Rhizopus soft rot

Taxonomy

Australia, Brazil, China, Colombia, Egypt, India, Italy, Japan, Kenya, Korea Republic, Pakistan, Peru, Romania, USA, USSR and Yugoslavia.

Economic importance

Soft rot caused by R. stolonifer is a postharvest disease that can be economically important without proper harvest and postharvest practices. It can destroy storage roots that are bruised or wounded during harvest, left overnight in the field after harvest, and are not properly cured before storage.

A water-soaked lesion starts to develop a few hours after the root has been wounded or bruised during harvest or handling. The lesion becomes covered by a web-like outgrowth with scattered pinhead-like structures which are the mycelia and sporangia of the fungus. In 4 to 5 days, the entire inner tissues of the root rot become soft, slimy and watery as they liquefy. However, the periderm remains almost intact except for small cracks and wrinkled appearance. Black brush- like domes, which are the sporangiophores, sporangia, and spores, emerge from these cracks, giving a  whisker-like appearance.

Rotting starts at either end where the fleshy root has been separated from the rest of the stem and the root system during harvest but any bruise or a slight scratch can initiate rotting.

Soft rot caused by Rhizopus has some similarities with bacterial soft rot, as both produce the watery, flesh decaying rot and both attract fruit flies. However, bacterial rot produces an unpleasant smell while Rhizopus rot emits a pleasant fermented odour that attracts fruit flies. Also, bacterial rot does not produce whiskers unlike Rhizopus soft rot

Morphology

The pathogen first appear as white cottony colonies on potato-dextrose-agar at 25°C becoming heavily speckled by the presence of sporangia and then brownish black in age, spreading rapidly by means of stolons fixed at various points to the substrate by rhizoids. The sporangiophores  measure up to 34 µm in  diameter and 1000-3500 µm in length, are smooth walled, non- septate, light brown, simple, and are in groups of 3-5 from stolons opposite rhizoids. The sporangia are 100-350 µm in  diameter, globose or sub-globose with somewhat flattened base, white at first, then black and have many spores. The columella measure 63-224 x 70-140 µm, are subglobose to dorsiventrally flattened, light brownish grey, and are umbrella-shaped when dehisced. The collar are either poorly defined or absent.  The apophysis is present and visible below young columellae. The rhizoids and stolons are transparent to dark brown. The brownish-black sporangiospores measure (5) 8-10 (26) µm, are irregular round, oval, elongate or angular in shape. They are heterotallic and strongly striate with homogeneous content. The zygospores are produced when compatible isolates are grown together.  They measure 103-180 (220) µm, are globose, or compressed between suspensors, brownish-black, thick walled and verrucose.  The suspensors measure 62-118 µm wide, swollen, and usually unequal and somewhat granular. No growth is produced at 37°C (J.A.Lunn, 1974).

Biology and ecology

Rhizopus spp. is known to be omnipresent in the air as a contaminant. Usually saprophytic, however, the infection of fleshy sweetpotato roots occurs in the field through wounds or bruises (made by harvesting implements) where the tissue is in a pre-necrotic process.

Once a spore lands on a wounded tissue, it germinates and starts growing on the surface, producing a thick mycelium which at the same time produces cell degrading enzymes (pectinases, amylases) that denature the tissues in advance thus leading to infection.

The infection process takes place in a wide temperature range of 20 to 30°C. Moisture is critical and there should be enough moisture (75 to 85%) for spores to germinate and mycelia to grow. Spores usually do not germinate at a higher relative humidity . The optimum temperature for sporulation, mycelial growth, and spore germination is between 23 to 28°C while rotting and tuber decay develop between 15 and 23°C.

Exposure to sunlight and chilling can also predispose the  storage roots to the disease. A long exposure of fleshy roots to sunlight produces burns where the development of infecting mycelia can occur. Chilling has a similar effect as it damages the cell walls, through which the fungus can penetrate.

Since the disease does not affect the sweetpotato plant or roots while in the field, the influence of environmental factors during the growing season is not significant. However, conditions at harvest are important. For example, heavy rain and low temperature at harvest make the possibility of infection higher.

Other species found in soft rot of sweetpotatoes are Rhizopus oryzae, Mucor racemosus, M. circinelloides, and M. piriformis. All these fungi are saprophytic so they initially produce pectic enzymes that break down sweetpotato tissues.

Primary hosts are Cucurbitaceae and Solanum melongena (eggplant/aubergine).

It  also attacks Abelmoschus esculentus (okra), Anona muricata (soursop), Arachis hipogaea (groundnut), Arthrocarpus heterophyllus (jackfruit), Cajanus cajan (pigeon pea), Carica papaya (papaw), Cicer arietinum (chickpea), Cucumis melo (melon), Datura metel (Hindu datura), Dacryodes edulis (Canarium saphii), Ficus carica (fig), Fragaria spp. (strawberry), Gossypium spp. (cotton), Ipomoea batatas (sweetpotato), Lufa aegiptica (loofah), Lycopersicon esculentum (tomato), Malus domestica (apple),Oxalis tuberosa (oca),Pastinaca sativa (parsnip), Prunus armeniaca (apricot), Prunus avium (cherry), Prunus dulcis (bitter almond), Prunus domestica (plum), Prunus persica (peach), Phyllantus emblica (Indian gooseberry), Pyrus communis (European pear), sorghum, Tropaeolum tuberosum (mashua), Ullucus tuberosus (ulluco), Vitis vinifera (grapes), Vicia faba (broad bean), Vigna radiata (mungbean), Zea mays (maize) and Zingiber officinale (ginger).

The fungus usually infects storage roots after harvest and a good diagnostic sign is the presence of whiskers.

In the laboratory Rhizopus can be detected by placing a small piece of wounded tissue in a moist chamber, and observing a mycelial mat outgrowth after one to two days.

Rhizopus grows in any substrate containing starch and/or any kind of sugar but grows better on PDA (Potato-Dextrose-Agar) plates.

Prevent unnecessary wounding during harvest and handling, and sorting out wounded storage roots.

Not leaving sweetpotatoes in the field after harvest.  This will prevent unnecessary exposure to direct sunlight, chilling in the field and other unfavourable conditions.

Avoid harvesting during or after rainfall.

Cure storage roots immediately after harvest and storing them at 12 to 15°C. This counts only for places where the fleshy roots are stored. 

Host-Plant Resistance

Some resistance has been reported but it may be compromised by excessive wounding while susceptibility in various degrees has been observed.

2,6-dichloro-4-nitroaniline (DCNA) is used routinely as protective fungicide for dipping roots to protect against rotting caused by Rhizopus.

Bouwkamp, J.C., Scott, L.E., and Kantzes, J.G. 1971. Control of soft rot (Rhizopus spp.) in cut root pieces of sweetpotatoes with 2,6-dichloro-nitroaniline. Plant Dis. Rep. 55:1097-1099.

Clark, C.A. and Moyer, J.W. 1988. Compendium of sweet potato diseases. APS Press. 74 p.

Gatumbi, R.W. 1990. sweetpotato diseases in Kenya. Proceedings of a National workshop held in Mombasa, Kenya, 7-ll May, 1990: 41-43.

Lunn, J.A. 1977. Rhizopus stolonifer. CMI Descriptions of Pathogenic Fungi and Bacteria No. 524.

McClure, T.T. 1959. Rhizopus decay of sweetpotatoes as affected by chilling, recuring and hydrowarming after storage. Phytopathology 49:359-361.

Palomar, M.K, Solis, A.D., and Bandala, H.S.1980. Sweet potato tuber rot disease in the Philippines. Ann. Trop. Res. 2:111-121.

Contributed by: Teresa Ames

Taxonomy

Economic importance

Geographical distribution

Symptoms

Morphology

Biology and ecology

Host range

Detection

Management

References

Web-like outgrowth on storage root (W. Martin, APS).