Over the past month or so I have observed more than a few cases of Leptosphaerulina blight (pronounced “Lep-toe-sphere-u-line-a”). Based on my experience, damage appears mostly on stressed or declining turf, and often manifests itself as ill-defined patches of dead and decaying grass blades (Figure 1). In Figure 1, the patterns correspond to patches of annual bluegrass in a creeping bentgrass fairway. The diagnostic sign is the presence of numerous fungal structures on the collapsed leaves. The structures (called pseudothecia) look like small volcanoes under magnification (Figure 2).
Leptosphaerulina is not an aggressive pathogen, and some may argue that it is not a pathogen at all. It is more of an opportunistic fungus that thrives on stressed turf under certain environmental conditions. This explains why symptoms are more likely to occur on senescing annual bluegass than creeping bentgrass. It is not unusual to find a few leaves with some of the structures in any sample of decaying turf, regardless of turf species. However, when they occur in great abundance and without any obvious evidence of other pathogens, the likely conclusion is that the Leptosphaerulina is at least responsible for hastening the decline of turf. Unlike the dollar spot and brown patch pathogens that spread via mycelium, this fungus spreads via rain-splashed spores–you can see the dark spores near the “volcanoes” in Figure 2. I think the steady rains in early-mid summer (dispersing potentially infectious spores) contributed heavily to the extraordinary amounts of Leptosphaerulina on Poa species.
Information on the nature of Leptosphaerulina blight is relatively scarce. The good news is that infection on healthy turf is unlikely. Symptomatic turf is expected to recover (albeit slowly) without any special treatment as conditions that favor turf growth return at summer’s end. Given the lack of strong evidence of the infectious nature of the Leptosphaerulina, remedial treatment with fungicides is not advised.