Density-related establishment and reproduction of Aphelenchoides besseyi (Nematoda: Aphelenchoididae) populations on Oryza sativa

Aphelenchoides besseyi Christie (Nematoda: Aphelenchoididae) is a seed-borne ectoparasite of Oryza sativa rice and causes ‘white tip’ disease. When seeds are soaked in water, the nematodes emerge from them, arrive at rice plants by swimming in flooded paddy fields, and reproduce on them. Two separate experiments were conducted to determine the relationships of the nematode abundance in water to the arrival (infection) and subsequent multiplication of nematodes, the disease incidence, and the yield of rice grains. Experiment 1 showed that, as the A. besseyi population (Pi) surrounding the seedlings increased, both the disease incidence of plants and the number of nematodes (Pf) in all seeds harvested per plant increased, but the rate of increase in the nematode population (Pf/Pi) decreased. Negative relationships were found between the Pi level and the estimated probability of nematode arrival, although it was extremely difficult to isolate all arrivals from plants (experiments 2 − 5). However, the number of arrivals was estimated to increase with the Pi level. It is considered from these results that the density-dependent decrease in the rate of increase (Pf/Pi) was determined largely by the density-dependent reduction in the rates of nematode establishment (colonization) and/or post-establishment reproduction on plants.