EFFECTS OF CROP SPECIES, CROP CULTIVARS AND ISOLATES OF VERTICILLIUM-DAHLIAE ON THE POPULATION OF MICROSCLEROTIA IN THE SOIL, AND CONSEQUENCES FOR CROP YIELD

In a micro-plot experiment the development of the inoculum density of Verticillium dahliae in soil was studied and consequences for yields o f potato cvs Element, Ostara, Astarte, and Mirka, faba bean, flax, pea , barley, sugar beet and onion were evaluated. In May 1991, 75-litre c ontainers were filled with sterile soil. Soil was infested with 2 or 2 00 microsclerotia per g of a potato isolate or a faba bean isolate of V. dahliae. The same crop species and cultivar was grown on the same p lot in 1991 and 1992. For flax, a non-pulled crop was compared with a pulled crop. Fallow plots were included as a control. In 1993, potato cv. Element was grown on all plots. During 3 years the soils were samp led, and the soil inoculum densities were assessed by plating the soil on a semi-selective medium. In 1991, yield differences in dry matter per plot between isolates were only significant for potato cv. Element (lower yield when infected with potato isolate), and potato cv. Astar te, faba bean, and sugar beet (lower yield when infected with faba bea n isolate). In 1992, isolates and infestation levels did not affect th e dry matter yields per plot. In 1993, haulm dry matter yield per plot of potato cv. Element was affected by the isolate, the inoculum densi ty and the crop grown in the previous years. Plots previously cropped to potato cvs Element, Astarte and Mirka gave the lowest dry matter yi elds in 1993. No decrease of inoculum density occurred under any crop species or cultivar grown. Increase of inoculum densities was signific ant for potato cvs Element, Ostara, Astarte, and faba bean at the low initial soil infestation level. At the high initial soil infestation l evel the effect of crop species and cultivar on the inoculum density d epended on the isolate used. Potato cultivars seemed to be more suscep tible to potato isolate than to faba bean isolate and faba bean seemed to be more susceptible to faba bean isolate than to potato isolate. R emoving flax culms from the field diminished the increase of the inocu lum density in treatments infested with potato isolate at a high level . In 1993, the concentration of microsclerotia in the haulm debris was positively related to the inoculum density in the soil, and the haulm dry matter yield was negatively related to the inoculum density in th e soil.