Chlorotic leaf distortion

Taxonomy

Economic importance

Yield of storage roots was not affected in studies in the USA using cultivars that varied from mild to severe symptom development. The potential effect of CLD on quality of vines for use as food or feed has not been reported.

Geographical distribution

The disease has been reported from Brazil, Kenya, Peru, and USA. However, the causal fungus was isolated from botanical seed from a number of additional countries. It appears that the disease is widely distributed and it is likely that the disease occurs in many places from which it has not yet been reported.

Symptoms

The youngest 2-3 leaves at the tips of vines become generally chlorotic, sometimes becoming bright yellow, almost bleached in appearance . As the leaves mature, they regain most of their normal colour. On cultivars that normally have purple leaves, the leaves may become pink when affected with CLD. Following extended periods of conducive weather (sunny and humid), marginal necrosis may develop on affected leaves (Fig. A). Mycelia of the causal fungus may be seen growing out from between halves of leaves that have not yet opened (Fig. B) or as scattered clumps on the upper surface of more mature leaves (Fig. C). Associated with the mycelia may be phialides that produce typical Fusarium macroconidia and microconidia. The latter clumps may appear similar to salt deposits. On some cultivars affected leaves may be twisted or distorted (Fig. D).

Fig. A	Fig. B
Fig. C	Fig. D

Morphology

Polyphialides up to 20 μm long may be found in aerial mycelia or on sporodochia. Conidia are 0 to 5-septate with the 0-septate averaging 2.5-3.5 x 7-10 μm and the 3-septate averaging 3.5-4.0 x 35-40 μm. Chlamydospores are not produced.

Biology and ecology

The relationship between the causal fungus and plant host is most unusual in that the fungus primarily colonizes the surfaces of the growing vine tip without invading the plant. Mycelia are found on apical meristems and between halves of developing leaves that have not yet opened. Once the leaves open and expose the fungal mycelia, the mycelia appear to stop growing. As a result, individual leaves appear to recover as they mature. Symptom development is strongly favoured by warm, sunny, humid weather and Chlorotic leaf distortion r. It is not known how the fungus overwinters in temperate areas.

Host range

The disease is only known to occur on sweetpotato, although other Convolvulaceous plants can be infected by artificial inoculation.

The unique symptoms and the presence of fungal mycelia and characteristic Fusarium macroconidia are adequate for diagnosis.

Management

Since CLD does not appear to affect storage root production, management efforts have not been considered necessary. It is difficult to eliminate the pathogen from planting material, but it has been successfully eliminated by using the same meristem-tip culture techniques that are used for virus elimination.

References

Clark, C. A. 1992. Histological evidence that Fusarium lateritium is an exopathogen on sweetpotato with chlorotic leaf distortion. Phytopathology 82:656-663.

Clark, C. A., and Hoy, M. W. 1994. Isolation of Fusarium lateritium from sweetpotato seed. Plant Dis. 78:585-587.

Clark, C. A., Valverde, R. A., Wilder-Ayers, J. A., and Nelson, P. E. 1990. Fusarium lateritium, causal agent of sweetpotato chlorotic leaf distortion. Phytopathology 80:741-744.

Icochea, T., Duarte, V., and Clark, C. A. 1994. Occurrence of sweetpotato chlorotic leaf distortion in Brazil and Peru caused by Fusarium lateritium. (Note) Plant Dis. 78:754.

Kihurani, A. W., Skoglund, L. G., and Gatumbi, R. W. 1993. Occurrence of sweetpotato chlorotic leaf distortion caused by Fusarium lateritium in Kenya. Plant Dis. 77:1260-1261.

Kim, B-K., La Bonte, D. R., and Clark, C. A. 1996. Studies on sweetpotato with chlorotic leaf distortion, caused by Fusarium lateritium. Internat. J. Pest Management 42:67-70.

Contributed by: Christopher Clark

Taxonomy

Economic importance

Geographical distribution

Symptoms

Morphology

Biology and ecology

Host range

Diagnosis

Management

References

Youngest leaves are generally chlorotic and may be distorted (C. Clark).

Youngest leaves may have almost bleached appearance (C. Clark).

Pink appearance of affected young leaves on normally purple-tipped genotype (C.Clark).