Leaf and stem scab

Economic importance

Scab is the most severe fungal disease of sweetpotato in South East Asia and the Pacific. In Queensland, Australia, 20% yield losses were reported for the cultivar ‘Puerto Rico’. In the Philippines, yield losses reached 50%, and in Papua New Guinea losses of up to 57% were noted.

When plants are affected at an early stage, the yield of marketable roots is severely reduced.

Australia, Brazil, Brunei, Cambodia, Caroline Islands, China, Cook Islands, Fiji, French Polynesia, Guam, Hong Kong, Indonesia, Japan, Malaysia (including Sabah and Sarawak), Mexico, Micronesia, New Caledonia, New Guinea, Niue, Papua New Guinea, Northern Mariana Islands, Puerto Rico, Solomon Islands, Taiwan, The Philippines, Tonga, USA and Vanuatu.

Symptoms

The first evident symptoms are tiny spots or lesions on the leaves and stem.  These may be circular, elliptical or elongate, yellowish to reddish brown, and may be initially sunken but become raised and scabby, as they spread and merge with each other.  The leaf veins on the lower surface are most commonly infected.  Scab tissue contains the structures of the causal agent intermingled with the leaf or stem tissue. 

When expanded leaves are infected, lesions are present without leaf deformity.  However, newly formed leaves are most susceptible, and these become greatly distorted as affected tissue stops growing.  In particular, the petiole does not flex at the base of the leaves, so they retain the upright presentation of unopened leaves. Leaves are generally cupped and shrivelled in a claw-like manner.

Early infection leads to considerable yield reduction.

Similar symptoms on the leaves are also caused by the thrips Dendrothripoides innoxius. 

The ascomata formed below the epidermis are dark brown to black and solitary to aggregated. They measure up to 150 µm in diameter, composed of pseudoparenchymatic tissues, and contain numerous monoascus locules. The asci are globose or ovoid, 8-spored, thick walled and measure 18-25 x 12-25 µm. The ascospores are hyaline, smooth, transversely 1-3 septate, constricted at the midseptum, and measure 12-18 x 4.5 µm. The acervulus is colourless, 12-16 µm in diameter. The  conidiophores are short, simple to rarely branched and measure 10 x 3 µm; the conidia are hyaline, smooth, aseptate, oblong, and measure  4 - 9 x 2.5-3.5 µm.

Biology and ecology

The disease is transmitted by infected cuttings and through rain splash that carries masses of spores from infected to healthy plant parts of the same plant or to neighbouring plants. The disease is widespread in places with a high incidence of rain, mist and dew or in places where sprinkle irrigation is used.

Under controlled conditions, a temperature range between 25 and 30°C is optimal for fungal growth.

The fungus remains from one season to the next in crop refuse in the form of ascomata. When the temperature rises and there is enough moisture, the ascomata release asci and ascospores, which are the structures that initiate infection of young leaves and stems. Once in the plant tissue, the fungus grows and produces conidia, which are the secondary inoculum that spreads the disease in a field.

Host range

Sweetpotato is the primary host but the disease has also been found in Ipomoea aquatica, I. gracilis, and I. triloba.

Plant resistant cultivars.

 Burning or burying of infected vines after harvest.

Use of disease-free healthy planting material.

Non-use of overhead irrigation.

Plant rotation crops that are non-host of the disease.

The cultivars Centennial ’83 and Beerwah Gold were found  highly resistant and moderately resistant, respectively. Philippine cultivars V2-1, V2-3 and V2-30 and a number of AVRDC cultivars were also found to be highly resistant in Taiwan. It has been found that varieties with a thicker cuticle and fewer stomata are more resistant to fungus invasion.

The fungicides benomyl and chlorotalonil reduce disease incidence.

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

Lenné, J.M., Sweetmore, A., and Burstow A. 1994. Morphological and pathogenic characterisation of Elsinoe batatas : causal agent of sweet potato scab. 4th International Conference of Plant Protection in the Tropics, March 1994. Kuala Lumpur, Malaysia Tropical Plant Protection Society, 64-66.

Palomar, M.K. and Juntilla, R.F. 1983. Disease management series: Sweet potato stem and foliage scab. Plant Pest Clinic Advisory Bulletin No. 12. ViSCA, Baybay, Leyte, Philippines.

Ramsay, M., Vawdrey, L.L., Hardy, J. 1988. Scab (Sphaceloma batatas) a new disease of sweet potatoes in Australia: fungicide and cultivar evaluation. Australian Journal of experimental Agriculture, 28: 137-141.

Sivanesan A., and Hyde, K.D. 1992. Elsinoë batatas. IMI Descriptions of fungi and bacteria No. 1124.

Vasquez, E.A. and Sajise, C. E. 1989. Pests of sweet potato: Insects, mites and diseases. PRIS and PRCRTC, Visayas State College of Agriculture, Baybay, Leyte, Philippines, 66 p.

Contributed by: Teresa Ames and Jane O'Sullivan

Taxonomy

Economic Importance

Geographical distribution

Symptoms

Morphology

Biology and ecology

Host range

Management

References

Cupped, distorted and upright leaves showing the confluent scabby lesions along veins and petioles (J. O'Sullivan).

Early infection showing little leaf distortion, but upright leaf presentation (J. O'Sullivan)

Brown, raised scaby lesions on veins under leaf (E. T. Rasco).

Stem lesions at an early stage of infection (J. O'Sullivan).