Red Ring Disease of the Coconut Palm, Inoculation Studies and Histopathology

Abstract

Coconut, Cocos nucifera L., and cabbage palm trees, Roystonea oleracea F. O. Cook, aged approximately 4-7 years developed red ring disease and died following inoculation with Rhadinaphelenchus cocophilus through wounds in their stems and roots. When roots of 12 year old coconut trees were inoculated with R. cocophilus, they became infested and decayed but trees did not die from red ring disease. R. cocophilus incited red ring disease in both coconut and cabbage palms 3-7 years old when inoculum was placed on roots which were not artificially damaged. R. cocophilus also incited red ring in the coconut palm but not in the cabbage palm when inoculum was placed in their leaf axils. R. cocophilus occurred mostly in discoloured ground parenchyma tissue and in smaller numbers in the adjacent white parenchyma. Nematodes were present only intercellularly in the upper part of the stem where lesions were discrete, but in the discoloured zone, they were also present intracellularly and parenchyma tissue was damaged. A few cells in such areas were completely destroyed, forming microscopic cavities which were teeming with R. cocophilus. Though the nematode was not found in the vascular tissue, xylem vessels in the discoloured area were found to be occluded with tyloses. R. cocophilus appeared to enter coconut trees through cracks in the stem near leaf bases. Suggested approaches to control are to seal such cracks mechanically or to place a granular preparation of a nematicide in leaf axils.