Figure 1. As the rhizopus head rot develops in sunflower, a watery soft rot will develop on the backs of the heads.  (Photos by Bob Harveson)	Figure 2.  The disease can appear on the flower side of the sunflower head as a grayish, fuzzy substance covered with sporangia, the small, black reproductive fruits of the fungus.

July 26, 2012

A Disease Affiliated with Mechanical Damage to Sunflower Heads

Head rot disease is caused by several species in the genus Rhizopus, including R. stolonifer, R. arrhizus, R. oryzae, and R. microsporus. These species may occur singly or in a complex, and all are found naturally in soil or as a common airborne contaminant. This fungus is well known for causing soft rots of fruits, vegetables, and root crops, especially in postharvest storage situations.

Rhizopus head rot is one of the few diseases caused by this group of fungi that occurs in field crops during the cropping season. It has historically been considered to be of minor importance in the United States and Canada; however, under the right environmental conditions (see Disease Cycle, below) it has been shown to be a serious disease in the High Plains, sometimes causing 100% loss in severely affected fields.

Symptoms

The disease first appears as dark spots on the back of ripening heads, followed by a watery, soft rot that later turns dark brown (Figure 1). As the disease progresses, heads dry prematurely and shrivel, and tissues appear to shred. The fungus can be observed inside shredded heads as coarse, threadlike mycelial strands that are later followed by the fungal reproductive (fruiting) structures called sporangia. These look like small black dots about the size of pinheads. The disease also can be recognized on the flower side of the head by the appearance of a grayish, fuzzy substance (Figure 2) covered with sporangia. These dark fruiting bodies are sacks filled with spores. They are easily broken open to release spores that are spread on wind currents to infect neighboring plants.

Disease Cycle and Damage

Even though the spores of Rhizopus spp. are found almost everywhere in the environment, mechanical injury on the heads is a prerequisite for infection and disease development. Once the head is infected, the disease will develop in warm, humid environments, especially under irrigation. The cycle continues under high humidity conditions as windborne spores are carried to sunflower plants and infection is initiated through wounds created by hail (Figure 3), birds or insects.

In addition to the actual wounding and infection, damage and economic losses are highly dependent upon when the wounding and infection occur. Research shows that head rot rarely occurs before flowering, so it appears that mature tissues are required to support the growth of Rhizopus spp.

Yields are reduced because seeds in infected heads will fail to fill properly and will have reduced weights. Yield also can be affected, even if infection occurs late in the season, when seeds from shredded heads fall to the ground and can’t be harvested. If the peduncle (stem below head) becomes severely infected, the head can fall off completely, increasing the potential for severe yield loss.

Lastly, oilseed growers may also be adversely affected by head rot due to bitter or poor quality oils obtained from infected plants.

Management

Follow these steps to limit damage to your sunflowers from Rhizopus spp:

• Avoid mechanical damage after flowering.
• Control head moth infestation before or at flowering. (See how in a related CW story, Sunflower Head Moth Outbreak is Coming on Strong)
• Remove wild sunflowers that may serve as reservoirs for insects and pathogens before they produce seed.
• Control bird feeding:
  —  Select varieties with head types that turn down after flowering.
  — Avoid planting sunflowers near water that consistently harbors many birds.

Robert Harveson
Extension Plant Pathologist, Panhandle REC, Scottsbluff