The unfab five
As a turf-care specialist, you face many challenges in the quest for that perfect lawn for your clientele. However, your clients may present the greatest challenge in lawn care. Most, but not all, know little about growing turf. Often, the homeowner does the mowing and irrigating, and you do the fertilizing and pest control. But proper mowing and irrigating are as critical to growing healthy, vigorous turf as fertilizing and pest control. If done improperly, they may compound disease problems. Therefore, customer education is the key.
In the past 25 years, the two most dramatic changes in cool-season turf diseases that we've seen are the increased incidence of brown patch and the reduced incidence of leaf spot and melting out. Both changes reflect a shift in lawn-care management. We have seen the interest in turf-type tall fescues grow and with it, the incidence of brown patch. More than ever, you need to educate your clients of the concept of blending.
The following sections outline the five most important diseases in cool-season turf. You will find tips for recognizing and managing them. The management aspect focuses on an integrated approach that highlights preventive practices. For a list of chemical control options, consult the Fungicide Update on page 38.
Brown patch affects all commonly cultivated cool-season turfgrasses. However, cultivars differ in susceptibility. Tall fescue, Kentucky bluegrass and perennial rye are the primary turfgrass hosts. The causal agent of brown patch is Rhizoctonia solani.
Symptoms. Look for circular patches of dead and dying grass. They may encompass large portions of the turf. The turf in these patches appears “sunken.” The center of diseased patches may appear less affected, and it may show the frog-eye symptoms commonly associated with summer patch. However, look for the characteristic brown-patch leaf spot on individual blades. It will distinguish brown patch from summer patch. Also, brown-patch-affected turf appears less matted.
Look at green plants within the affected turf. A dark-brown margin will surround long, irregularly shaped grayish-colored leaf spots. Your diagnosis should include not only the characteristic leaf-spot symptoms but also gross symptomatology of the affected turf. If you are having diagnosis troubles, contact a plant disease diagnostic laboratory. They will isolate and culture the pathogen to determine the species.
Factors favoring brown patch. Look for brown patch in dense, heavily fertilized, irrigated turf during hot (above 85°F), humid weather when nighttime temperatures remain above 60°F. When diagnosing, look for poorly drained soils and thick thatch. These factors contribute to brown-patch incidence. Leaf wetness contributes to disease formation and high levels of nitrogen and low levels of phosphorus or potassium may cause increased disease severity. Also, if your mower blades are dull, you may be contributing to the incidence of this disease. Dull blades fray grass tips, causing excessive wounding and enhancing infection.
Managing brown patch. To prevent brown patch, avoid high nitrogen application rates during the summer. The nitrogen stimulates lush growth that is more susceptible to brown patch. Avoid watering practices that keep the turf wet for more than 6 hours. Irrigate in the early morning so that the leaf blades dry quickly. Also, aerify and de-thatch to keep thatch to less than 0.5 inch. Plant or overseed with resistant cultivars — contact your seed distributor for more information.
Begin fungicide treatments when symptoms first appear. Rescue or curative treatments often fail, so you must monitor your sites for early diagnosis. If you manage your turf properly, it can recover quickly from brown patch.
Summer patch is one of the most destructive diseases of Kentucky bluegrass. The causal agent of summer patch, the soil-borne fungus, Magnaporthe poae, infects the plant roots. Therefore, early diagnosis can be difficult.
Symptoms. Begin monitoring early in the season as soil temperatures reach the low 60s. Look for scattered plants or patches of bluish-green, wilted plants. Summer patch damages the roots and causes the plants to succumb to drought stresses. Because the fungus is underground, it often goes undetected until the plants begin to die.
Monitor sites prone to heat stress. Southerly-exposed slopes or turf near concrete driveways will be the first to exhibit moisture stress. Look for 6- to 20-inch circular, semi-circular or serpentine patches. They will give the area a pockmarked or doughnut appearance. Look for matted light-tan, dead turf. Also, look for a tuft of green grass in the center (the “frog-eye” pattern). The leading edge of an affected area usually shows the most characteristic “frog-eye” symptoms. Plants at the edge of the patches will appear unhealthy. Dig up some roots. If you see roots that are dark and partially rotted, the plant is infected,
Factors favoring summer patch. Look for summer patch from mid-June through September. You may not see symptoms during cool periods when moisture stress is not exhibited, but the symptoms will reappear when hot weather returns. You usually will see summer patch break out during hot, dry weather after a wet period. The fungus is most active in irrigated turf or when frequent rains occur.
Other factors that favor summer patch include heavy thatch, low mowing, unbalanced fertility, improper irrigation, compaction, sites exposed to heat, steep slopes and poorly adapted grass varieties.
Managing summer patch. Eliminate plant stress during the summer. It may be the most important factor affecting symptom development. Avoid management practices that promote rapid top growth at the expense of root development. Integrate good cultural practices with fungicide treatments as outlined in the sidebar on page Contractor 20.
Dollar spot, caused by Sclerotinia spp., is another disease that is common in Kentucky bluegrass, bentgrass, fescue and ryegrass turf. If you properly maintain your turf, it will usually recover from dollar-spot injury. Poorly maintained Kentucky bluegrass may be severely injured, and it may not recover even if you make fungicide treatments.
Symptoms. The name dollar spot comes from the silver-dollar-sized, light-tan or grayish spots that occur on bentgrass putting greens. However, in taller turfgrass, look for 4- to 6-inch patches of mottled, straw-colored turf. Affected areas may merge and become large as the disease spreads. Dollar spot can be severe when nitrogen fertilization is less than optimum.
In residential lawns, look for symptoms that resemble melting-out disease, grubs or drought. Fortunately, you can easily distinguish dollar spot by the characteristic lesion on the leaf blades. Look for lesions that are first chlorotic (yellow), then water-soaked and finally bleached-out or light tan. They may be up to an inch in length, and they usually span the width of the blade. You will typically see reddish-brown bands on either side of the lesions.
Factors favoring dollar spot. You will begin to see dollar spot during periods of warm days and cool nights that produce dew and high humidity in the turf canopy. Look for cobweb-like mycelium growing on the grass blades. Grass clippings, mowers and foot traffic will easily spread dollar spot. Be aware of these potential sources of infection.
Managing dollar spot. To prevent dollar spot with minimal fungicide applications, properly fertilize to ensure vigorous growth. Also, irrigate during morning hours only. Aerify to control thatch and to reduce compaction. To hasten recovery, implement practices that promote vigorous, but not lush, growth and reduce plant stress. If necessary, make fungicide applications when symptoms first appear.
Leaf spot and melting out
Twenty-five years ago, leaf spot and melting out were the most serious diseases of Kentucky bluegrass turfs. Thankfully, researchers have developed resistant, improved cultivars, and these diseases are no longer as important as they once were. The leaf spot pathogen, Bipolaris sorokiniana, attacks bluegrass, bentgrass, ryegrass and fescue. Melting out, caused by Drechslera poae, is mainly a disease of Kentucky bluegrass. However, it also occurs on ryegrasses and fescues.
Symptoms. You will see two phases of leaf-spot symptom development. They correspond with changing temperatures during the growing season. Look for the leaf-spot stage when temperatures are between 70° and 85°F. When temperatures are above 85°F, look for necrosis of the entire leaf blade and the resulting leaf blight. On Kentucky bluegrass and fine and tall fescues, you will first see small, dark-purple to black spots on the leaf blade. As the black spots age, you will see them as round to oval spots that have buff-colored centers. These lesions are surrounded by a dark-brown to dark-purple margin, and they may merge and girdle the leaf blade. You will see the blade turn yellow or reddish-brown and die back from the tip. When leaf blighting progresses, the turf will fade to a brownish color. During hot, humid weather, leaf sheaths, crowns and roots become infected. This causes thin, open areas in the turf. Plants with severe crown and root rot usually die from drought stress.
The leaf spot symptoms of melting out (D. poae) on Kentucky bluegrass are nearly identical to those of leaf spot caused by B. sorokiniana. You will first see the symptoms appear on the leaf blades as small dark lesions that develop into oval spots with buff centers and dark purplish-black margins. Once the fungus colonizes the leaf sheath, you will see the leaf turn yellow and then tan. Eventually the leaf drops from the plant. This stage is known as melting out. As with the leaf-spot disease, you will notice the symptoms progress from leaf spotting through melting out. Eventually, the crowns and roots rot. Ultimately, you will see the affected areas become brown and thin.
Factors favoring leaf spot and melting out. Look for both diseases during dry periods following prolonged cloudy, wet weather. Leaf spot is most active when temperatures are between 70° and 85°F. The optimum temperatures for melting out are 65° to 75°F.
Managing leaf spot and melting out. Use a combination of improved cultivars, good turfgrass management practices and fungicide applications for the most effective control. Use improved, disease-resistant cultivars when establishing or renovating turf. Assess your fertilization program, and incorporate a program that does not stimulate lush growth. Manage your thatch and irrigate in the morning. Adjust mowing frequency to correspond with the growth of the grass. When necessary, apply a fungicide in April followed by two or three additional applications spaced 3 to 4 weeks apart or as label directions dictate.
Powdery mildew is caused by the fungus Erysiphe graminis. You will see it on a variety of cultivated cool-season turfgrasses. You will most commonly notice it in Kentucky bluegrass and fine-leaf fescues growing in shade. It is a highly specialized plant pathogen that primarily lives on the outer surface of the host plant. Powdery mildews have a high demand for nutrients and obtain them from their hosts. Although powdery mildew rarely kills a plant, infections reduce plant vigor and lower aesthetic value.
Symptoms. Powdery mildew is named for the grayish-white to light-tan, powdery mat that forms on the leaf blade. What you see is both mycelium and masses of spores. You will easily recognize the mildew mat. Look for plants that appear to have been dusted with lime.
Reaction of grass plants to powdery mildew varies considerably. Age of host tissue at the time of infection, susceptibility of the cultivar, rate of plant growth and environmental and cultural conditions all determine the extent of symptoms and injury.
Factors favoring powdery mildew. During cloudy, humid periods, look for this disease when the days are warm and nights are cool. Unlike most foliar blights or leaf-spot diseases, powdery mildew does not require wet foliage. However, high humidity is important for infection.
Managing powdery mildew. Selectively prune shade trees to increase light penetration and improve air movement. This will lower humidity in the grass canopy. In shade areas, use resistant cultivars of Kentucky bluegrass and fine-leaved fescue. Use fungicides to treat severe outbreaks or turf that has persistent mildew problems. Make one or two applications in spring, late summer or autumn or as label directions state.
The field of turfgrass management has experienced tremendous growth in the past 25 years. Integrated pest management has blossomed, and we no longer just mow and fertilize. Turf managers have evolved into a group of knowledgeable, highly skilled specialists trained in all aspects of turfgrass management.
John Watkins is an extension plant pathologist with the University of Nebraska (Lincoln, Neb.).
PREVENTIVE PRACTICES FOR SUMMER PATCH
Avoid heavy, early spring and summer nitrogen fertilization.
Develop a fall fertilization program supplemented with a light (half-rate), mid-summer fertilization.
Water deeply and infrequently when the soil begins to dry out. Irrigate less, but more frequently, when summer patch is active.
Syringe-irrigate heat-exposed turf during mid-day in July and August.
Aerate in early fall or mid-spring.
Remove thatch by aerating, vertical mowing or power raking. Aeration is the preferred method.
Increase mowing height to 3 inches during July and August.
Overseed annually with a blend of cultivars adapted to your region.
Apply preventive fungicide treatments in the spring when soil temperatures, at a depth of 2 inches, reach 62°F. Repeat the treatment in 30 days or as label directions indicate.
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