Symptoms:

Despite the different names, these two diseases have essentially identical symptoms. As the names suggest, symptoms of necrotic ring spot and summer patch include rings and circular patches of dead or dying turfgrass. Sometimes there is a center of relatively healthy grass in the "doughnut" or "frog-eye" patch. In addition, weeds or non-susceptible turfgrass species may colonize the centers of patches where the diseased turfgrass has died. Both diseases are most severe on Kentucky bluegrass lawns that were seeded or sodded two to four years previously. Fine-leaf fescues are also susceptible. Bentgrass and perennial ryegrass lawns are not susceptible to these diseases. Symptoms often become obvious in warm to hot, dry weather when the lawn becomes stressed. Areas along driveways, walls, sidewalks and other places that are heat- and water-stressed will often show symptoms first. When the disease first begins, rings or patches of blue-green wilting turfgrass will develop in the heat of the day, but this early stage is not commonly observed. With time, the patches may coalesce, obscuring the circular pattern of the damage. In lawns containing mixed turfgrass species, symptoms are not as distinct, but circular patterns of diseased turfgrass are generally present. Both diseases are caused by root-infecting fungi. Above-ground symptoms may be confused with other diseases such as Rhizoctonia brown patch. However, the roots and crowns of plants with necrotic ring spot or summer patch will be brown to black and rotted.

Necrotic ring spot	Summer patch (photo from the Maryland Cooperative Extension)

Season:

Leptosphaeria korrae (necrotic ring spot) infects turfgrass in spring and fall in wet weather, but symptoms become severe when environmental stress conditions impact the diseased plants from early summer on.

Magnaporthe poae is the fungus which causes summer patch. It is more active in hotter weather, so summer patch symptoms are most severe in mid-summer, especially following a heavy rainfall. The times of occurrence of these diseases overlap, so it is not always possible to separate the diseases strictly by season or field symptoms.

Disease Cycle:

Both fungi produce very similar dark brown, microscopic "runner hyphae" that are found along the roots, crowns, and rhizomes of turf-grass plants (it is not possible to separate the diseases by looking only at these hyphae). In both diseases, infection occurs before symptoms appear. The fungi spread from plant to plant with the runner hyphae and are spread longer distances when feet and mowers spread spores, soil, or infected plant parts.

Cultural Management:

Severe damage is most common on sodded Kentucky bluegrass lawns that have been planted on poorly prepared sites with poor drainage and compacted soil. It is important to properly prepare a lawn site with well-drained topsoil, correct pH, and balanced fertility. When possible, plant Kentucky bluegrass cultivars with resistance to these diseases or consider using sod with perennial ryegrass in the mix. This will help mask areas of infection. Similarly, overseeding with resistant cultivars and perennial ryegrass will improve already damaged areas. Once an area has been seeded or sodded, avoid stressful growing conditions. Mow at a recommended height and frequency. Water deeply, especially in areas of heat and moisture stress. Some research indicates that light mid-day irrigation in addition to routine irrigation will relieve plant stress and improve natural microbial activity against the fungus responsible for necrotic ring spot. Maintain balanced fertility according to a soil test and avoid high rates of nitrogen before hot weather. There is evidence that slow-release nitrogen and some organic fertilizers reduce disease severity. Ammonium-based nitrogen sources lower root zone pH which has been shown to reduce summer patch. Remove excess thatch and aerate compacted areas.

Chemical Management:

Penetrant, systemic fungicides can help prevent these diseases but are not generally cost-effective for use on lawns and grounds. Considerable damage to the root system has usually already occurred by the time symptoms are noticed, so curative treatments with fungicides may offer disappointing results. Because the infection occurs in roots, foliar applications of contact fungicides are not effective. Because of the expense and difficulty in fungicide applications to turfgrass roots, the cultural controls described above are strongly recommended for lawns.

Adapted from the UMass Extension, 2000