Figure 1

Figure 2

Figure 3

Figure 4

Warm, wet and humid weather is stressful to the cool season turf grasses we grow. Common lawn care practices such as close mowing, applications of herbicides, excessive nitrogen fertilizer, or newly laid sod that lacks good root contact with the underlying soil can predispose lawns to disease outbreaks and exacerbate disease problems that are already present.

Symptoms
Because large turf areas may become uniformly blighted or appear patchy (Fig. 1), diagnosis of Leptosphaerulina blight can be confused with the "patch diseases." However, careful examination of individual leaves will reveal a pattern where the grass begins dying back from the tip down to the leaf sheath (Fig. 2), unlike other foliar disease problems, which cause scattered leaf spots throughout the blade.

Signs
Careful examination of the dead and dying leaf blades will reveal numerous reproductive structures or fruiting bodies (perithecia) where spores (Fig. 3) are produced that allow the disease to spread. Conclusive diagnosis can only be established through the observation of spores by hand lens or microscope (Fig. 4).

Epidemiology
The spores are produced and released in warm, wet weather and are then blown, splashed, and transported by shoes or lawnmowers to healthy leaves. The spores germinate and penetrate the leaf blades in a film of moisture or enter through wounds. The life cycle is repeated, enlarging dead patches as long as the weather permits. Ultimately, this fungus overwinters as perithecia and mycelia in dead grass tissue.

Management
Leptosphaerulina leaf blight is considered a weak pathogen that attacks stressed lawns. Implementing the proper cultural practices can manage this disease. Poorly laid sod, thick thatch layers, or low spots promote the development of Leptosphaerulina. Fungicides containing the active ingredient Iprodione are available to prevent further outbreaks, but are of limited use once disease symptoms are visible. With the appropriate environmental conditions, this disease will flare up on a regular basis if the underlying cultural disorders are not corrected. Cultural practices that maintain healthy turf and prevent establishment of Leptosphaerulina blight include:

• Dethatching: Thatch is the layer of dead grass that develops between the soil and the grass. All lawns have thatch, however, to keep lawns healthy, you need to dethatch in the spring or fall (preferably fall) if the thatch layer is over ½" thick. Dethatching may need to be repeated for several years to reduce the thatch layer. Dethatch in early fall when the temperatures are cooler; dethatching in hot weather can cause severe damage to lawns.

• Aerating: A core aerator removes plugs of soil and lawn and deposits them on top. Although many homeowners find this unsightly, aeration will reduce compaction, increase nutrient and water penetration, and increase microbial activity, which helps decompose the thatch layer. Aeration is a particularly important step to rectify underlying root development problems due to poorly laid sod.

• Fertilizing: Properly fertilize lawns by following the recommendations of a soil test. Tests are available from the U of MN Soil Testing Lab (612-625-3101). Increased nitrogen fertility (2-3 times per year) in the form of a slow release fertilizer will help reduce disease. Do not apply fertilizer during periods of drought or high temperature.

• Watering: Water throughout the summer only when necessary. Water the lawn to a depth of at least 6 inches at each irrigation. Repeat every 7 to 10 days if the weather remains dry. Avoid light sprinklings, especially in late afternoon or evening, which favors the fungus that causes this disease.

• Pruning the overhead canopy: Heavy shade has been associated with severe outbreaks of Leptosphaerulina. Selectively pruning dense trees, shrubs, and hedges bordering the turf area. This increases both light penetration and air movement to the lawn, which encourages drying of the grass surface, preventing disease.

• Mowing: Mow grass at a height of 2½" to 3½" in the summer, and never remove more than 1/3 of the leaf blade (If your lawn is kept at a height of 3 inches, mow when it reaches 4.5 inches). Keep mower blades sharp and do not mow when grass is wet.

• Reseeding: Large, dead patches of grass will likely require reseeding. Reseeding dead areas with resistant varieties of bluegrass may expedite the recovery process. Although there are no reported grass varieties having resistance to Leptosphaerulina, many varieties of Kentucky bluegrass are available with good growth characteristics and resistances to other diseases that may predispose turf to Leptosphaerulina. Some suggested cultivars are Adelphi, America, Eclipse, Kelly, and Midnight. Be cautious when using weed control products during reseeding, as most can damage grass seed.

• Fungicides: Fungicides do not cure damaged grass. Fungicides are only applied as preventative treatments to keep the disease from spreading by preventing new infections. Fungicides containing the active ingredient Iprodione are labeled for use in home lawns. A professional applicator must apply this fungicide.

References
Compendium of Turfgrass Diseases, Second Edition. APS Press. p.19.
Managing Turfgrass Pests. Thomas L. Watschke , Peter H. Dernoeden , and David Shetlar. Lewis Publishers. pp121-22.

P327L
10/02