Pythium blight, also known as grease spot, can be a highly destructive lawn grass disease, especially on rye grasses. Severe outbreaks can completely destroy the lawn grass within a few days if weather conditions favour disease development.
Grass leaves appear water-soaked in appearance at first, then shrivelled. Affected patches fade to a light brown or grey or black colour. Groups of affected spots frequently join together to form a large affected patch. At times, the shape of the 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. In high humidity during early morning or through the day, diseased leaves may be covered with the white, cobweb, growth of the Pythium fungus.
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