1. Turfgrass Pest Management, Category 3A Diseases of Turfgrass Chapter 8
2. Diseases, which are considered a continual disturbance of normal plant function, are the most difficult plant disorders to diagnose and manage.
3. Diseases are either non-infectious or infectious. Non-infectious diseases cannot be spread between plants. They are the diseases that are caused by cultural or environmental conditions, such as responses to salt injury or flooding.

Infectious diseases can be spread between plants and are caused by pathogens.

6. With adequate temperature and moisture, fungi enter plants through wounds or natural openings in leaves, stems, and roots. After growing in the host for days or weeks the fungi produce seed-like spores for reproducing. The resting stage of most fungal diseases of turfgrass is found in the leaves, stems and roots of turfgrass. Fungi also overwinter in thatch and near the soil surface.

 

7. Nematodes are very tiny, slender round worms 1/50 to 1/10 inch long. The are commonly found living in soil, water, and other organisms. They reproduce by eggs and are spread whenever nematode-infested plants or soil is moved.

Of the 1,500 plant-parasitic nematodes, only a few feed on turf roots.

8. Nematodes feed by inserting their needle-like mouthpart, called a stylet, into plant cells and consume the contents. Nematodes impair the root’s ability to absorb water and nutrients, thereby reducing the turf plant’s growth and vigor. Nematode-infested turf does not respond well to fertilizing or other maintenance practices. The turf tends to be more susceptible to stresses and other pests.
9. To confirm that turfgrass has a nematode problem, pull a sample of living turf including the soil surrounding its root system. A golf course cup-cutter works well for pulling samples. Send the sample to the MSU Plant and Pest Diagnostic Clinic. The address is listed in the turf study manual.
10. For a plant to become infected by a pathogen, certain conditions must exits. If one of the conditions is not right, the pathogen cannot infect the plant.
11. The development of an infectious disease requires all of the following:

1. A susceptible host plant.
2. Presence of a disease-causing agent.
3. An environment favorable for infections and development of the disease (this usually means a certain temperature and moisture condition).

6. When plant injury is noted, the first thing a good plant diagnostician does in determines whether the damage is caused by an infectious disease, or by cultural or environmental conditions. If an infectious disease is the cause, we must determine which one.
7. Because pathogens are microscopic, disease diagnosis is often difficult. Typically in the field we rely on the injury symptoms rather than trying to directly determine the pathogenic species. Pathogens can be associated with the type of host plants that they infect and the environmental conditions needed for development. Therefore, diagnosis also involves determining the type of host (Kentucky bluegrass, fine fescue or rye grass) and consideration for recent and current environmental conditions (humidity, day and night time temperatures, etc.).
8. When symptoms and all other pieces of information do not provide a clear diagnosis for the causal agent, samples can be sent or brought to the MSU Plant and Pest Diagnostic Clinic. The current phone number is (517) 355-4536.
9. There are several common turfgrass diseases. We will proceed through this list one disease at a time. The components of the disease triangle for each pathogen will be discussed. As you now know, one component of the triangle is the pathogen itself, plus the host susceptible to it, and the environmental conditions conducive to the pathogen’s infection and development.
10. Anthracnose of annual bluegrass causes the turf to turn yellow-bronze to reddish-brown in nondescript areas. Air temperatures in the 80’s and high humidity are conducive to this disease. With a hand lens, black, spiny-cushions can be seen on older blighted leaves. Anthracnose becomes severe when annual bluegrass is stressed.
11. To manage anthracnose, provide adequate fertility levels and reduce all types of stress including compaction, poor drinage, and improper mowing heights. To control an existing infection, use a fungicide labeled for anthracnose control.
12. Dollar spot can be found on a few turfgrass hosts-bentgrass, bluegrass and perennial ryegrass. The disease gets it name because it causes small bleached areas of turf about the size of a quarter of silver dollar. Under severe conditions, the spots mere and blight large areas. In the early morning, white fluffy fungal strands can be seen on spots. This is actually one of the easier diseases to diagnose in the field. Look at individual infected grass blades. Look for a tan lesion with dark-brown to reddish-brown borders that girdles the leaf blade.
13. Maintaining adequate nitrogen levels will reduce dollar spot disease symptoms. On golf courses, fungicide treatments are usually necessary to keep this disease in check.
14. Fairy rings are caused by fungi that live the soil and break down organic matter. Initially, a dark green ring is produced which may develop mushrooms. The mushrooms typically appear after heavy rains or irrigation. The size of the rings varies from a few inches to greater than 50 feet in diameter. Fairy rings are usually tolerable on consumer lawns but cause serious problems on golf courses greens and other high maintenance turf.
15. Fairy rings are difficult problem to eliminate. Most turf managers attempt to mask the dark green rings with fertilization. Fumigation or soil removal is necessary when trying to eliminate the problem.
16. Pink snow mold is caused by a fungus that lives in turf thatch and residue. When conditions are cool (45 F) and wet, whitishgray or reddish-brown spots develop. The rounded spots are 2 inches to 2 feet in diameter, and can develop with or without snow cover. Shortly after snow melt, pink fungal strands can be seen on the edges of the spots. Annual bluegrass, perennial ryegrass and bentgrass are highly susceptible to pink snow mold, now also know as Michrodochium path.
17. To reduce the chance of pink snow mold, allow the grass to harden off before going into the dormant season. Be careful with the timing of fall nitrogen applications. Preventive fungicide treatments may be necessary for valuable, susceptible turf stands. If spots occur, rake and replace the matted grass.
18. Leafspot and melting-out diseases are caused by a variety of fungi. Although they can be active throughout the year, leafspot diseases are most active during the cool, moist weather of spring and fall. During periods of drought stress, the fungi may spread to the blight crowns and roots causing "melting-out". The leafspots first appear as small, purple to black lesions on the leaf blades. As more spots develop, they join together to form large, elongated areas of infection. Leafspot is mainly a problem on creeping bentgrass and fine leaf fescues, while melting-out injures mostly Kentucky bluegrass.
19. Turfgrass varieties are often rated on their susceptibility to leafspot and melting-out. When seeding, select resistant varieties. For established turf stands:

1. limit stress by moving at an appropriate height of cut,
2. avoid promoting excessive lush growth in the spring,
3. limit moisture on foliage by avoiding evening in and night waterings, and
4. promote air circulation.

6. Necrotic ring spot is a serious disease of Kentucky bluegrass. Symptoms become obvious in midsummer, when turf is drought stressed. Wilted, dying or dead turf in spots 2 to more than 12 inches wide appear first. The straw-colored patches may grow together forming streaks or circles in the turf. Often, healthy grass is surrounded by a ring of dead turf creating a "frog-eye" pattern. Necrotic rings spot is favored by periods of warm, wet weather followed by periods of heat and drought. The disease often inflicts turf in hot, compacted areas.
7. Some varieties of turf are more resistant to necrotic ring spot than others. Determine what these varieties are and use them from new seeding. This is one disease in which the symptoms can be managed culturally. Provide adequate nitrogen, irrigation and mow at the appropriate height to avoid the onset of symptoms. Keep thatch at a minimum and provide light, daily, late morning irrigation during the summer months. Fungicides are available if the cultural practices are not sufficient in managing necrotic ring spot.
8. nematode-infested turfgrass is thin, lacks vigor, and is frequently pale yellow, stunted and slow-growing. Water and fertilizer do not generate much response and severely infected plants wilt midday, and later die in irregular patches. Roots may be coarse and shallow, and may have swollen areas or galls.
9. If nematodes are suspected, use diagnostic lab services to confirm the infection. If managing a nematode-infested turf stand, provide light watering at midday to avoid wilting. Nematicides may be a management tool that you consider after a positive diagnosis and getting input from other turf professionals.
10. This disease develops white, powdery patches of mildew on grass blades, especially in shaded or poorly drained areas. Powdery mildew on Kentucky bluegrass is common in the spring and fall when nights are damp and cool and days are mild and cloudy. Powdery mildew reduces plant growth and causes diseased leaves to turn yellow, wither and die. Heavily infested plants may die.
11. Avoid creating conditions favorable to powdery mildew. Plant more shade tolerant grasses such as fine fescues. To increase sunlight and air circulation, consider trimming trees and shrubs.
12. Pythium blight is somewhat predictable because it is a hot-weather disease. Pythium blight rapidly develops when daytime temperatures are in the 90’s and night temperatures stay above 75 F. Since these conditions are infrequent in Michigan for extended periods of time, pythium blight is often confined to areas with poor drainage.

Round to irregular, water-soaked, "greasy," sunken patches up to 12" wide are first to develop. When humidity is high, a fluffy white mold growth may be associated with the leaf lesions. Pythium blight often appears in elongated streaks following water drainage or mowing patterns.

13. Pythium blights can be reduced by improving soil drainage. Avoid over-watering, thick thatch, compaction , and excessive nitrogen fertilization. When an outbreak occurs, a fungicide application can control the problem. By watching the weather conditions, you can predict when this disease may become active and you’ll be prepared for locating problematic areas on your site.

 

14. Red thread is a fairly distinguishable turf disease that develops during prolonged humid weather. You typically see reddish-pink fungal strands protruding from the leaves. From a distance the disease causes the turf to develop irregular to circular light tan to pink patches. Large infected areas appear bleached-tan or yellowish and scorched.
15. Turf that is lean, or has had low fertility is very prone to this disease, especially perennial ryegrass. When red thread appears, provide adequate nitrogen levels and remove the clippings to reduce the inoculum. The turf usually recovers without the need for fungicide applications.
16. Rhizoctonia Brown Patch attacks all types of turf in Michigan It is very likely to occur when temperatures are above 70 F degrees during the day with humid, overcast conditions and when the night temperatures remain above 68 F. the patches can reach up to 2 to 3 feet across. When moist, such as in the morning, a grayish-black "smoke" ring of wilted and blighted grass may be evident on the patch edge. On perennial ryegrass, the disease has a slimy appearance.
17. Succulent turf is most vulnerable to brown patch. Therefore, avoid high nitrogen levels during periods of weather favorable to the disease. Removing early morning dew and incrasing air circulation helps reduce injury. Several fungicides are also available for managing brown patch.
18. Rust is a fungal problem for bluegrass and ryegrass in Michigan. Heavily infected turf becomes reddish-brown to yellow-orange because of the numerous fungi pustules growing on grass blades. The rust material easily rubs off onto fingers, shoes, and clothing. A severe rust infection results in thin, weakened turf that is more susceptible to winterkill, drought, and other diseases. Rust is typically a problem during hot, dry periods when grass growth is reduced.
19. Poor seed quality, and low fertility are primers for rust infestations. To aviod problems with rust, use resistant varieties of bluegrass and ryegrass and maintain adequate fertility and irrigation practices, especially during dry periods of weather.
20. Slime molds are harmless fungi that feed on decaying organic matter. During warm weather and after heavy rain or watering, multiple colors of slimy masses grow over the grass blades. These slime molds develop in streaks or small patches that smother, rather than consume, otherwise healthy turf plants. The masses dry out and form peculiar ash-gray crusty mats.
21. Slime molds rarely occur more than once a season and quickly disappear when they do develop. If you must take action, raking brushing or spraying the mold with a strong stream of water will remove the mold from the grass blades. chemical controls are not necessary.
22. Stripe smut is a major cool-weather disease of Kentucky bluegrass and creeping bentgrass in Michigan. Because the symptoms are subtle and difficult to detect, the disease is often not recognized until damage is severe. Turf infected with strip smut is stunted and slow to start growing in the spring.

Infected grass blades are pale green with long black strakes of pustules along the veins. These streaks are not apparent on short-mowed turf. As pustules open, shoes and clothes become coated with black spores. Later infected blades fray, twist, and turn white. The foliar symptoms almost disappear in the summer, making diagnosis difficult.

23. When establishing turf areas, select stripe smut-resistant varieties. In established stands infections are difficult to control since fungicides provide only temporary control. It is important to follow good turf management practices and to keep infected turf growing during the summer. If the grass goes dormant while infected, it will die.
24. Typhula blight or gray snow mold can be problem where snow cover remains for longer than three months without melting. As the snow melts, circular gray to dark brown spots, typically 6 to 12 inches wide, appear in the turf. Along the margins of the spots, grayish-white fungal strands can be seen. When turf goes into winter lush and snow falls on unfrozen turf, gray snow mold can be severe.
25. To reduce the likelihood of gray snow mold, avoid fall nitrogen applications that might encourage new, tender growth. Use resistant turfgrass varieties when possible. If conditions favor typhula blight, it may be necessary to protect valuable turf with a fungicide treatment.
26. Predicting disease activities requires you to be aware of when the three components of the disease triangle for each possible turf disease might occur. Keep in mind the type of turf you are managing and the diseases it is prone to. Stay in tune with the weather patterns, including recent past conditions, current conditions and the forecasted conditions. Further, be familiar with those areas on your site(s) that may be more conducive to disease development. Those areas may be shady, or low areas with limited air movement, or open, high and dry areas.
27. When we address turfgrass disease management, our efforts focus on preventing the disease from occurring, or lessening the effects of disease when it shows up.
28. There are three primary strategies used in turfgrass management. They include resistance, avoidance and protection.