Sclerotial blight

Economic importance

The disease is quite serious especially in sweetpotato seedbeds. In the field the disease is present at any stage of growth, but mostly scattered in patches affecting several plants. Affected plants are girdled and usually die. Losses are 5 - 20% in tropical areas due to dead plants.

The fungus attacks many hosts and is present worldwide.  However, there were only few reports on sweetpotato.

The disease usually appears as isolated patches in seed beds and in the field, mainly when plants have grown to the stage where the foliage is dense enough to cover the soil surface.

Plants initially show yellowing of the lower leaves and wilting. In the stems,  depressed, water soaked, sunken lesions are usually observed at or below the soil surface, and they soon become covered with a white fan-like mycelium that radiates from the lesion. Soon after, the stem is girdled and the plant eventually dies. The fungus continues to grow in dead plants in the field, and when they start drying, numerous sclerotia are formed.  They are initially white but become light brown, having the appearance of mustard seeds.

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 favours hot weather condition and prefers high temperatures (28-30°C). It is present in tropical and subtropical areas of the world where the temperature and relative humidity are high most of the time.

The fungus 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.

During the growth period when senescent leaves drop to the soil, the fungus starts growing on them and from there invades the base of the plant.

The pathogen survives in the soil for several years in plant residues in the upper 10 cm or slightly below the soil line. 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.

There is no significant difference in the growth of the pathogen in soils of various textures and pH, except in clay loam soils, due to its conductivity. It has also been suggested that soil pH is important for sclerotial formation.

Differences in virulence have been found. These might be due to the strain present, because several strains have been identified based on oxalic acid and enzymatic production.

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

Deep plowing. The sclerotia  do not survive when deeply buried into the soil.

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

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

Solarization. Prior to establishing seedbeds, the soil may be covered with black polyethylene sheets for a 6 week period during the hottest months of the year.  The top soil layers become very hot, effectively controlling the fungus.

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.

The most commonly cited fungicides to control S. rolfsii in several crops are captan, quitozene, thiram, carbendazin, triadimenol, benomyl and oxicarboxin; most of these can be used in sweetpotatoes as protectants where soils are infected,  and for dipping the cuttings before planting.

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

Taxonomy

Economic importance

Geographical distribution

Symptoms

Morphology

Biology and ecology

Host range

Inspection and diagnosis

Management

References

Severely infected plant after stems are girdled by Sclerotium rolfsii (C. Clark, APS).

An infected plant showing yellowing of lower leaves and the characteristic white hyphae of the fungus (C. Clark, APS).

An affected patch within a seed bed, where sprouts have been killed, and brown granular sclerotia are formed on the mycelial mat (W. Martin, APS)