This disease is caused by a complex of soil-borne fungi. The spores germinate in the soil. Seedlings become infected following germination. Spores are produced on diseased tissue and are spread by wind, water, cultivation, and infected seeds. Further infections continue throughout the growing season with new spores being produced on plant parts as they mature.
Patchy emergence is usually the first indication that damage has occurred.
Infections start on the roots and sub-crown internodes and move to the leaves. Seedlings may die before or soon after emergence even though they only show slight damage. Drought and warm weather cause more severe infections. Nutritional stress can also favour infection. Secondary infections by other soil-borne pathogens may mask the initial symptoms.