If the concentration of a particular nutrient, such as nitrate, is higher in the soil water than in its root, water from the root will move out of the root and into the soil. If this movement is rapid, the cells collapse and die. This is referred to as burning. Plants may also respond to this effect by wilting. 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.