Rhizopus

Members of Rhizopus belong to a large group of fungi called Zygomycetes true fungi with well-developed mycelium and chitin in their cell walls. Zygomycetes produce nonmotile spores called sporangiospores. These fungi are terrestrial and saprophytes or weak pathogens causing soft rots and molds. Members of this group also include parasites of insects, nematodes, mushrooms, and humans, as well as dung fungi.

Members of the genus Rhizopus grow in soil and on fruits, other foods, and decaying organic materials. Characteristic feature of Rhizopus is the presence of rhizoids (short root-like appendages) at the base of spore-bearing structures.

Plant Pathogens

In sunflowers Rhizopus causes head rot. Initial symptoms of Rhizopus head rot are small- water-soaked spots on the back of the diseased head. The spots gradually enlarge and the mycelium invades the plant tissue. During wet weather, woolly masses of white fungus can be seen. Cultivars with upright head are more susceptible than cultivars with nodding heads.1



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Rhizopus solonifer is a weak parasite on ripe fruit - peach, fig, strawberries, citrus, persimmon, pear, avocado, and melons. A coarse cottony mold appearing over the surface is covered with pinhead-like structures (sporangia), white when young, black when mature. A watery fluid with an offensive odor leaks from the soft fruit.3 In sweet potatoes the fungus causes soft rot disease. Storage roots become soft, wet, and stringy, often starting at one end. A strong, alcohol-like odor is produced. Fungal aerial mycelium is commonly seen on the surface of rotting roots. The disease is spread by infested soil or air-borne spores that enter wounds.2 Rhizopus solonifer also causes a serious disease of strawberries called "leak."

Picture of Rhizopus fungus under stereo microscope
Rhizopus Micrograph by Larysa Johnston

Rhizopus arrhizus and Rhizopus solonifer infect peanuts. When seeds are sown, decay of seeds is very rapid. In about 5 days, seeds are indistinguishable from the surrounding soil.

Human Pathogens

R. arrhizus, R. microsporus, R. rhizopodiformis are human pathogens associated with zygomycosis. The subcutaneous deep tissue fungal infection occurs predominantly in people with suppressed immune system, leukemia, patients treated with steroids, trauma and burns, metabolic acidosis, deferoxamine therapy in patients receiving haemodialysis, and those with HIV and transplants. Rhinocerebral zygomycosis occurs in those with poorly controlled diabetes. R. arrhizus causes 60% of all cases and and 90% of rhinocerebral infections. Rhizopus microsporus is the second most common pathogen, being especially responsible for the skin and gastrointestinal forms of mycosis; it is also known to be hospital-acquired.4,5 The onset of the disease is rapid and includes the invasion of blood vessels, causing thrombs and tissue necrosis. The disease can spread to whole body organs, causing meningitis and death within 2 to 10 days. Standard therapy for mucormycosis includes reversal of the underlying predisposing factors (if possible), wide-spread surgical cleaning of the infected area and antifungal therapy. In the absence of surgical removal of the infected area (such as removal of the eye in patients with rhinocerebral mucormycosis), antifungal therapy alone is rarely curative.6,8



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Species of Rhizopus:

  • Rhizopus americanus
  • Rhizopus caespitosus
  • Rhizopus homothallicus
  • Rhizopus lyococcus
  • Rhizopus microsporus
  • Rhizopus niveus
  • Rhizopus schipperae
  • Rhizopus sexualis
  • Rhizopus stolonifer
Picture of rhizoids, root-like structures that attach to substrates
Rhizoids (root-like structures that attach to substrates)
Micrograph by Larysa Johnston
Picture of Rhizopus; mature  sporangia with umbrella-shaped collapsing columellae; compound microscope.

Mature sporangia with collapsing columellae
Micrograph by Larysa Johnston

Rhizopus keratitis occasionally is also associated with contact lens wear and poor contact lens hygiene. While Rhizopus is known to cause rapid and severe destruction of the extraocular and rhinocerebral soft tissues, it behaves less aggressively in the cornea, perhaps due to the avascularity of this tissue. The mold is highly sensitive to topical amphotericin when administered following a relatively prompt diagnosis.9

References

  1. Field crop diseases By Robert F. Nyvall
  2. Sweetpotato: major pests, diseases, and nutritional disorders By Ames de Icochea, T. (CIP)., T. Ames, International Potato Center
  3. Westcott's Plant Disease Handbook By Cynthia Westcott
  4. Coronary disease in women: evidence-based diagnosis and treatment By Leslee J. Shaw, Rita Fran Redberg
  5. Pathogenic fungi in humans and animals By Dexter H. Howard
  6. Living with the earth: concepts in environmental health science By Gary S. Moore
  7. Plant pathology: concepts and laboratory exercises By Robert Nicholas Trigiano, Mark Townsend Windham, Alan S. Windham
  8. An integrated genomic and transcriptomic survey of mucormycosis-causing fungi. Marcus C. Chibucos et al.
  9. Rhizopus Keratitis Associated with Poor Contact Lens Hygiene. David B. Warner, Hugh E. WrightIII, and Eric R. Rosenbaum

 

 

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