Evaluating Verticillium dahliae for biological control of Ophiostoma novo‐ulmi in Ulmus minor

The ability of isolate Vd‐48 of Verticillium dahliae to induce resistance against subsequent challenge with Ophiostoma novo‐ulmi was examined in Ulmus minor . In the first experiment, conditioning inoculation of 5‐year‐old elm trees (2–3 m in height) with Vd‐48 15 days prior to challenge inoculation with O. novo‐ulmi significantly reduced wilting ( P ≤ 0·05) compared with trees not conditioned with Vd‐48. However, in another experiment on 6‐year‐old trees (2–3·5 m in height), no protection was achieved when the length of time between conditioning and challenge inoculations was 45 or 60 days. In a further experiment, inoculations with Vd‐48 alone resulted in severe wilting in 22 out of 118 trees (6–10 years old and 4–7 m in height). Across the 2 years of this last trial, nine trees showed massive feeding wounds made by Scolytus sp., while O. novo‐ulmi strains were isolated from six trees. Vd‐48 provided a variable prophylactic effect against O. novo‐ulmi in U. minor . Major difficulties with this approach to control Dutch elm disease are discussed.