INTERLABORATORY COMPARISON OF METHODS TO QUANTIFY MICROSCLEROTIA OF VERTICILLIUM-DAHLIAE IN SOIL

In a comparison of different methods for estimating Verticillium dahli ae in soil, 14 soil samples were analyzed in a blinded fashion by 13 r esearch groups in seven countries, using their preferred methods. One group analyzed only four samples. Twelve soil samples were naturally i nfested, and two had known numbers of microsclerotia of V. dahliae add ed to them. In addition, a control was included to determine whether t ransport had an effect on the results. Results differed considerably a mong the research groups. There was a 118-fold difference between the groups with the lowest and highest mean estimates. Results of the othe r groups were evenly distributed between these extremes. In general, m ethods based on plating dry soil samples gave higher numbers of V. dah liae than did plating of an aqueous soil suspension. Recovery of V. da hliae from samples with added microsclerotia varied from 0 to 59%. Mos t of the variability within each analysis was at the petri dish level. The results indicate the necessity to check the performance of detect ion assays regularly by comparing recoveries with other laboratories, using a common set of soil samples. We conclude that wet plating assay s are less accurate than dry plating assays.