IPM Identification and Mangement Of Turfgrass Diseases

Author(s)

Ascochyta leaf blight

Pathogen: Ascochyta spp.
Hosts: Kentucky bluegrass, tall fescue, perennial ryegrass

Symptoms and signs

Ascochyta leaf blight symptoms can 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 (Figure 1). The overall appearance of the disease resembles drought stress, except that the symptoms of Ascochyta leaf blight appear quickly (sometimes over-night). Although the blighting within an area appears complete from a distance, healthy leaves are interspersed within the patch (Figure 2). Blighting is usually restricted to the leaves. Bluegrass crowns and roots typically are not killed.

Leaves infected with Ascochyta spp. often exhibit a bleached tip dieback that extends about a third to halfway down the leaf blade (Figure 3). The margin between healthy and diseased tissue is abrupt and slightly pinched, with a dark brown to purple banding that is generally wider than that associated with dollar spot. Sometimes spots with distinct borders occur elsewhere on leaf blades. In other cases leaves may exhibit white banding or be entirely collapsed and shriveled. These foliar symptoms resemble heat or drought stress.
Ascochyta spp. produce minute yellow to dark brown, flask-shaped fruiting bodies (pycnidia) in diseased leaf tissue (Figure 4). These fruiting bodies, which are easy to view with a hand lens, are peppered throughout the dead leaves and can be useful as a diagnostic sign distinguishing Ascochyta leaf blight from dollar spot, abiotic causes and white grub damage. With white grub damage, the turf can be easily peeled back to observe the grubs. Conidia (spores) produced within a pycnidium are hyaline (colorless) with one to two septa (Figure 5). Septate condia are often slipper-shaped (Figure 5 inset).

Conditions

Ascochyta spp. can be found on senescing or dead leaves of several turfgrass species; however, the disease appears to be most serious on Kentucky bluegrass.

Ascochyta spp. survive as conidia in pycnidia on dead leaves or clippings remaining in the thatch. The pycnidia are highly resistant to breakdown by drought or extreme temperatures. Thousands of conidia ooze from a single pycnidium during wet weather and are dispersed by splashing rain, irrigation, mowing or other management activities.
Conditions that favor Ascochyta blight development are poorly understood. The disease can occur in late spring or summer on drought-stressed turf resulting from water restrictions or poor irrigation system coverage. However, the disease also develops during periods of hot weather preceded by unusually wet soil conditions induced by excessive rain or overirrigation. Frequent mowing and dull mower blades contribute to disease severity by creating more wounds that serve as infection sites.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.