Spring and early summer bring leaf spot diseases to cool-season grasses in home lawns, parks and grounds, on sports fields, and occasionally on golf courses. These diseases are caused by several fungi in the genera Curvularia, Bipolaris and Drechslera. Every cool-season turfgrass species has at least one leaf spot disease associated with it, but the most common and well researched of the spring leaf spot diseases is Leaf Spot and Melting Out of Kentucky Bluegrass, so it gets the most attention here.

Symptoms and Signs

The causal fungi initially penetrate and infect leaf tissues producing small brown spots. These spots on infected leaves expand and develop dark purplish or reddish-brown oval borders with tan centers. The spots are often described as “football” shaped (Photo 1). Infected leaves yellow as the disease progresses and senesce prematurely. This process can thin turf stands, but most modern turfgrass varieties can tolerate minor infections and the loss of a few leaves, so the thinning is generally minimal (Photo 2).

Photo 1: “Football” shaped leaf lesions on Kentucky bluegrass caused by Drechslera poae. Photo: Sabrina Tirpak, Rutgers PDL

Under ideal conditions for disease development, or with very susceptible cultivars, the crowns and roots of infected plants also become diseased, which results in a brown or black rot of the crown tissue. Once the crown becomes dysfunctional, individual plants begin to fail as temperatures rise and the evapotranspiration demand on the grass increases into the summer months. This usually results in severe thinning of the turf stand and is known as the “melting-out” phase of the disease (Photo 4).

Photo 2: Yellowing and thinning Kentucky bluegrass by Drechslera poae. Photo: Richard J. Buckley, Rutgers PDL

Conidia (asexual spores) produced by the these fungi grow on stalks (conidiophores), are olivaceous (dark), and are shaped like cigars (Photo 3). They are only visible with the aid of a microscope or a high-magnification hand-lens. Trained diagnosticians can determine the genus and species of each leaf spot fungus based on spore germination characteristics as well as the size and shape of the spores.

Photo 3: Conidia of Drechslera siccans on a conidiophore. Photo: Richard J. Buckley, Rutgers PDL

Disease Cycle

The causal fungus of Leaf Spot and Melting Out of Kentucky Bluegrass, Drechslera poae, survives the winter in crowns and roots of infected turfgrass plants and/or on turf debris (thatch). This fungus begins to grow and reproduce during cool, wet weather in late-winter and early-spring beginning in March. Conidia produced by the fungus are splashed by rain or irrigation water to newly emerging grass leaves causing leaf spots. Successive generations of spores and leaf spot symptoms are produced during mid-to-late spring with the highest concentrations of conidia found on leaf litter during May at temperatures between 50 and 65^oF. Research suggests that conidial production ceases at temperatures over 68^oF. If conditions are favorable for disease development in late-spring, infections of the crowns result in the melting-out phase of the disease by summer. Melting-out causes large areas of previously thinning turfgrass to fail.

Be aware that not all of these closely related pathogens follow the same disease-cycle. Differences among the various causal fungi are generally related to host and the temperature range necessary for disease activity. Table 1. outlines the group.

Cultural Conditions that Favor Disease

Cultural conditions favoring leaf spot disease include: mowing at low cutting heights, light and frequent irrigation cycles, and excessive nitrogen fertilizer applications in early spring. These diseases can also be severe on turfgrasses with nitrogen deficiencies. There is evidence that turf stands (KBG) with excessive thatch accumulations can have more severe disease outcomes. Although many improved cultivars of Kentucky bluegrass and perennial ryegrass have a high degree of leaf spot resistance, other older cultivars—often found in inexpensive seed mixtures or as components in tall fescue sod—are susceptible (Photo 4). These cultivars can sustain severe damage and will suffer recurring disease outbreaks in a single season and over multiple years.

Management

Genetics is everything with leaf spot diseases. The single, most effective cultural practice for preventing severe damage is to plant leaf spot-resistant turfgrass cultivars whenever possible. Lists of leaf spot-resistant grasses can be found on the National Turfgrass Evaluation Program website (NTEP.org). Another important cultural practice is to apply only moderate amounts of nitrogen fertilizer at a time (0.5 lb. nitrogen/1,000 sq. ft.), particularly in the early spring. If higher rates are necessary, using products that are formulated with at least 50% of the available nitrogen in a slow release form are desired to avoid succulent and over-stimulated foliage. Mowing lawns at cutting heights above 2.5 inches during spring may help to reduce the severity of leaf spot and melting-out. If irrigation is required, watering inputs should be made to prevent drought stress and sustain growth, without creating an overly wet canopy. Thatch accumulations should be mechanically removed.

Photo 4: Melting out of Kentucky bluegrass due to Drechslera poae. Note the damage to the older cultivar compared to the improved cultivar. Photo: unknown

Many fungicides are labeled and efficacious for the control of leaf spot diseases. Timing matters for disease outcomes and preventative fungicide applications provide the best results. Make applications to susceptible grasses in April at the first evidence of leaf infection. Products containing iprodione, chlorothalonil, mancozeb, fluazinam, fludioxonil, azoxystrobin, or penthiopyrad have demonstrated good control of leaf spot diseases in University Trials. Be aware that some research trials have shown enhanced leaf spot disease activity with thiophanate-methyl products, even though they are labeled for leaf spot control. For a complete list of labeled materials and their efficacy, see University of Kentucky’s information bulletin:

Chemical Control of Turfgrass Diseases 2020. Bruce B. Clarke, Department of Plant Biology, Rutgers University; Paul Vincelli, Department of Plant Pathology, University of Kentucky; Paul Koch, Department of Plant Pathology, University of Wisconsin-Madison; Gregg Munshaw, Department of Plant and Soil Sciences, University of Kentucky.