Wayne Nishijima, Extension Plant Pathologist
Department of Plant Pathology
University of Hawaii at Hilo
Rhizopus stolonifer has a large host range. Many fruits and vegetables are susceptible to this pathogen. Some susceptible genera include: Allium, Ananas, Brassica, Cucumis, Cucurbita, Fragaria, Lycopersicon, Phaseolus, Pisum, Solanum, and several more.
However, this summary only discusses the affect of this fungus on papaya.
A common postharvest disease of papayas. The disease is important only during storage and transit of papayas and is rarely seen in the field. The disease, when it occurs on fruits packed in cardboard boxes, can be an unsightly mess due to the watery leakage from fruits causing the boxes to get wet and collapse.
Rhizopus soft rot of papaya is characterized by a soft and watery rot that quickly collapses the entire fruit but leaves the cuticle intact. The fungus can grow out through any break in the cuticle and spread rapidly to adjacent fruits, often destroying the entire contents of a box within a few days. The infected fruit is often covered by coarse, gray, hairy mycelia that form a mass of black sporangia at their tips. The affected fruit becomes quickly colonized by yeasts and bacteria and emanates a sour odor.
The most common cause of Rhizopus soft rot is R. stolonifer (syn.: Rhizopus nigricans Ehrenb.), but R. arrhizus A. Fischer (syn. R. oryzae Went & Geerlings) has also been reported from Pakistan and India. Both fungi are fast growing, cottony white at first then darkens to gray to black depending on the amount of sporulation. The hyphae secrete pectinolytic enzymes that break down the middle lamellae of infected tissue and causes a soft, watery rot. The fungi lack cutinases and, therefore, can enter host tissue only through wounds.
Both fungi are ubiquitous and are commonly found in soil, compost and other decaying plant matter. Rhizopus spores are airborne and can be found throughout orchards and packing houses. Because they are incapable of penetrating through sound, uninjured fruit surfaces they can only enter through wounds that occur during harvesting, transporting, postharvest treating and handling. Puncture wounds are more readily colonized than either abrasions or bruises. Rhizopus stolonifer is capable of utilizing lesions caused by other fungi as courts of entry into papaya fruits.
The incidence of Rhizopus soft rot usually increases during rainy weather due in part to greater inoculum, higher humidity and an increase in the number of fruit lesions caused by other fungi. High humidity and temperatures of about 25ûC during storage or transit are optimum for Rhizopus soft rot development.
Rhizopus spores are airborne and can be found throughout orchards and packing houses. Because they are incapable of penetrating through sound, uninjured fruit surfaces they can only enter through wounds that occur during harvesting, transporting, postharvest treating and handling.
The most important control measure is sanitation in and around the packing plant. Rotting fruit in packing plants should be removed and destroyed. Water tanks used to unload bins, wash or move papaya should be chlorinated to prevent buildup of this and other pathogens. Conveyer belts, rollers, bins and other equipment that papaya fruit come in direct contact with should also be regularly sanitized. Wounds to papaya fruits should be minimized from the time of harvest until the time it reaches the consumer.
Heat treatment used for quarantine purposes is effective in killing Rhizopus mycelia but does not kill all the spores.
Preventive field fungicide sprays control Rhizopus soft rot by reducing field inoculum levels. Fungicide sprays also reduce the incidence of fruit lesions, caused by other fungi, that Rhizopus can use as courts of entry into the papaya fruit.
Alvarez, A. M., and Nishijima, W. T. 1987. Postharvest diseases of papaya. Plant Dis. 71:681-6
Brodrick, H. T., Jacobs, C. J., Swartz, H. D., and Mulder, N. J. 1972. The control of storage diseases of papaws in South Africa. The Citrus Grower and Sub-Trop. Fruit J., 467:5-21.
Nishijima, W. T., Fernandez, J.A., and Ebersole, S. 1990. Factors influencing development of postharvest incidence of Rhizopus soft rot of papayas. Pages 495-502 in R.E. Paull, Ed. Symposium on Tropical Fruit in International Trade, Honolulu, Hawaii, Acta Horticulturae 269.