Investigation of Some Coconut Leaf Spots in Papua New Guinea.

Abstract

Examination of coconut leaf spot collections from high and from low rainfall areas in Papua New Guinea showed that Drechslera incurvata and Pestalotiopsis palmarum were the two species most commonly found sporulating on the leaf spots considered in this study, the former being the most common species on young spots. There were twice as many non-sporulating spots on collections from low rainfall areas as on a collection from a high rainfall area. P. palmarum was the species most commc ly isolated from surface-sterilized non-sporulating spots from low rainfall collections, being isolated five times more frequently than D. incurvata. Inoculation tests showed that D. incurvata was pathogenic to young leaves of coconut seedlings, infecting the host via stomata on the abaxial leaf surface and producing spots comparable with young non-sporulating spots observed on field collections. P. palmarum was not able to infect uninjured coconut seedling leaves but was able to colonize D. incurvata spots and was responsible, at least in part, for changes in the size and appearance of D. incurvata spots. Fourteen seed sources were tested in the field for their reaction to Drechslera leaf spot. Reactions were variable within any one seed source. Rennell, Buka, Karkar and Madang seed sources were scored as the most susceptible and Ralabang (dwarf), New Hebrides and Kieta as the most resistant. Evidence is presented which suggests that the susceptibility of young coconut palms to Drechslera leaf spot decreases with age.