Quantitative isolation of Verticillium spp. from soil and moribund potato haulm

SUMMARY A technique to isolate a high proportion of the propagules of important Verticillium spp. from soils is described with modifications for quantitatively distinguishing between the microsclerotial and conidial fractions of V. dahliae and estimating the microsclerotial population and its viability in dried potato stems. In pots, soil populations of V. dahliae with less than c. 40 propagules/g failed to induce disease in potato plants; with larger populations there was some relation between disease severity and fungal population. In soils artificially infested with V. dahliae (c. 50000 propagules/g dried soil) the viable population decreased within 6 months to c. 100 propagules/g and then remained stable for more than a year. From fields in South Wales and East Anglia large populations of V. nigrescens and V. lateritium and small populations of V. dahliae were obtained.