Rhizopus head rot is the most common fungal disease of sunflowers, and it can result in losses of up to 100%.
It starts with a tiny wound and ends with the loss of the entire head.
Rhizopus head rot is caused by three different fungi: Rhizopus stolonifer, R. oryzae (syn. R. arrhizus), and R. microsporus. These fungi are everywhere. They are found in the soil and are easily disturbed and spread on the wind. These are the same fungi that cause bread molds, and soft rots in carrots, melons, raspberries, sweet potatoes, and many other crops.
Rhizopus head rot symptoms
The fungal spores that cause Rhizopus head rot first make their way into your lovely sunflowers through wounds caused by birds, hail, rubbing, and head moth and other insect feeding. At first, these wounds look like small holes or dark spots on the back of ripening heads. Those spots start to rot, eventually drying to a dark brown. As the disease progresses, heads dry prematurely, and the interiors take on a shredded appearance.
You can differentiate this disease from others, such as bacterial head rot or Sclerotinia head rot, by the presence of grey threads (mycelium) and tiny black reproductive structures (sporangia) within the shredded tissue.
Rhizopus head rot management
High temperatures and high humidity set the stage for this disease. There isn’t much you can do about those besides avoiding overhead watering. There are no chemical treatments for this condition.
Wound prevention is the best way to prevent this disease from robbing you of all those delicious seeds. That means monitoring for bird and insect damage and possibly staking plants to prevent rubbing. If you live in areas with hail, there isn’t much you can do short of providing your sunflowers with umbrellas.
You can reduce the likelihood of Rhizopus head rot in your sunflowers by removing rouge plants that may harbor the pathogen and insects most likely to feed on sunflowers. While there are no resistant cultivars, sunflower varieties with more upright heads seem to be more susceptible.
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