These pathogens produce motile spores, zoospores, that move freely in water. Zoospores infect the root behind the root tip. If root growth is reduced and slowed, these pathogens have plenty of time to germinate and penetrate into the root. If root growth is optimal, the growing root may outgrow the germinating zoospore and escape infection. Stressed and injured roots are more susceptible to root infection than healthy roots.
Roots are leaky.
They are constantly exchanging water, sugars, and gases with the surrounding soil. Injured or burned roots are especially leaky. These roots tend to attract root pathogens because some, like the water molds, can follow a chemical gradient in the water to the injured root. Wounds also provide a direct entry point for many pathogens. Some root pathogens, such as Fusarium and Rhizoctonia , are favored by stress conditions including excessive drying and rewetting of the soil medium.