Turfgrass Diseases: Leaf Spot and Melting-Out Diseases (Causal Fungi: Bipolaris and Drechslera spp.)

These diseases are incited by fungi in the genera Bipolaris and Drechslera. Every cool-season turfgrass species has at least one leaf spot and melting-out disease associated with it, but Kentucky bluegrass and perennial ryegrass are most severely affected. Although many improved cultivars of Kentucky bluegrass and perennial ryegrass have a high degree of leaf spot resistance, other cultivars (often found in inexpensive seed mixtures) are susceptible and suffer injury on an annual basis.

Symptoms and Signs

The causal fungi of leaf spot and melting-out diseases penetrate and infect leaf tissues, producing small brown spots. As the disease worsens, spots on leaf blades expand and develop dark purplish-red or brown oval borders around tan centers. Yellowing of leaf tissue often occurs in zones surrounding spots. Spots enlarge until the entire width of the leaf blade is blighted and the leaf turns yellow, tan, and/or brown. The leaf-spotting phase is less damaging to turfgrasses than the melting-out phase of the disease. In melting-out, the crowns and roots become diseased, resulting in a brown or black rotting of tissues and severe thinning of turf. Spores produced by Bipolaris and Drechslera spp. are shaped like cigars and only visible with the aid of a microscope. Trained diagnosticians can determine the genus and species of leaf spot and melting-out fungi based on spore germination characteristics as well as the size and shape of spores.

Disease Cycle

This disease cycle outlines the most problematic leaf spot disease in Pennsylvania, spring leaf spot and melting-out of Kentucky bluegrass. The causal fungus, Drechslera poae, survives the winter in crowns and roots of turfgrass plants or on turf debris on or in the soil and becomes active during the cool, wet weather of spring (April to early May). Spores produced by the causal fungus are released and splashed by rain or irrigation water to newly emerging grass leaves. The spores germinate, producing hyphae that penetrate and infect leaves, causing new leaf spots. Successive generations of spores and leaf spot symptoms are produced during mid-to-late spring. If conditions are favorable for disease development, spores wash onto the crowns and roots, inciting the melting-out phase of the disease, usually by late spring or early summer. Not all leaf spot diseases cause injury in spring. During summer, leaf spot and melting-out fungi can infect and cause disease in nearly every cool-season turfgrass species.

Disease Development

Leaf spot and melting-out diseases can develop at different times of the growing season, and under a variety of environmental and cultural conditions. As previously mentioned, spring leaf spot and melting-out of Kentucky bluegrass typically occur during wet and cool conditions. Other leaf spot and melting-out diseases take place in warm, wet weather, including those affecting perennial ryegrass, tall fescue, the fine fescues, and creeping bentgrass.

Cultural conditions favoring leaf spot and melting-out diseases include infrequent 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. Turfgrass cultivars with a high degree of susceptibility to leaf spot and melting-out diseases often sustain severe damage and can suffer recurring disease outbreaks in a single season and over multiple years.

Cultural Control

The best cultural practice for preventing severe leaf spot and melting-out disease damage is to plant leaf spot-resistant turfgrass cultivars whenever possible. Another important cultural practice is to apply only moderate amounts of nitrogen fertilizer (0.5 to 1.0 lb nitrogen/1,000 sq ft) in early spring. Frequent mowing of lawns at heights of 2.5 to 3.0 inches during spring may help to reduce the severity of leaf spot and melting-out. If irrigation is required in summer, watering cycles should be long enough to wet the soil thoroughly so that the frequency of watering to prevent wilting and drought stress is reduced.

Chemical Control

Fungicides effectively control leaf spot and melting-out diseases, but correct application timing and product selection are critical. For spring and summer leaf spot, preventative fungicide applications, or applications in the early stages of disease development provide the best results. Products containing iprodione, chlorothalonil, mancozeb, fludioxonil, azoxystrobin, or penthiopyrad typically provide good control of leaf spot diseases. Some research trials have shown enhanced leaf spot disease activity with thiophanate-methyl products, even though they are labeled for leaf spot control. Fungicides applied at the melting-out stage usually produce little improvement. Recovery of the turf at this stage becomes a matter of encouraging regrowth of the thinned turf or overseeding.

*FRAC is an abbreviation for Fungicide Resistance Action Committee. The FRAC code/resistance group system consists of numbers indicating classes or groups of fungicides based on mode of action, and letters that refer to broad classifications of fungicides (P = host plant defense inducers; M = multi-site fungicides; and U = unknown mode of action and unknown resistance risk). Due to the risk of fungicide resistance, turf managers should avoid excessive use of fungicides within the same FRAC code/resistance group and alternate products among different FRAC codes/resistance groups.

**Plant mobility classification refers to a fungicide’s ability to penetrate plant surfaces or remain on plant leaf or stem surfaces without penetration. Fungicides that penetrate plant surfaces and are translocated mostly upwards through plant xylem tissues are called acropetal penetrants (acropetal = toward the apex). Fungicides that enter plant cuticles or move limited distances in internal plant spaces, but do not translocate through vascular tissues (xylem and/or phloem) are called local penetrants. Contact fungicides do not penetrate plant surfaces and only inhibit fungal pathogens residing on leaf and stem surfaces.

***Follow label precautionary statements, restrictions, and directions regarding tolerant turfgrass species, rates, and timing of applications.

References

Buhler, W. Fungicide spraying by the numbers.

Clarke, B.B., P. Koch, and G. Munshaw. 2020. Chemical control of turfgrass diseases 2020. University of Kentucky, Rutgers University, and University of Wisconsin.

Latin, R. 2011. A practical guide to turfgrass fungicides. American Phytopathological Society Press, St. Paul, MN.

Smiley, R.W., P.H. Dernoeden, and B.B. Clarke. 2005. Compendium of turfgrass diseases, 3rd Edition. American Phytopathological Society Press, St. Paul, MN.

Smith, J. D., N. Jackson, and A.R. Woolhouse. 1989. Fungal diseases of amenity turfgrasses. 3rd ed. E. and F. Spon, London.