack the pathogen that causes NRS biologically.
Research has consistently shown that using a slow-release form of nitrogen improves the symptoms of NRS. Properly managing the irrigation and fertility of an affected area will mask the symptoms of this disease during stressful periods throughout the season.
This disease is probably the second most important disease found in Kentucky bluegrass and perennial ryegrass. A customer usually notices this disease when the orange pustules that erupt on leaf surfaces begin to cover their shoes, mowers and other equipment with “rust.” The pustules that erupt contain thousands of tiny spores of the pathogen that are dislodged to cover nearly everything. However, this disease is generally a nuisance rather than a true problem that needs the power of fungicide applications.
Rust infections generally occur during the late fall months as turfgrass begins to slow down its growth and harden off for the approaching winter. As a result, the infection can take hold because infected turf is not growing fast enough to be mowed off with regular mowings. The orange pustules that erupt on leaf surfaces are the diagnostic sign of rust. The spores are spread to other plants via wind and mechanical transport.
Once you discover and diagnose this disease, apply a quick-release fertilizer so that the plant can outgrow the symptoms of the infection. Apply enough fertilizer should to encourage some growth (¼ to ½ pound N/1,000 per square feet). However, exercise care because rapid growth at this time of year also results in a corresponding decrease in the hardening ability of the plant that can open turf up to devastating snow mold infections. If the outbreak occurs early in the fall, it would be justified for you to apply a fertilizer. But as it gets later in the season, it's better to leave it alone because the disease will disappear as winter approaches.
This disease is a particular problem anywhere perennial ryegrass grows. Red thread can cause significant damage and is a particular problem during the cool, wet springs common in much of the northern climes of this country. Symptoms include rapid decline of leaves in the affected area and leaf browning. Infected areas often develop in circular patches that can merge with other patches to become large areas of infected turf. The major diagnostic criterion for this disease is a sign that you can observe within affected areas from which the disease gets its name. At the end of infected leaf blades the fungus produces its fruiting structures, which appear as “threads” of fungal tissue that are colored a deep pink-reddish color. It is these “red threads” that give this disease its name.
Once you identify and diagnose this disease, you should increase fertility to promote growth of the affected plants. You should also withhold irrigation until the cool, wet conditions subside. You can effectively manage red thread with a dormant fertilizer application (1 pound N/ 1,000 square feet), and a good spring fertilizer program (˜1 pound N/ 1,000 square feet/month). Red thread is usually only a problem where nitrogen fertility is lacking.
Like rust, powdery mildew is more of a nuisance disease that causes an unsightly appearance rather than severe damage. However, it can make problems worse for plants that are in a shaded condition and already under stress. This disease is most severe in humid and shady conditions that are often present in heavily wooded landscapes. Turf growing under these conditions is already stressed due to a lack of adequate sunlight, and this disease compounds the problem. You can also sometimes see powdery mildew infecting turf on the north sides of buildings because of the lack of sunlight.
The disease most readily infects Kentucky bluegrass and can be seen on leaf blades as a fine, white, cottony growth on them. The white growth is actually the pathogen that penetrates the leaves (but does not kill them). The fungus actually benefits from not killing, as it can harvest nutrients from the plant. Eventually, the fungus matures and this white, cottony growth gives rise to spores that are easily dislodged from leaf surfaces. To effectively rid this disease, you must resolve the shading situation. An alternative to removing the shading situation is to plant mildew resistant species. The best grasses for these situations are the creeping red fescues. These grasses make excellent stands even under heavy shading. Also, the fescues are not susceptible to powdery mildew, so they perform very well in these stressful conditions.
USE YOUR RESOURCES
Answering the questions outlined earlier each and every time you arrive on the site of a potential disease problem generates the information you will use to make diagnoses. Knowing the diagnostic characteristics of the four common turfgrass diseases in this article, coupled with your knowledge about the site, will allow you to approach the correct diagnosis a great deal of the time. So, what about those situations where the site conditions don't correlate with the diagnostic criteria for one of the four diseases? A good diagnostician has the ability to say, “I don't know.” Many university programs have a diagnostic clinic that can properly diagnose these cryptic problems. Also, your own reference library may have images of possible problems that look similar to the disease in question and these can be an invaluable resource for you. The information that you generated by asking the questions about the site will help the diagnosticians at the clinic zero-in on the problem and help reduce your cost for having a sample analyzed. Don't be afraid to take advantage of these services when you need them; they often can be excellent sources of information on new and emerging problems. You also can improve your diagnostic abilities by double checking your diagnosis by sending in a sample for analysis. So the next time someone calls you about an area of damaged turf, the questions that run through your head should be those that help you assess the situation and diagnose it rather than those questions that cause you to think about how up-to-date your resume is.
Brandon Horvath, Ph.D., is an instructor in the Department of Plant Pathology at Michigan State University (E. Lansing, Mich.).
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