Distribution pattern and mortality of the white tip nematode, Aphelenchoides besseyi (Nematoda: Aphelenchoididae), among rice seeds

Abstract Aphelenchoides besseyi is a seed-borne nematode and the causal agent of rice white tip disease. To determine the relationships between nematode numbers per seed, size of seed harbouring nematodes, and nematode mortality, 18 panicles were collected from 12 paddy fields showing three different levels of white tip disease. The panicles were investigated after storage for 30-82 days at 4°C. There was no nematode mortality within seeds during storage. Mean nematode number per seed increased and mean degree of seed swelling decreased as the paddy field infestation of white tip disease increased. For paddy field means, there was a significant, negative correlation between the mean nematode number per seed and the mean degree of rice seed swelling. Intriguingly, for individual seeds, the mean degree of swelling increased up to that typical of well-developed seeds with increasing nematode number per seed. Nematode mortality occurred in an inversely density-dependent fashion within individual seeds. The nematode exhibited a clumped distribution among seeds in each paddy field. Such ecological features of the nematode might contribute to its persistence in rice plant population.