Leaf scorch decline of coconut in Sri Lanka: a review

Leaf scorch decline (LSD) is regarded as an unknown disorder of the coconut palm, despite intensive research in the past four decades. It was first reported in the southern province of Sri Lanka in 1955 and is now rapidly spreading to the major coconut growing areas of the country resulting in considerable economic losses to the coconut industry. The most characteristic visible symptom is the gradual scorching of leaflets from tip to the midrib of the frond and progressing from the lower to the upper fronds. In the course of the disorder, which in 2-6 years makes the palm completely unproductive, the trunk tapers, fewer inflorescences are produced, fronds become shorter, the nuts appear elongated, and eventually the crown diminishes and withers off. Extensive decaying of roots has also been observed. Histological studies of root sections have revealed blackening of cortical tissue and blockage of xylem vessels. Mycological and bacteriological studies of leaves and roots have not indicated that any pathogens are consistently associated with the disorder. Studies on the association of viruses, viroids and phytoplasma were inconclusive but merit further research. Associated pathogenic nematodes and insects have been eliminated as possible causes for LSD. Although major nutrients do not cause the disorder, the role of micronutrients is uncertain. LSD has always been associated with adverse soil physical and chemical conditions which contribute to the extensive decaying of roots of affected palms. The LSD-affected palms appeared to be in a physiologically stressed condition. They have a high internal water deficit due to decreased transpiration, increased stomatal diffusive resistance and low leaf water potential. Increase in polyphenolic compounds in leaves has also been reported. A complex of factors appears to be involved in the manifestation of LSD. A well co-ordinated, multidisciplinary research thrust is urgently needed to identify the causes and to prevent its further spread.