Answer: Necrotic ringspot is caused by a soil-borne fungus called Ophiosphaerella korrae. The fungus survives from year to year on dead, colonized bluegrass roots and crowns or on the surface of living roots. The fungus actively colonizes the outside of roots at soil temperatures between 65? and 80? F. These temperatures occur from mid-May through mid-September along the Front Range, but may vary somewhat on the Western Slope and at higher elevations. The fungus eventually penetrates roots and colonizes the root cortex. This either debilitates or kills roots and leads to a decrease in water and nutrient uptake by the plant. Plants with extensively damaged root systems are more prone to injury or death, particularly during periods of high temperature or drought stress. Thus, NRS symptoms often don?t show up until July or August even though root colonization begins in May and continues throughout the summer.
Necrotic ring spot continues to be a very difficult disease to manage because of its perennial nature and inconsistency in control with fungicides. Banner Max, Spectator, Chipco 26019 and Fungo are fungicides that should help.
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