In summer when the growing sunflower roots come in contact with sclerotia, the sclerotia germinate and infect the roots. The fungus grows upward in the infected root, into the tap root, and forms a canker at the base of the plant. The plant wilts and dies. This is called Sclerotinia wilt or stalk rot.
Sclerotinia wilt can occur any time that sunflower is planted into an infested field.
Its development is independent of weather conditions, with infection occurring at almost any soil moisture capable of supporting sunflower growth. When excessive rainfall saturates the soil, the sclerotia germinate to produce tiny mushroom-like bodies the size and shape of golf tees. The spores infect sunflower during wet weather at flowering time producing middle stalk rot and head rot. The spores do not infect healthy plant tissue, but need dead plant matter as a food source in order to begin growth. Dead flower parts are the preferred food source.