Rhizopus Soft Rot

Rhizopus soft rot is one of those diseases that can come into your garden from the grocery store.

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Foods may look perfectly healthy when you bring them home. What you may not see are thousands of bacteria, fungal spores, or viruses that quickly make themselves at home in your garden. Some of these pathogens can take years to get rid of.

Rhizopus stolonifer on a strawberry showing black sporangia (Eric McKinzie) CC BY 3.0 au

Rhizopus soft rot hosts

Also known as black bread mold, Rhizopus soft rot is caused by Rhizopus stolonifer fungi. This fungal disease is commonly seen on sweet potatoes, strawberries, pears, peaches, melons, and mangos. These delicious crops have a high sugar content and are easily damaged. When these fungi appear in damaged almonds, we call it hull rot.


Rhizopus soft rot symptoms

Tiny wounds or bruises slowly become water-soaked areas. Those areas then turn soft and start to rot. When conditions are right, white thread-like mycelia appear with little black knobs (sporangia) on top. These fungi produce enzymes when they germinate that help them penetrate plant cell walls.

Rhizopus stolonifer (WDKeeper) CC BY-SA 4.0

Rhizopus soft rot behavior

The fungi that cause Rhizopus soft rot are surprisingly athletic. Like many plants in the garden, they spread rapidly using stolons or runners. These fungal runners can move up walls and tree trunks, as well as horizontally across branches, stems, and soil. These fungi thrive when temperatures are 68°F–86°F and they can survive in weather as hot as 140°F.


Rhizopus soft rot prevention

Once fruit begins rotting, there isn’t much you can do besides trim out the bad bits and eat the rest. There are several things you can do to prevent Rhizopus soft rot:

• Buy certified disease-free seeds and seedlings
• Plant resistant varieties
• Ensure good drainage
• Use crop rotation

If you are growing sweet potatoes, curing them properly after harvesting is the best way to prevent Rhizopus soft rot. This means storing them at 84°F at 90% humidity for 5 to 7 days.