PRE-PENETRATION EVENTS AFFECTING HOST-SPECIFICITY OF VERTICILLIUM-LECANII

The pathogenicity of two isolates of Verticillium lecanii towards non- target invertebrates and six species of aphid was tested in bioassays. There was no evidence of infection in any of the 20 non-target invert ebrates. All target aphid species were susceptible to at least one iso late. The level of susceptibility varied between aphid species, with M yzus persicae and Brachycaudus helichrysi showing the greatest suscept ibility. Studies on initial spore deposition, retention and germinatio n on two susceptible aphid species and four resistant non-target insec ts revealed differences in the behaviour of the pathogen between the t wo groups. Spore density mm(-2) was greater on susceptible insects bot h immediately post-inoculation and following 24 h incubation. There wa s no loss of spores from M. persicae over 24 h. All other species show ed more than 50% spore loss in the same period. However, even after 90 % spore loss, large numbers of spores remained on the bigger insects s uch as Agonum dorsale. Germination and germ-tube growth were possible on such resistant non-target insects, indicating that resistance to in fection occurs after this stage. There was little difference in rate o f germination on the two groups. However, differences in germ-tube gro wth were evident, with the production of longer and narrower germ-tube s on the resistant insects.