Incidence of fungal species associated with leaf rot disease of coconut palms in relation to weather and the stage of lesion development

Abstract

Summary.Leaf rot disease (LRD) of coconut occurs in Kerala State, India, and is generally severe during the monsoon, a time of high rainfall and r.h. and low maximum temperature. The pathogenic fungi Colletotrichum gloeosporioides, Exserohilum rostratum, Gliocladium vermoeseni, Fusarium solani, F. moniliforme var. intermedium, Thielaviopsis paradoxa and Rhizoctonia solani were among 14 species isolated from spindles of palms with LRD in multiple samplings of two experiments, each lasting 1 year. Co‐occurrence of fungi was observed in 70–76% of spindles although leaf pieces yielded mostly individual fungi irrespective of the stage of lesion development. C. gloeosporioides occurred most frequently on early lesions. The incidence of C. gloeosporioides was most strongly correlated with rainfall and r.h. and, negatively, with maximum temperature and hours of sunshine. It was the fungus isolated most commonly during the monsoon and occurred on early lesions more frequently than on advanced lesions (determined in one experiment). C. gloeosporioides is thus implicated as the principal pathogen involved in initiation of the disease during monsoons. Its relatively low incidence in the dry season suggests a quiescent phase between periods of parasitic activity. E. rostratum was isolated less frequently and its incidence was less strongly or consistently correlated with weather or with the stage of lesion development. Fusarium spp. and R. solani were associated with dryer weather and higher temperatures. The Fusarium spp. were the fungi isolated most commonly during part of the dry season (January‐March) and occurred more on early than on advanced lesions at one sample (January) when they may have been primary disease agents. R. solani was also more frequent on advanced lesions and may be principally a secondary coloniser of established lesions. The incidence of other fungi, which occurred less frequently and more sporadically, was not associated with weather or consistently with different stages of lesion development.