The relationship between soil inoculum density and plant infection as a basis for a quantitative bioassay of Verticillium dahliae

Using potato, eggplant and thorn apple as test plants, the relationship between soil inoculum density and plant infection was studied as a basis for the development of a quantitative bioassay of Verticillium dahliae. A linear relationship was demonstrated (P < 0.05) between soil inoculum density and population density on roots for all three test plants and for soil inoculum density and population density in sap extracted from stems for eggplant. Correlation coefficients were higher with densities on or in roots (R² varying from 0.45 to 0.99) than with densities in stems (R² varying from 0.04 to 0.26). With eggplant, population densities on/in root and in sap extracted from stems were significantly correlated at 20 and 25°C with Pearson's correlation coefficients of 0.41 and 0.53, respectively. For potato, root colonization was higher at 15 than at 20°C, whereas the reverse applied to eggplant. Stems of potato were less colonized than stems of eggplant. The pathozone sensu Gilligan (1985) was calculated to be <300 µm, indicating that infection was caused by microsclerotia which were located close to the roots. To assess the density of V. dahliae in plant tissue pipetting infested plant sap on solidified ethanol agar medium without salts yielded higher densities than using pectate medium or mixing sap with molten agar. A bioassay for determining effects of (a)biotic factors on development of V. dahliae in the plant is recommended with eggplants as a test plant, grown in soil infested with 300 single, viable microsclerotia g^-1 soil at a matric potential of –6.2 kPa, and incubated at 20°C for 8 weeks.