Verticillium longisporum is a soil-borne fungal pathogen causing vascular wilt of Brassica crops. This study was conducted to enhance our knowledge on the host range of V. longisporum. Seven crop species (barley, oat, oilseed rape, pea, red clover, sugar beet and wheat) and five weed species (barren brome, black-grass, charlock, cleavers and scentless mayweed) all common in southern Sweden were evaluated for infection by response to V. longisporum. Oat, spring wheat, oilseed rape, scentless mayweed and charlock inoculated with V. longisporum in a greenhouse showed stunting to various degrees close to the fully ripe stage. Based on the extent of microsclerotia formation, explants were separated into four groups: for pea and wheat, <5% of the samples had formed microsclerotia; for scentless mayweed, 5-10%; for oat, 10-20%; and for charlock and oilseed rape >80%. The results suggest that plant species outside the Brassicaceae can act as reservoirs of V. longisporum inoculum. Soil inoculum densities in nine fields were monitored over a period of 12 months, which ranged from 1 to 48 cfu g )1 soil. Density of microsclerotia was lowest just after harvest, reaching its maximum six months later. No significant correlation between inoculum density in soil and disease incidence on oilseed rape plants was found. However, the data suggest that a threshold of 1 cfu g )1 soil is needed to cause disease on oilseed rape. Species identification based on microsclerotia morphology and PCR analysis showed that V. longisporum dominated in soil of seven, and V. dahliae in two of the nine fields studied.

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Verticillium wilt can cause high losses in tree nurseries. To be able to predict disease and unravel disease dynamics over time and space, the relationship between verticillium wilt and soil inoculum densities of Verticillium dahliae and the nematode Pratylenchus fallax was studied in two 4-year field experiments with Acer platanoides and Catalpa bignonioides in the Netherlands. Best-fit regression equations showed that pre-planting inoculum densities of V. dahliae can be used to predict verticillium wilt over a period of at least 4 years. Pratylenchus fallax contributed significantly to disease severity in A. platanoides in some years. Disease can already occur at the detection limit of the pathogens. The 5% infection thresholds for V. dahliae were at 1 (A. platanoides) vs. 3 (C. bignonioides) colony-forming units (CFU) g −1 soil. Analysis of spatial relationships indicated that diseased plants had a higher influence on neighbouring plants at low V. dahliae inoculum densities (<5 CFU g −1 soil) than at high densities (≥5 CFU g −1 soil). Seventy-four percent of the diseased plants recovered during the following year.After that year, recovered plants had a significantly higher probability of becoming diseased again than plants that were healthy during the two previous years, at high inoculum densities of V. dahliae, indicating that inoculum density in the soil, rather than incomplete recovery, was the most important factor for disease development.

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Two hypotheses which might explain a recent increase in the incidence of verticillium wilt of chrysanthemums in glasshouses in the Netherlands were investigated, viz whether selection for increased resistance to elevated temperatures has occurred due to frequent steaming of soils in the glasshouses, or whether the strains of Verticillium dahliae occurring in chrysanthemum glasshouses are particularly virulent towards this host. Following artificial inoculation, five isolates of V. dahliae from chrysanthemum were pathogenic on chrysanthemum but five isolates from potato were non-pathogenic for this host. When inoculated onto potato plants, all isolates caused early senescence with no significant difference between the two groups of isolates. In amplified fragment length polymorphism analysis, the potato isolates formed a cluster distinct from all other isolates. As a group the chrysanthemum isolates were no more diverse than the potato isolates but did not form a cluster distinct from 12 other isolates tested. This suggests that high pathogenicity to chrysanthemum has developed on several occasions but that the group of potato isolates were possibly monophyletic. Microsclerotia produced in vitro from the chrysanthemum isolates had significantly lower average lethal temperature tolerance than those from the five potato isolates suggesting that being able to resist the effects of soil sterilisation by steam is not a factor in wilt of chrysanthemums in the Netherlands.