Circular spot

Economic importance

An important and widespread problem, particularly on land that is frequently cropped.

The fungus attacks many hosts and is present worldwide.

Circular lesions, typically 1-2 cm wide, with sharply defined margins are observed at harvest. Lesions are usually shallow, about 1-5 mm deep, but on particularly susceptible cultivars, an extensive rot may develop. Externally, the spot is brown, often with a slightly darker margin. Tissue within the lesion is yellowish brown, soft and wet at harvest.  It becomes dark brown, sunken and leathery as it dries out in storage, often cracking in the centre. 

Affected roots have a distinctive bitter flavour.

The size of the lesion does not increase in storage as a rule.  Normally the lesion dries out within a few days, and may eventually be peeled off exposing a healed wound surface.  Only in very moist, warm storage conditions can the fungus continue to spread, and white hyphae spread from the centre of the lesion across the root surface.  

Infection is usually at a late stage of development of the crop.  Hence lesions rarely cause any deformity in the shape of the storage roots.

The disease can also cause an unrestricted wet, soft rot, common when infected storage roots are used as seeds in plant beds while the lesions are still wet.

The mycelia are white in early stages of development become tan as they mature and finally form strands of brown pigmented hyphae.  They measure 6-9 µm wide, have clamp connections and numerous dikaryotic nuclei. The sclerotia resemble mustard seeds, are round to irregular in shape, generally tan outside and have a white medulla.

The basidial stage has not been reported in sweetpotato.

Biology and ecology

S. rolfsii is a saprophyte that kills the plant tissue in advance, producing pectolitic enzymes and cellulase, which macerate the tissue. In the process,  the cell walls and middle lamellae are destroyed which facilitate infection. The fungus also produces high amounts of oxalic acid.

The pathogen survives in the soil for several years in plant residues in the upper 10 cm or slightly below the soil line.  It prefers hot weather conditions (28-30°C) and high humidity. It spreads by the mycelium growing in the soil surface, or by mycelial fragments and sclerotia, by surface water, or by mechanical means. Sclerotia can survive passage through the  digestive tract of cattle.

Circular spot is caused by the same pathogen that causes Sclerotial Blight, but the two disorders are not necessarily concurrent in the field, possibly because different strains are involved.  The incidence of circular spot is quite variable and unpredictable, and conditions favouring the disease are poorly understood.

S. rolfsii has several antagonists that live in the soil which can be important for integrated disease management. Some of the  most important species tested in the laboratory and field are: Trichoderma harzianum, T. hematum, Aspergillus flavus, A. niger, Gliocladium virens, Serratia marsescens, Pseudomonas fluorescens, Xanthomonas campestris and Bacillus subtilis.

The fungus is omnivorous and is recorded to live on more than 500 plant species. Sweetpotato (Ipomoea batatas) is considered a primary host together with soybean (Glycine max), maize (Zea mays), sunflower (Helianthus annuus), rice (Oryza sativa), bean (Phaseolus vulgaris), groundnut (Arachis hipogaea), cotton (Gossypium hirsutum), pea (Pisum sativum), black pepper (Piper nigrum), sugarcane (Sacharum officinarum), tomato (Lycopersicon esculentum), bell pepper (Capsicum annuum), carrot (Daucus carota), potato (Solanum tuberosum), sugarbeet (Beta vulgaris var. saccharifera), wheat (Triticum aestivum), lentil (Lens culinaris subsp.culinaris), chickpea (Cicer arietinum), broad bean (Vicia faba), taro (Colocasia esculenta), flax (Linum usitatissimun), turmeric (Curcuma longa), etc.

Secondary hosts are numerous, among them food crops and ornamentals, most of them economically important. There are also a variety of weed hosts.

Inspection and diagnosis

Symptomatic plants or plant parts are placed in a moist chamber for 3 to 4 days; abundant fan-like mycelia grow from them and in few a more days, sclerotia start forming.

Management

Crop rotation with non-host  crops could be an effective control for sweetpotato.

Early harvest may reduce the extent of the disease.

Use of soil amendments. Amendments can be added to the soil to stimulate the increase of antagonistic microorganisms.

Very little is mentioned about resistance of sweetpotato plants to S. rolfsii. However, differences have been found in the reaction of certain cultivars to artificial inoculation.

Aycock, R. 1966. Stem rot and other diseases caused by Sclerotium rolfsii or the status of Rolfs’ fungus after 70 years. N.C. Agric. Exp. Stn. Tech. Bull. No. 174 202 p.

Clark, C.A., and Moyer, J.M. 1988. Compendium of sweetpotato diseases. The American Phytopahological Society. APS Press, Minnesota, USA. 74 p.

Mordue, J.E.M. 1974. Corticium rolfsii. CMI Descriptions of Pathogenic Fungi and Bacteria No. 410. 2 p.

Contributed by: Teresa Ames and Jane O'Sullivan

Taxonomy

Economic importance

Geographical distribution

Symptoms

Morphology

Biology and ecology

Host range

Inspection and diagnosis

Management

References

Circular lesion with defined margin on storage root ( W. Martin, APS).

Circular spot lesions after a period of storage, dried and sunken and cracked from the centre out (C. Clark, APS).

Circular spot lesion after several weeks in storage.  It has dried out and begun to peel away (C. Clark, APS).

An infected root after incubation in high humidity, showing growth of the fungal mycelium out of the lesions (C. Clark, APS).