It first appears as dark spots on the back of ripening heads, followed by a watery, soft rot that later turns dark brown. As the disease progresses, heads dry prematurely, shrivel, and tissues appear to shred. Inside the shredded heads the fungus can be observed as coarse, threadlike mycelial strands that later are followed by the appearance of small, black dots about the size of pinheads. The disease also can be recognized on the flower side of the heads by the appearance of a grayish, fuzzy substance covered with small dark dots. These dark fruiting bodies are sacks filled with spores that are easily broken open and released through wind currents to infect neighboring plants.
Symptoms often can seem to appear simultaneously on opposite sides of the head.
The disease has also been shown to cause a systemic effect on the plant on the side where the head became infected. If the peduncle becomes infected, heads can completely fall off. This disease is more severe in warm, humid environments, especially under irrigation. Even though spores of Rhizopus spp. are found everywhere, mechanical injury on heads is a prerequisite for infection and disease development. In Israel, it was determined that bird feeding on seed heads was the primary method for disease spread and development.