Rhizopus Head Rot
Rhizopus Head Rot (Rhizopus species)
Rhizopus head rot is a sporadic, but often destructive disease when it does occur.  The disease has been reported from all sunflower-producing countries around the world. Although it has historically been considered to be of minor importance in the United States and Canada, it has also been documented causing severe damage annually to sunflowers grown in Israel.  Furthermore, it commonly poses a serious threat to production in the high plains of the U.S.  Under the right environmental conditions, up to 100 percent losses have been noted from severely affected fields.
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Figure 1. Severe yield loss from Rhizopus head rot.
Rhizopus head rot is caused by several species in the genus Rhizopus, including R. stolonifer, R. oryzae (syn. R. arrhizus), and R. microspores, but R. oryzae is the primary species found in field infections. This fungus is well known for causing soft rots of fruits, vegetables, and root crops, particularly in postharvest storage situations. However, it is one of the few diseases caused by this group of fungi affecting field crops during the cropping season.
Symptoms of disease first appear on the backs of heads as dark spots of varying sizes as a result of some form of wound 
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Figure 2. Early infection of Rhizopus head rot, originating from a wound
followed by a watery, soft rot that later dries and turns dark brown.
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Figure 3. Infection advancing that becomes water, soft and brown.
As disease progresses, heads dry prematurely, shrivel and infected tissues become shredded.
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Figure 4. Dry and skeletonized head.
At this point, symptoms could easily be confused with other diseases such as bacterial, Sclerotinia, and Botrytis head rots.  Rhizopus head rot is distinguished from the others by exhibiting coarse, dirty white to grayish, threadlike strands of mycelial growth within affected heads.
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Figure 5. Coarse, dirty white to gray, threadlike fungal growth of Rhiozpus spp. in sunflower head.
The pathogen later produces reproductive structures (fruiting structures) called sporangia that look like small black dots about the size of pinheads.  These dark fruiting bodies are sacks filled with spores that are easily broken open to release spores spread on wind currents to infect neighboring plants. The disease can also be recognized on the flower side of the head by the appearance of the grayish, fuzzy mycelium that has grown through the heads after first entering on the back side.
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Figure 6. Infected unflower head covered with gray mold of Rhizopus spp.
The spores of Rhizopus spp are ubiquitously found in soils and are readily released into the air in most field environments.  Nevertheless, for infection and disease development, mechanical injury of some form is required to open wounds, allowing the fungal pathogen an entry point.
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Figure 7. Wound on sunflower head providing a point of entry for Rhizopus spp.
The disease is more severe in warm, humid environments, particularly under irrigation. Thus, as spores are carried to sunflower plants through air currents, and under conditions of high humidity, infection may be initiated through wounds created by a number of sources, including hail, birds, or insects.  In the Central High Plains of the U.S, summer hailstorms are common and will often contribute to initiation of wounding with subsequent infection.
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Figure 8. Severe damage to back of sunflower head caused by hail, which commonly will facilitate Rhizopus head rot infection.
Economic losses and yield damage are highly dependent upon several primary factors, including the number and severity of wounds, a humid environment, and the age of the plants at time of infection.  Head rot rarely occurs before flowering, so it is assumed that mature tissues are needed for optimal growth of Rhizopus spp.  Infected heads may still fail to fill properly producing reduced weights, thereby resulting in economic losses to confectionary seed growers as their payment is based on seed size.  Furthermore, if infection progresses down to the stalk, the head can fall completely off thereby becoming unable to harvest, increasing the potential for yield reductions.
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Figure 9. Drooping and decapitation of sunflower head caused by progression of Rhizopus spp. into neck and stem.
Very few definitive management recommendations can be made for this disease. No known effective fungicide or resistant hybrids are available. The primary techniques for managing Rhizopus head rot presently include avoiding mechanical damage to heads by controlling insect and bird feeding after flowering. Controlling weed sunflowers on field perimeters and ditch banks will also remove potential reservoirs of sunflower insects and pathogens. 
Images
Figure 1. Severe yield loss from Rhizopus head rot (Bob Harveson, University of Nebraska).
Figure 2. Early infection of Rhizopus head rot, originating from a wound (Bob Harveson, University of Nebraska).
Figure 3. Infection advancing that becomes water, soft and brown (Bob Harveson, University of Nebraska).
Figure 4. Dry and skeletonized head (Bob Harveson, University of Nebraska).
Figure 5. Coarse, dirty white to gray, threadlike fungal growth of Rhiozpus spp. in sunflower head (Bob Harveson, University of Nebraska).
Figure 6. Infected unflower head covered with gray mold of Rhizopus spp. (Bob Harveson, University of Nebraska).
Figure 7. Wound on sunflower head providing a point of entry for Rhizopus spp. (Bob Harveson, University of Nebraska).
Figure 8. Severe damage to back of sunflower head caused by hail, which commonly will facilitate Rhizopus head rot infection (Bob Harveson, University of Nebraska).
Figure 9. Drooping and decapitation of sunflower head caused by progression of Rhizopus spp. into neck and stem (Bob Harveson, University of Nebraska).
Additional Resources
Report from a 2017-2019 NSA-funded study ‘Quantification of Yield Loss from Rhizopus Head Rot in Sunflower’

Powerpoint presentation of a 2017-2019 NSA-funded study ‘Quantification of Yield Loss from Rhizopus Head Rot in Sunflower’
Other NSA Resources
Disclaimer statements
Information based in part on and reproduced from Kandel, H., Endres, G. and Buetow, R. 2020. Sunflower Production Guide. North Dakota Extension Publication A1995. Informational updates made possible by the Sunflower Pathology Working Group, and is/was supported by the USDA National Institute of Food and Agriculture, Crop Protection and Pest Management Program through the North Central IPM Center (2018-70006-28883).
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