Pythium blight first appears as small, irregularly shaped spots ranging from 12 mm to 100 mm in diameter. Leaves appear water-soaked in appearance at first, then shrivelled. Diseased patches fade to a light-brown or grey colour.
Groups of spots frequently join together. At times, the shape of the affected areas may resemble elongated streaks. Both the presence and pattern of these streaks seem to be determined by the flow or presence of surface water.
With high humidity in early morning or throughout the day, diseased leaves may be covered with the white, cobwebby, moldlike growth of the causal fungus that can grow very rapidly.
Primarily a warm, wet weather disease, turf blighting and disease development will be most rapid and severe at air temperatures from 25°C to 30°C. As the air temperature approaches 30°C, destruction of grass strands can occur in a very short time. The disease develops more rapidly when nitrogen levels are high, and more slowly under moderate or low nitrogen fertiliser programs.
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