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ISSUE 12  JULY 22, 1999



    After a major storm event in midsummer, many of the cool-season grasses (annual bluegrass,
Kentucky bluegrass and fine leaf fescue) will manifest this troublesome disease. Excess or heavy irrigation
during this period will also bring on these symptoms. Modeling studies have shown a direct correlation
between moisture pushing the oxygen out of the soil and the development of symptoms. Hence, one of
the first recommendations for control being core aeration, and the reduction of normal watering cycles.

    The other key factor in the development of symptoms is a soil temperature above 70 degrees F at a
depth of 2 inches for at least 48 hours. The disease activity actually begins when the soil temperature
reaches 65 degrees at this depth in late spring, but little to no damage is noted at that time. It is when
the heat stress of mid summer hits that the symptomatic conditions develop. Peat backed sod or turfgrass
that has very heavy thatch build-up are vulnerable to this disease. The peat sod is affected because of the
particle size interface difference between the sod and the heavier soil which interferes with the percolation
rate of the soil, causing moisture to collect and therefore deprive the soil of oxygen. Basically, the heavy
thatch accomplishes the same stress - oxygen deprivation - causing the turfgrass to re-root in the thatch
and not the soil.

    With oxygen deprivation, the turfgrass plants deprived of nutrients which weakens the plant, and allows
the pathogen to invade the vascular tissue which further interferes with water and nutrient uptake, and the
movement of photosynthates from the foliage to the roots.

Methods of Control:

    The summer patch organism (Magnaporthe poae) can be controlled via 3 avenues of management:
fertility; irrigation; and chemical means.

    With fertility, it is important to maintain adequate levels of nitrogen throughout the growing season.
This calls for pound of nitrogen per 1000 square feet every 3-4 weeks being required. The slow
release carriers (Restore, Milorganite, IBDU) have been more effective than the quick release forms
of nitrogen like urea.

    Secondly, irrigation is usually the most abused management practice in turf care. When the turf begins
to discolor as a result of this or other diseases, the natural response is to apply more water. This simply
compounds the problem by driving more oxygen out of the rootzone. What is needed is frequent, light,
applications of water through the hot part of the day. This would involve about a 2 - minute cycling of
each station on the irrigation clock at 10, 12, 2, and 4 during each hot day. Such applications would
not contribute to the "water load" in the root zone, but simply act as a "cooler" to help reduce the heat
stress on the turfgrass. This treatment acts more as a preventative than a curative, but should keep the
problem from intensifying.

    Chemical treatments come either as a preventative or curative. The preventatives must be applied
before symptom development occurs. Select from triadimefon or fenarimol, applying when the soil
temperature reaches 65 degrees F at 2 inches, and a second application 30 days later. For curative
treatments, use benomyl or thiophanate methyl, reapplying a second time about 3 weeks later. To be
effective, these fungicides need to be drenched into the rootzone.

    Remember, the objective in controlling this disease is to get oxygen into the root zone. Annual core
aeration followed by light, frequent nitrogen fertilization will help get the turfgrass back on track. Cures
do not come overnight, nor likely in a single season. With persistence in following the recommendations
outlined here, the disease should gradually subside.

Ron C. Smith
Extension Horticulturist and Turfgrass Specialist

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