Much of New England experienced unusually hot, dry weather throughout the month of June. During this time, both irrigated and unirrigated turf stands endured abnormally severe heat and drought stress. Cool season grasses typically react to such conditions by going dormant, and thus some shoot dieback is likely to be observed. Many of us hoped to see the grass perk up a bit when needed rain finally arrived in early July, but what we saw instead was rapid decline and a sudden, distinctive blackening of the turf. What in the world is going on?
On closer laboratory inspection of multiple turf samples, abundant Curvularia was observed on dead shoot tissue. Curvularia is a ubiquitous and talented saprophyte, subsisting on dead plant material and remaining unnoticed most of the time. Like most other fungi, Curvularia also appreciates wet, humid weather, and is at its most active under such conditions.
So here is the scene: as heat and drought stressed turf dies back, Curvularia quietly moves into the dead blades. When the rain finally comes, the fungus is ready to take advantage of the situation and proliferates like crazy, quickly producing dark spores and spore-bearing conidiophores all over the place, which turns the grass black. The stressed turf is not able to respond to the rain as quickly, and initially it appears as if the turf has been killed by the fungus. Eventually, turf crowns that survived the environmental stress suffered in June will produce new growth, while any crowns that have died as a result of drought stress will not. In one case, after dead material had sloughed off, 75% of turf crowns produced new shoots when irrigated regularly.
It is important to note that other things will also cause turf to turn black, including Pythium blight and smut diseases (caused by pathogens such as Ustilago spp. and Urocystis spp). While both of these have also been observed at low levels, they do not appear to be involved in the more widespread incidents of blackening turf.