Verticillium wilt in nursery trees: damage thresholds, spatial and temporal aspects

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.