Spread from plant to plant as mold spores form and disperse, pythium disease is caused by lingering bacteria in your soil, which may have been thriving down there for years - it’s hard to get rid of, and can resurface even when it seems dead.
Known to thrive in especially hot or humid weather and climates, you’re more likely to be susceptible to Pythium Blight when daily temperatures are hitting between 80 and 95 degrees Fahrenheit on a regular basis.
That being said, even when it’s colder and there is continuous rainfall, Pythium can still thrive: it’s just more likely to appear during what’s known as the “150 rule” when adding up temps for the day and night gives 150+ degrees Fahrenheit.
Slowly infecting leaves or grass one single blade at a time, it will appear in distinctive circular patches, often with trails leading away as a result of foot traffic, lawnmowers, or other vehicles dragging water that contains the mold spores across your lawn.
Areas with soil that is especially rich in nitrogen, where plants grow lush and free, are also more likely to be susceptible to Pythium; likewise, alkaline soles or those lacking in calcium can also be more easily affected.