Population dynamics of Verticillium species in cauliflower fields: Influence of crop rotation, debris removal and ryegrass incorporation

Verticillium wilt, caused by the soil-borne fungus Verticillium longisporum, leads to economic losses in cauliflower production in Belgium. Development of sustainable control measures of the disease is necessary. Previous studies in our laboratory have shown that incorporation of ryegrass can reduce the viability of V. longisporum microsclerotia in soil. However, field experiments are lacking in Belgian conditions. To gain knowledge about the population dynamics of V. longisporum and its relationship with Verticillium wilt, experiments were conducted from 2006 to 2010 in two cauliflower fields (Oppuurs and
Ardooie) with history of Verticillium wilt. Three main crop systems (fallow, cauliflower and cauliflower followed by removal of debris) and two cover crop systems (no cover crop and ryegrass) were tested. The results of this study showed that (i) crop rotation intensity of cauliflower cultivation does not affect the inoculum density of microsclerotia forming Verticillium species in soil, (ii) amendment of lignin-rich ryegrass may reduce the soil inoculum, and (iii) seasonal fluctuations of inoculum densities occur but any soil inoculum variation above a level of one microsclerotium per gram of soil does not affect disease
levels. Furthermore, we found that Verticillium wilt of cauliflower in Ardooie was reduced in soil containing Verticillium tricorpus-like organisms. The Verticillium isolate Vt305 obtained from the cauliflower field in Ardooie was morphologically similar to V. tricorpus, but its rDNA ITS region showed 100% identity with the non-pathogenic species Verticillium isaacii, recently described in literature.