Pathogenicity of radopholus similis on coconut (Cocos nucifera L.) seedlings under greenhouse and field conditions Koshy P. K., Sosamma V. K. Nematology Laboratory, Central Plantation Crops Research Institute, Regional Station, Kayangulam, Krishnapuram-690533, Kerala, India. Abstract The pathogenic effects of Radopholus similis on coconut seedlings was established through two experiments, the first with a duration of 5 years, using nematode inoculum extracted from roots of naturally infested coconut palms under field conditions and the second using axenic nematode population, reared on carrot discs under greenhouse conditions over a period of one year. In both cases, reduced growth of plants and intensity of root lesions and rotting were directly proportional to the increase in initial inoculum used. An initial inoculum density of 62, 500 nematodes per seedling caused 48, 21,76 and 79 per cent reduction over control in height, girth, shoot and root weight respectively, over a period of 5 years, in sandy loam soil in pots under field conditions. However, the threshold inoculum density causing significant reduction of various growth parameters was 100 nematodes per seedling or one nematode in 576 cm3 or 800 g sandy loam soil over a period of five years. In the greenhouse an initial inoculum density of 100,000 nematodes caused 40, 55,20,62,71,57 and 51 per cent reduction over control with regard to height, shoot weight, number of leaves, leaf area, number of lateral roots, volume and root weight respectively over a period of one year. No appreciable damage was noticed in plants inoculated with the fungus, Cylindrocarpon effusum alone. Plants inoculated simulataneously with the fungus and nematode recorded reduced rate of multiplication of the nematode. This study established the potential of the burrowing nematode as a pathogen of coconut. The pathogenic threshold level of R. similis was 1000 nematodes per plant or one nematode per 10 cm3 of sandy loam soil under greenhouse conditions. Top Key words Radopholus similis, coconut, pathogenicity. Top |