Acronym: SPLL phytoplasma

Other names: Ishuku-byo; witches' broom

Crop losses are unknown. However, the disease causes greatest losses in dry areas, which favour high leafhopper populations. Some infected plants do not survive until harvest or they produce few harvestable roots.

Widespread in Asia, Australia and the western Pacific. The distribution pattern correlates closely with that of its principal leafhopper vectors .

In Asia, witches' broom has been reported in Bangladesh, China (Fujian, Taiwan), India, Indonesia, Japan (Ryukyu Archipelago), Korea, Republic of Korea, Malaysia, and  Philippines. In Oceania, it is found in Australia (Northern Territory), Belau, Federated states of Micronesia, New Caledonia, New Zealand, Niue, Papua New Guinea, Solomon Islands, Tonga, and Vanuatu.

The initial symptoms consist of a transient vein clearing followed by the development of new leaves that are distinctly smaller and more chlorotic (paler) than normal.  Generally only scattered individual plants are infected, and they are conspicuously smaller than adjacent uninfected plants. Leaves may have a more rounded shape, often curling at the leaf margins. The growth habit tends to be more erect than in healthy plants, internodes are shortened and there is a proliferation of axillary shoots which, together with a greatly reduced root system, result in weak plants with a compressed or bushy appearance. The number and quality of storage roots are reduced, with few or no harvestable storage roots being produced on severely affected plants. Production of latex in vines and roots is also reduced.

Characteristic pleomorphic bodies ranging from 0.1 to 1.0 µm in diameter, with a well defined unit membrane.

The SPLL phytoplasma can be transmitted by the leafhopper Orosius lotophagorum ryukyuensis and Nesophrosyne ryukyuensis in a persistent manner. Low annual rainfall and prolonged dry seasons favour the vector and, under these conditions, the disease can reach epidemic proportions. Infected planting material is also important in the dissemination of the disease. As the disease has an exceptionally long incubation period in sweetpotato (up to 283 days) following graft transmission, infected planting material can appear healthy.

In small, local cultivations, vines may be taken from more vigorous plants in order to establish a subsequent crop. While this practice may assist in selecting plants that are better able to tolerate infection by the phytoplasma, it probably contributes to the promulgation of the disease in the crop; this situation may be further exacerbated where crop plantings overlap or where the previous crop is not removed.

Primary natural host is sweetpotato (Ipomoea batatas). Wild hosts (Ipomoea nil, Pharbitis purpurea, Ipomoea pes-caprae) act as reservoirs.

Alternative experimental hosts of SPLL phytoplasma are Ipomoea setosa, I. triloba, I. indica, I. ericolor, Pharbitis purpurea, P. nil, and Catharanthus roseus.

Management

Sanitation (removal of all previous crop debris from the field and use of healthy planting material) has provided the best control to date. The removal and destruction of diseased plants and wild hosts reduces the spread of the disease.

Insecticides could prove useful in the production of nursery stock or in commercial plantings, but not when sweetpotato is grown as a subsistence crop.

Tetracycline causes remission of symptoms leading to the production of disease-free young shoots, and may be of use in breeding and selection programmes; however, it is unlikely to be commercially significant.

There is little evidence of resistance to the disease. Only one clone from Solomon Islands and another from Taiwan showed moderate resistance, but these clones are affected by SPLL isolates from different geographical regions.

Phytoplasma-free vegetative material should be transferred as in-vitro plantlets.

Chen, M.J. 1972. Electron microscopic studies of plant pathogenic mycoplasma found in Taiwan. Proceedings of the National Science Council (Republic of China), 5:61-77.

Dabek, A.J., Sagar, C. 1978. Witches' broom chlorotic little leaf of sweet potato in Guadal-canal, Solomon Islands, possibly caused by mycoplasma-like organisms. Phytopathologische. Zeitschrift, 92: 1-11.

Gibb, K.S., Padovan, A.C., Mogen, B.D. 1995. Studies on sweet potato little-leaf phytoplasma detected in sweet potato and other plant species growing in Northern Australia. Phytopathology, 85(2):169-174.

Green, S.K., Luo, C.Y., Lee, D.R. 1989. Elimination of mycoplasma-like organisms from witches' broom infected sweet potato. Journal of Phytopathology, 126(3):204-212.

Jackson, G.V.H., Zettler, F.W. 1983. Sweet potato witches' broom and legume little-leaf diseases in the Solomon Islands. Plant Disease, 67(9):1141-1144.

Kahn, R.P., Lawson, R.H., Monroe, R.L., Hearon, S. 1972. Sweet potato little leaf (witches'-broom) associated with a mycoplasma-like organism. Phytopathology, 62:903-909.

Ko, H.C., Lin, C.P. 1993. Development and application of cloned DNA probes for a mycoplasmalike organism associated with sweetpotato witches'-broom. Phytopathology, 84:468-473.

Shen, W.C., Lin, C.P. 1993. Production of monoclonal antibodies against a mycoplasmalike organism associated with sweetpotato witches' broom. Phytopathology, 83: 671-675.

Summers, E.M. 1951. "Ishuku-byo" dwarf of sweet potatoes in the Ryukyu Islands. Plant Disease Reporter, 35:255-267.

Zhang, B-C., Huang, Y-C. 1990. A list of important plant diseases in China. Review of Plant Pathology 69(3), 97-118.

Contributed by:  Segundo Fuentes and  Luis Salazar  

Taxonomy

Economic importance

Geographical distribution

Symptoms

Morphology

Biology and ecology

Host range

Management

References

Shortened internodes and compressed growth habit are typical symptoms of witches' broom (S. Fuentes & L. Salazar).

Proliferation of shoots and reduced leaf size in infected plant. (F. Zettler, APS).

General or mottled chlorosis and yellowing of oldest leaves often accompanies the reduction in leaf size (E. Coleman).

Reduced size of whole plant and of storage roots, in a plant infected with SPLL (right) compared with the adjacent uninfected plant (E. Coleman).