Environment
Pythium aphanidermatum infects plants via motile zoospores, and because zoospores need to swim in order to infect the host, moist conditions facilitate the most rapid spread of the disease. Temperature also has an effect on the rate of pathogen propagation. The protist can cause disease in cool temperatures (55-64°F) but ideal conditions are between 86°F and 95°F, a characteristic which distinguishes it from other Pythium species. Potential host plants that are stressed are more susceptible to infection. Factors that may cause stress in plants and therefore increase the likelihood of infection include high saline conditions, drought, nutrient deficiencies, and excessive moisture around the plant. High saline content in the soil can promote infection at lower temperature and humidity than is ideal for the pathogen. Excessive nitrogen fertilization will also increase chance of disease because the nitrogen decreases the function of the plant’s innate defense response, and it also damages the ends of the roots, which are the primary mode of infection. Furthermore, the medium in which plants are grown dictates affects the severity of Pythium infection. Sterile soilless cultures are the most susceptible, while increasing soil content inhibits disease progression due to bacteria present in the soil. Finally, seedlings and plants that are germinating have greater susceptibility to the pathogen, and often experience damping off.
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