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Disease Cycle and Epidemiology

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Pythium aphanidermatum and P. ultimum survive as oospores in turfgrass root zones and thatch, and as vegetative mycelium in turfgrass leaves and roots. When environmental conditions are not conducive for disease development, survival as sporangia, zoospores and mycelium is considered to be short-lived. Under favorable conditions for disease development mycelium can resume growth and infect a grass plant in 1-2 hours. Infection of turfgrass leaves occurs by direct penetration from vegetative hyphae or zoospores. Zoospores, which are released from vesicles produced by sporangia, require free water to move and infect other turfgrass plants. Pythium blight can also spread to other adjacent susceptible grass plants by mycelial growth. Infested material in the form of soil and thatch can spread the disease when transported to other areas. Movement of Pythium propagules can also occur on infested mower clippings, as well as dispersal of zoospores on equipment when conditions are wet.

Epidemiology

Pythium blight can occur during periods of cool (13 - 18°C / 55 - 64°F), wet weather, but the disease is most severe during periods of hot (30-35°C / 86 - 95°F), humid, rainy or cloudy weather. Pythium aphanidermatum is most actively pathogenic when temperatures are between 30° and 35°C (86 - 95°F) and nighttime temperatures remain above 21°C (70°F). The highest frequency of infection occurs during periods of high relative humidity (>90%) and prolonged leaf wetness.

The susceptibility of some turfgrasses to Pythium sp. can be enhanced by specific environmental factors. For example, high soil salinity predisposes creeping bentgrass to infection by P. aphanidermatum. Soil salinity is normally a problem in arid environments where salt levels fluctuate throughout the year based on water quality and the frequency and duration of irrigation events. During periods of high salinity, Pythium blight can occur at air temperatures and atmospheric humidities that are lower than those normally thought to be favorable for disease development. In addition to soil salinity, drought stress may enhance turfgrass susceptibility to Pythium infection. For example, creeping bentgrass is more susceptible to attack from P. ultimum when grown at low soil moisture (-1.5 MPa) than at field capacity (-0.033 MPa).

Turfgrass site factors that favor the development of standing water enhance the spread of Pythium spp. Fertility can also have an impact on Pythium blight. High levels of nitrogen fertilization enhance the severity of Pythium blight by creating a lush turf canopy. Variations in calcium nutrition, particularly a calcium deficiency, can predispose plants to Pythium blight.