Evaluation of tactics far managing resistance of Venturia inaequalis to sterol demethylation inhibitors

The impact on the selection and control of subpopulations of V. inaequalis resistant to the sterol demethylation inhibitor (DMI) fenarimol or to dodin e were evaluated with respect to several tactics of apple scab control. Exp eriments were conducted in an experimental orchard with elevated levels of DMI and dodine resistance over a period of three consecutive seasons. The D MI-resistant subpopulation was poorly (14%) controlled at a fenarimol rate of 15 mg/liter (sprayed to run-off), whereas control was significantly impr oved (54%) at twice that rate. Mancozeb mixed with the low rate of fenarimo l also improved the control of DMI-resistant isolates, but the improvement was due to the indiscriminate control of both the DMI-sensitive and -resist ant populations provided by mancozeb. The selection of fenarimol-resistant isolates resulting from poor control of the resistant subpopulation by the low rate of fenarimol was equivalent whether fenarimol was applied singly o r in mixture with mancozeb. Consequently, the use of high DMI rates in mixt ure with a protective fungicide is expected to delay the build-up of resist ant subpopulations by limiting their increase through two separate principl es of control. For dodine in mixture with fenarimol, it was found that each mixing partner applied alone selected both fenarimol- and dodine-resistant isolates. This selection pattern was partly explained by the possibility t hat one of the multiple genes underlying fenarimol and dodine resistance co nfers resistance to both fungicides, in addition to the selection of double -resistant isolates. Regardless, a mixture of fenarimol with dodine each em ployed at a low rate controlled both the fenarimol- and the dodine-resistan t subpopulation at least as effectively as the individual components at twi ce their mixture rate, and an accelerated selection of double-resistant iso lates was not detected. In commercial orchard trials, mixtures of DMIs with either a protective fungicide or with dodine provided equivalent control e ven when levels of DMI resistance, dodine resistance, or both were moderate ly elevated. With the exception of orchards with high levels of DMI or dodi ne resistance, dodine might be an alternative to protective fungicides as a mixing partner with DMIs.