Although most of the important (very damaging) turf diseases happen during the warmer parts of the growing season, there are a few that do quite well under prolonged cool, wet weather. Among the most difficult cool-weather diseases to identify are Fusarium patch (also called pink snow mold or Microdochium patch) and yellow patch (also called cool-weather Rhizoctonia). Both are foliage pathogens that have been observed in Illinois over the last month or so, and they are easily confused with each other.

Fusarium patch (not to be confused with summer patch or Fusarium blight, which are root diseases!) is common and can be troublesome any time of the year during cool to cold, wet weather. In fact, in 1997, this disease was active into early June. The pathogen affects a wide range of turfgrass species but may be particularly damaging on annual bluegrass, bentgrasses, Kentucky bluegrass, and ryegrasses. The disease first appears as roughly circular, water-soaked spots, 1 to 3 inches in diameter, which soon turn yellow or reddish to dark brown. With continued cool, wet weather, these patches may enlarge to a foot or more in diameter. You may notice small amounts of white or pale pink fungal growth within or at the leading edge of the patch, particularly following any snow melt or cold, rainy weather. Fusarium patch is often most damaging when there is prolonged snow cover or when the turf remains moist through much of the winter and early spring.

Fungicide applications at this point are probably not warranted because the damage is already done and warmer, drier conditions, which will shut down the pathogen, are sure to return. However, some management practices will reduce the potential damage of this disease in the future. In fall, it is important to continue to mow the turf until it stops growing. In addition, maintain adequate phosphorus (P) and potassium (K) soil-test values, but avoid overstimu-lation with any source of nitrogen. Research indicates that high soil pH (alkaline) increases disease potential. Where the disease continues to be a perennial problem, resistant varieties of ryegrasses, fine-leaf fescues, and bentgrasses should be considered. Although no varieties are available that are considered highly resistant to this disease, some level of resistance is better than none.

If all else fails to provide adequate results, preventive fungicide applications may be justified, beginning in the fall. Rather than using a “blanket approach,” the applications should target perennial problem areas. Refer to page 14 of the Illinois Commercial Landscape and Turfgrass Pest Management Handbook, 1998–1999 for a listing of labeled fungicides and application information. For more information on Fusarium patch, consult Report on Plant Disease No. 404.

Yellow patch is caused by the fungus Rhizoctonia cerealis, which is related to the Rhizoctonia brown patch pathogen, R. solani. However, yellow patch is a much less destructive species, which operates at cooler temperatures. Like Fusarium patch, yellow patch can be troublesome any time of the year during cool to cold, wet weather. The pathogen can affect a wide range of turfgrass species but is most commonly associated with bluegrasses and bentgrasses. The tan, to yellow, to bronze patches are often larger (a few inches to several feet in diameter) than that of Fusarium patch and appear in a variety of forms such as concentric rings, crescent shapes, or frog eyes (green turf in the center of the patch). Unlike Rhizoctonia brown patch, you will not see a smoke ring surrounding the patch; you may, however, see tan to white fungal growth down in the turf canopy. At the edge of patches, leaves of Kentucky bluegrass often have a reddish to purple appearance, beginning at the leaf tip. Damage is usually not severe, but there is often a thinning of the turf during prolonged cool, wet weather. A diagnostic feature for both species of Rhizoctonia is the presence of minute (1/4 mm to 2 mm), dark brown to black bulbils (sclerotia) on the surface of diseased grass tissues near the crown. If you’ve ever washed potatoes and noticed the very tiny, black pieces of “dirt” stuck to the skin, you have seen what I’m talking about here.

For the same reasons given above, fungicide applications at this point are probably not warranted. To reduce the potential damage of this disease in the future, maintain adequate phosphorus (P) and potassium (K) soil-test values, but avoid overstimulation with any source of nitrogen. In addition, reduce any excess thatch to 1/4 inch to 1/2 inch thick. Information about resistant varieties is limited. If all else fails to provide adequate results, preventive fungicide applications may be justified in the fall. Yellow patch is one of the more difficult diseases to control chemically, but Prostar is reported to be somewhat effective. Again, focus only on the perennial problem areas. For more information on yellow patch, consult Report on Plant Disease No. 411.