The Effect of Initial Gall and Plant Density On Gall Production By Anguina Funesta, the Vector in Annual Ryegrass Toxicity

Abstract

In a field experiment to determine the effects of initial gall and plant density on gall production by the nematode, Anguina funesta, the toxin-producing bacterium Corynebacterium sp. also developed. Final gall production, the proportion colonised by bacteria and the proportion of seed-heads with bacterial slime were analysed using weighted multiple linear regression analyses. Multiplication rate, as measured by the ratio of final to initial numbers of galls, was highest at low initial gall densities and decreased as the initial population increased. Under-population was not observed with the nematode densities used. Plant density was also important, particularly as the interaction with initial gall density was significant. The efficiency of gall production by the nematode increased to a maximum as initial plant density increased. The optimum ryegrass density for gall production increased as initial gall density increased. Maximum gall production per plant also increased as plant density increased to moderate levels, despite a decrease in production of seedheads/plant. Development of the Corynebacterium sp. may have restricted gall production. The effect may have been greater at low plant densities, as the percentage of galls colonised by bacteria increased as plant density decreased. Similarly, the proportion of seed-heads with bacterial slime increased as plant density decreased. However, at low plant densities the proportion of seed-heads with slime began to decrease. This was probably due to tillers being produced after the time when nematodes carrying the bacterium were invading the plants.