By: Kristi Waterworth
Lawns stretch out across suburbia like an endless grass sea, broken only by the occasional tree or flower patch, thanks to careful maintenance by an army of homeowners. When your lawn is healthy and green, it almost melts into the background, but as soon as brown, brittle grass appears, your lawn stands out like a neon sign. Blighted turf symptoms are common lawn troubles, often caused by turf stress and fungal diseases like ascochyta leaf blight.
Ascochyta leaf blight on lawns is caused by an infection by the fungal pathogen Ascochyta spp. Many grasses are susceptible, but Kentucky bluegrass, tall fescue and perennial ryegrass are the most common victims. Ascochyta leaf blight comes on quickly, causing large brown or bleached patches in lawns when the weather is quickly alternating between very wet and very dry, but the exact environmental trigger is unknown.
You can positively identify an ascochyta leaf blight infection by examining damaged grass blades with a hand magnifying glass. Look for minute yellow to dark brown, flask-shaped fruiting bodies scattered on discolored grass blades. If you find them, don’t panic, grass with leaf blight is rarely seriously injured since the fungus doesn’t attack the crowns or roots.
Because aschochyta blight is so transient, it’s difficult to time fungicidal treatments properly, but a good general care program can go a long way to helping your grass recover. Dethatch and aerate your lawn each year in the fall to increase water penetration and reduce hiding spots for fungal spores. Even irrigation throughout the growing season is recommended for grasses of all types, but don’t allow your lawn to get soggy or leave grasses in standing water.
Frequent, close mowing can increase the visibility of grass with leaf blight, so sharpen your blades and keep your grass at a height of 2 ½ to 3 inches. Reducing mowing frequency will give grass more time to heal between cuttings, reducing the opportunities for pathogens to enter the blades. Applying a balanced fertilizer can help strengthen grass, but avoid large applications of nitrogen, especially in the spring – excessive nitrogen increases the growth of new, succulent foliage that will require more frequent cutting.
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The "leaf spot" diseases are widespread on cool and warm-season turfgrasses and are caused by many different genera of fungi. Many of these diseases are minor in impact while others are capable of causing major destruction only the most important pathogens will be covered here.
Bipolaris sorokiniana affects all turfgrass species in the warm, wet summer months. Symptoms appear as small dark purple to black spots on leaf blades which enlarge with centers fading to tan, often with a yellow halo. The disease is confined to leaf blades in early stages, but sheaths, roots, and crowns can become infected during hot, humid weather. Leaf spots may coalesce causing large, necrotic areas and a general thinning of the turf. B. sorokiniana overwinters as dormant mycelium or conidia in infected plant tissues, thatch, and in plant debris. B. sorokiniana is favored by warm, wet weather and disease severity increases with rising temperatures. A period of drought stress followed by rewetting also encourages the pathogen. The life cycle and epidemiology of Bipolaris are similar to those of Drechslera poae.
Drechslera poae was once a very important disease of Kentucky bluegrass. It has become far less common in recent years due to the availability of resistant cultivars however, it may still be seen on golf courses and lawns where older KBG cultivars persist. The disease occurs in the cool, moist weather of spring as black to purple spots on the leaf blade. Older leaves are more susceptible to infection and lesions can encircle the entire leaf blade causing girdling and the death of individual blades or tillers. Eventually, the fungus invades leaf sheaths, crowns, and roots causing the "melting-out" phase of the disease.The turf thins, turns yellow to blackish brown, and roots, rhizomes, and crowns exhibit a reddish brown, dry rot. When the weather turns warmer and drier, the surviving turf will begin to recover and fill in. D. poae survives unfavorable periods as dormant mycelium in infected leaf tissue, thatch, and plant debris. When environmental conditions are conducive, the fungus sporulates profusely and can be spread by wind, rain, irrigation water, equipment, and people. Conidia may be produced at a wide range of temperatures, but 58-64°F (14-18°C) is the optimum. The pathogen is favored by extended periods of leaf wetness, cool, overcast or foggy days, poor air circulation, low light intensity, high nitrogen levels, and excessive thatch and leaf clippings.
Red Leaf Spot caused by Drechslera erythrospila is a disease of bentgrasses. Symptoms are small reddish brown, oval lesions which coalese, resulting in an overall reddish cast to affected turf. The disease occurs in randomly distributed, irregularly shaped patches. Individual blades are often girdled and die giving the appearance of drought stress. The disease is favored by warm, wet weather and its epidemiology is similar to D. poae.
Gloeocercospora sorghii (copper spot, zonate leaf spot) occurs primarily on bentgrass golf greens. Copper spot occurs in warm, wet weather as scattered, circular patches 1-3 in. in diameter that are salmon to copper color. Individual blades exhibit small red to brown lesions which may coalesce to blight the entire leaf. Copper spot can cause severe thinning, but seldom kills the plants. Infected turf recovers slowly in cool, dry weather. G. sorghii produces fruiting bodies (sporodochia) containing a gelatinous matrix of salmon-colored spores under wet conditions and tiny, black, spherical sclerotia develop in dead leaf tissue. The pathogen overwinters as sclerotia and when conditions are conducive for infection, disease development occurs rapidly. This disease is favored by prolonged leaf wetness, plant exudates from recently moved turf, high nitrogen levels, and acidic soils.
Other minor leaf spots are caused by species of Ascochyta, Curvularia, and Leptosphaerulinia trifolii. These pathogens are weakly virulent and often invade weakened or senescing tissues or occur as components of a disease complex. Their presence is typically indicative of turf stress and improved cultural practices are frequently sufficient for treatment.
Compost teas can be effective at fighting both early and late tomato blight. Compost tea is made by by mixing about one part well-aged compost that is at least 4 months old and 5 to 8 parts of water. The mixture is placed in a covered container and allowed to steep outside at temperatures between 60 and 70 degrees Fahrenheit for about five days. The tea must be stirred daily. On the fifth day, the material is poured through a sieve or cheesecloth and the strained tea applied to plants as a foliar spray. Compost tea should not be sprayed on the fruit if you plan to harvest in the following 2 to 3 weeks.
Ascochyta leaf blight is a common disease of Kentucky bluegrass lawns in Colorado. It may also occur on tall fescue and perennial ryegrass. The disease results in rapid development of large, straw-colored blighted areas of the lawn during the summer. The environmental conditions that trigger Ascochyta leaf blight are poorly understood.
Ascochyta leaf spot symptoms may develop throughout the growing season but are more common during hot, droughty periods that were preceded by cool, rainy conditions. Large irregular patches of turf rapidly turn a straw-color and appear dead. The overall appearance of the disease may resemble drought stress, except that the symptoms of Asochyta blight
appear quickly (i.e. sometimes overnight). Although the blighting within an area appears complete from a distance, healthy leaves are interspersed within the patch. Blighting is usually restricted to the leaves bluegrass crowns and roots typically are not killed.
Leaves infected with the Ascochyta fungus often exhibit a bleached tip dieback that extends approximately a third to halfway down the leaf blade. The margin between healthy and diseased tissue is abrupt and slightly pinched, but doesn’t have the dark brown to purple banding that is characteristic of another disease called dollar spot. In other cases leaves may exhibit white banding or entirely collapse and shrivel. These leaf symptoms resemble heat or drought stress.
The Ascochyta fungus produces minute yellow to dark brown, flask-shaped fungal fruiting bodies called pycnidia in diseased leaf tissue. These fruiting bodies, which are easier to view with the aid of a hand lens, are peppered throughout the dead leaves and can be very useful as a diagnostic feature.
Ascochyta species can be found on senescing or dead leaves of several turfgrass species, however the disease appears to be most serious on Kentucky bluegrass.
The Ascochyta fungus likely survives as spores in pycnidia on dead leaves or clippings remaining in the thatch. These pycnidia are highly resistant to drought and extreme temperatures. Thousands of spores may ooze from a single pycnidium during wet weather and be dispersed by splashing rain, irrigation, mowing or other management activities.
Conditions that favor Ascochyta blight are poorly understood. The disease occurs in late spring and summer and appears to be enhanced by soil moisture fluctuations, especially drought stress caused by watering restrictions and poor irrigation system coverage. However, the disease may also develop during periods of hot weather preceded by unusually wet
soil conditions caused by excessive rain or over-irrigation. Frequent mowing and dull mower blades may contribute to disease severity by creating more infection sites (wounds).
Ascochyta leaf blight can be managed by following good cultural practices that minimize stress in the lawn.
Remember that Ascochyta blight is primarily a foliage and not a root or crown disease. Therefore individual bluegrass plants are usually not killed. Given enough time, usually several weeks, depending on weather, new leaves will emerge from the surviving shoots. Be patient following a disease outbreak and maintain normal management practices.
1 N. Tisserat, Colorado State University professor, bioagricultural sciences and pest management department. 10/92. Revised 6/12.
If you are lucky enough to live in an area where it is hot enough in the spring with lots of sun before you plant this may help. I tried it in my northern climate but the jury is out on whether it actually worked.
Start with baking soda, one cup for each square yard. Work it in with a rake or hoe. There are fungus-spread blights and insect-spread blights, and the baking soda will help the soil of either. If you haven’t already, dig up all the affected plants and put them in a plastic bag to be carried off. Do not put the grass or plants in a compost heap in your yard. Probably, some sort of beetle or boll has brought the blight. With the removal of the plants, the bugs have probably moved on. But to be sure, use either a commercial blight killer from a farm and garden store, or use a heavy powdering of baking soda with vinegar sprayed over it. This will kill any roots, vines, plants, and most bugs in the area. (You can mix the baking soda and vinegar at 1 pound of soda and 1 cup of vinegar, but it will foam like crazy, so use a BIG bowl, allow it to settle, then pour into a quart spray bottle.) work this mixture into the soil. Wait at least 30 days, with re-sprinkling/powdering/spraying if needed, before you add new topsoil and work that in. Then you can replant. Best wishes, Donald, and keep baking soda and vinegar on hand for many uses. ☺️