Ascochyta Leaf Blight – Uncommon & Sometimes Devastating
Asco what?! That’s the immediate response I get from turfgrass managers when I tell them I just diagnosed their damaged tall fescue sample as Ascochyta leaf blight. Even though we don’t see this disease with the same frequency as brown patch, Pythium blight, or gray leaf spot in tall fescue, it demands respect in the world of turfgrass pathology. We typically diagnose a few cases of this disease every spring in North Carolina and have already confirmed a few positive sites within the past week. This might be the disease you’ve overlooked in past springs and is worth learning more about if you want to improve your management skills of tall fescue.
Ascochyta leaf blight is caused by nearly 80 different species of Ascochyta. This fungus has been documented on many turfgrasses, but more common on tall fescue, Kentucky bluegrass, and perennial ryegrass. As the name implies, the primary symptom is blighted leaves. The pathogen only attacks the foliage; therefore, damage is seldom permanent and recovery is quick under favorable growing conditions. As seen in Figure 1, stand symptoms may rapidly encompass large areas of uniformly blighted, straw colored turf. Individual leaves typically start dying from the tip back and bleached lesions may eventually develop along the entire leaf blade. The leaves will often appear shriveled and pointy, as seen in Figure 2. If you look closely with a hand lens at the affected portion of the leaf blade, you will see brownish, red, or black pycnidia (Figure 3). Pycnidia are the fruiting bodies that produce conidia (spores) for this fungus.
This fungus survives, like similar pathogens in turf, as mycelium and pycnidia in host debris. Conidia are typically released during wet periods, i.e. spring rains. This fungus will take advantage of freshly cut grass and even though mowing creates wounds for many fungi to enter, you shouldn’t stop mowing because of this disease. Even though the epidemiology of this disease is not well understood, it seems most severe during extreme weather changes like what we’ve experienced lately here in North Carolina where one day it’s 78 and sunny and the next it might be 50 and rainy.
In general, this is not a severe disease of tall fescue. However, if conditions are favorable, damage can become quite alarming and unsightly as seen in the photos. Fungicides are rarely recommended due to the high potential for rapid recovery during the spring when tall fescue grows best. This may take several weeks if the damage is extreme, but stands often recover. If you are managing a lawn with a known history or current outbreak, it’s best to mow the lawn with sharp blades at the correct height when the canopy is dry to minimize spread. Be sure to water deep and infrequently only when needed to prevent drought stress. Finally, be sure fertility is adequate and avoid excessive (nothing greater than 1 lb. N/M) applications of nitrogen fertilizer. There is limited information on fungicides, but it seems like most QoIs and DMIs would be effective against this fungus.
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