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Ascochyta Leaf Blight
Ascochyta leaf blight cause large irregular patches of turf to quickly turn straw-brown in color and appear dead. From a distance, the symptoms resemble drought stress. The difference is that the symptoms of Ascochyta may appear quickly or overnight. Infected leaves present a bleached tip dieback and the margin between healthy and diseased shows a slightly pinched appearance. Some leaves may have uniform lesions that affect the entire leaf blade.
There are approximately 80 species of fungi in the genus Ascochyta that can cause Ascochyta leaf blight. They prefer humid, wet weather. Other factors, such as thick thatch, poor soil, over or under-fertilization, and dull mower blades can contribute to disease severity.
The fungus survives as mycelium and fungal structures called pycnidia on dead leaves or clippings in the thatch. Hundreds of fungal spores emerge from pycnidium during wet weather and are dispersed by water, mowing, or other activities. The fungus generally enters plants soon after the grass is mowed and begins to grow from the freshly cut end toward the leaf base. Ascochyta blight is known to occur on drought-stressed turf and also during periods of hot weather preceded by wet conditions.
To manage the disease, reduce thatch by core aeration at least once a year to allow better water penetration. Maintain grass height between 2.5 and 3 inches. When disease is present reduce mowing frequency and increase mowing height. Avoid mowing during wet weather as it will harm the turf and provide more infection points for the pathogen. Also, it is recommended to avoid excessive nitrogen fertilization, especially in the spring, and try to maintain uniform soil moisture and good drainage. Collecting clippings to reduce fungal inoculum has not reduced disease severity. Ascochyta blight disease development is sporadic and rapid, so fungicide spray is not recommended. Turf may look dead from a distance, but the blight does not affect bluegrass crowns and roots. Therefore, grass will recover in 2 to 3 weeks or longer depending on weather conditions.
Lawns with this disease often quickly recover. See below for photos of affected lawns as they appeared on June 11 and how they looked on June 21.
Septoria Tip Blight
Septoria leaf spot has symptoms similar to Ascochyta leaf blight, but begins as gray-green lesions, and the grass does not appear pinched above the green lower blade of a diseased tiller. Also, while cool (60-75°F) wet weather is favorable for both diseases, Septoria leaf spot will not continue to afflict the grass during the heat of summer, but may return with cool wet conditions in autumn. Ascochyta leaf blight may continue throughout the summer under high humidity or otherwise moist conditions. Septoria produces a large number of speck-sized fruiting bodies near the tips of the diseased blades.
This disease is managed in the same way as Ascochyta Leaf Blight.
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