Effects of individual and combined use of bio-fumigation-derived products on the viability of Verticillium dahliae microsclerotia in soil

• Alternative products can reduce the level of Verticillium dahliae inoculum in the field.
• Combining two or three products resulted in efficacies less than the expected.
• These products are unlikely viable to control wilt on their own for strawberry.
• The value of combining products is questionable.

Verticillium dahliae is the causal agent of strawberry wilt. A microencapsulated terpene product containing cineole, camphor and borneol, digestate from anaerobic digestion, and BioFence™ derived from a mustard-based defatted seedmeal were tested for their suppressive activity against V. dahliae. First, naturally infested soil was amended with microencapsulated terpene, lavender waste pellet and BioFence™ (pellet) in a laboratory test to assess the efficacy against V. dahliae. Next, mini-field-plot experiments were conducted to evaluate the efficacy of individual and combined use of terpene, BioFence™ (liquid) and digestate against V. dahliae; sterile distilled water treatment and untreated control were also included. In the laboratory test, all treatments significantly reduced V. dahliae densities, with the control efficacy ranging from 27% (BioFence™) to 69% (lavender waste pellet). Although the lowest (1×) rate of terpene treatment resulted in a much lower control efficacy (35%) than the other two higher rates (3× – 55%; 9× – 53%), these differences were not statistically significant. In the field mini-plot trials, all treatments led to significant reductions in the V. dahliae density, with the efficacy ranging from 50% (digestate) to 78% (combined three-product treatment), irrespective of the initial wilt level. There were no significant differences in all comparisons of pairwise treatments except between digestate and combined three-product treatment. For the combined two or three-product treatments, the observed efficacy was significantly less than the expected efficacy on the assumption of Bliss independence. Furthermore, there were no significant differences between the observed efficacy of combined treatments and the best single component product efficacy. Although the observed efficacy for the combined three-product treatment was consistently higher than the best single component across replicate plots, such a difference was not statistically significant. The results indicate the value of these alternative treatments in practice but these are not likely to reduce V. dahliae inoculum sufficiently to eliminate the risk of strawberry wilt and question the value of combined treatments.