Head rot of sunflower is a disease caused by various species of the fungal genus Rhizopus, including R. stoloni- fer, R. arrhizus, R. oryzae, and R. microsporus. Rhizopus is well known for causing soft rots of fruits, vegetables, and root crops, usually in storage situations. Head rot is one of the few diseases caused by this group of fungi that occurs in field crops. It has also been documented as causing severe damage annually in Israel. Under the right environmental conditions, it can be a serious disease in the High Plains and has been known to cause up to 100 percent losses in certain fields.
Rhizopus is a commonly occurring organism present in soils and as airborne spores.
It is carried to sunflower plants via air currents. Under conditions of high humidity, it can initiate infection through wounds created by hail, birds, or insects. 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.