Rhizopus soft rot
| Rhizopus soft rot | |
|---|---|
| Causal agents | Rhizopus stolonifer |
| Hosts | sweet potato |
| EPPO code | RIZPST |
Rhizopus soft rot is a disease of the sweet potato. It is one of the most common to affect the sweet potato, happening during packing and shipping. The disease causes a watery soft rot of the internal portion of the storage root. Strategies to manage the disease include the development of resistant varieties, curing through the use of heat and humidity, and application of decay control products.
Sweet potatoes are susceptible to a number of diseases during the postharvest storage period and during shipping. The most common are Rhizopus soft rot (Rhizopus stolonifer), bacterial soft rot (Erwinia chrysanthemii), Fusarium root rot (Fusarium solani), Fusarium surface rot (Fusarium oxysporum), and black rot (Ceratocystis fimbriata).
R. stolonifer is a problematic pathogen as it infects fresh wounds occurring during packing and shipping. There is limited data on the exact losses attributed to Rhizopus soft rot. A study conducted in the New York City retail market found that the majority of culls due to disease were caused by Rhizopus soft rot (approximately 2% decay in survey). Anecdotal reports suggest that Rhizopus soft rot is unpredictably sporadic and generally results in heavy losses to entire shipments when it does occur.
R. stolonifer has a wide host range and can affect over 300 plant species including fruits, vegetables, and ornamentals.R. stolonifer (Ehrenb. ex Fr.) (syn R. nigricans) was first described in 1818 and first recognized as a pathogen on sweetpotato in 1890.
Symptoms of R. stolonifer infection of sweet potatoes include rapid development of a watery soft rot of the internal portion of the storage root with the periderm generally remaining intact. Infection can occur anywhere on the root but usually initiates at the ends due to the inevitable wounding resulting from harvest, or because a root's tapered ends are more likely to be injured. Rhizopus soft rot produces a characteristic fermentation odor. Roots may dry and mummify with only the periderm and root fibers remaining intact because of the inability of the fungus to break down the lignin in these components. Characteristic signs of Rhizopus soft rot include the production of tufts of white hyphae which break through the surface of the root and produce large numbers of brown-black sporangiophores (34 µm diam. by 1000-3500 µm length) which support a sporangium (100-350 µm diam.). Sporangiospores (4-11 µm diam) are produced in the sporangium and are unicellular, ovoid and brown. Sporangiospores serve as the primary and are passively released when the outer layer of the sporangium breaks down. Other R. stolonifer structures include stolons and rhizoids. Stolons arch over the surface and rhizoids grow into the substrate at each point of contact between stolon and substrate.
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