MSc

A survey of plant parasitic nematodes of tuber crops, cassava, sweet potato, coleus, colocasia/xanthosoma and diascorea in twenty five panchayats representing five agroclimatic zones of Kerala was conducted during December, 1988. This was the first attempt in this line of investigation. Meloidogyne sp. and Rotylenchulus sp. were more widespread with all the five tuber crops in the five agroclimatic zones of Kerala. These two genera were seen in soil and root samples. Rotylenchulus sp. was more predominant than Meloidogyne sp. in tapioca, sweet potato and diascorea than in colocasia /xanthoma and coleus. The two genera of nematodes are potential threat to cultivation of all the five tuber crops in Kerala. Pratylenchus sp. was also observed in soil samples associated with all crops but it could be collected from the root samples of diascorea and colocasia/xanthosoma only. In high population the genera may become a potent meance to both the crops since its pathogenecity on these crops have already been established. Helicotylenchus sp. and Radopholus sp. could be seen in soil samples associated with all the crops, Helicotylenchus sp. was more predominant in distribution and intensity in the tuber crops in all the agroclimatic zones. Hoplolaimus sp. was seen associated with diascorea alone in soil samples among the five crops in most cases. Tylenchorynchus sp. could be obtained from soil samples associated with all crops except diascorea. Heterodera sp. and Tylenchus sp. were recorded from colocasia/xanthosoma and sweet potato very sparsely.
The effect of varying levels of M. incognita on the growth parameters and yield of C. esculenta was assessed in a pot culture experiment. A population of 500 larvae per plant found causing 12 per cent yield reduction over control. With an increase in the initial population of the pest a progressive suppression in plant height, number of leaves, shoot weight, root weight and weight of tubers and an increase in trend in pest population, root knot and percentage of galls (weight basis) were observed. But these were not linear. Apparently healthy tubers harvested from the nematode inoculated treatments suffered spoilage in storage. Histopathological observation of M. incognita infested colocasia roots showed that the damage was similar to those reported in colocasia roots showed that the damage was similar to those reported in coleus and sweet potato.
A pot culture experiment revealed that the extent of adverse effect on the tuber crops did not show wide variations at the normal and water stress conditions. In coleus the adverse effect on the growth of the plant by nematodes was more with higher levels of irrigation. Less frequent irrigation in sweet potato and coleus caused higher reduction in the population of the pests compared to that in dioscorea. Even with lesser population of nematodes more damage can occur in a crop under stress and the response was found to vary with crops.