Resistance management in Vf apple scab resistant organic apple orchards

Summary

Modern Vf scab resistant apple varieties open the way for organic growers to lower fungicide input, higher yields, better skin quality, more biological control for mites and insect pests and better consumer acceptance of their management practices. Manny examples in the past years have shown however that the Vf resis-tance can be easily overcome by local scab populations in north-western Europe. Discussions during the meetings of the IOBC working group Diseases in Orchards in 2000 in Fontevraud (France) and in 2003 in Lindau (Germany) lead to a set of management practises necessary to prevent gene-fulx, and selection to-wards Vf virulence in the local apple scab populations.
Eleven orchards of the Vf resistant apple variety Santana that where planted between 1998 and 2000 where monitored for apple scab lesions on fruits form 2002 to 2005. The results where evaluated in respect tot the applied resistance management practices. We conclude that fungicide treatments on the major primary scab infections are the key factor in the resistance management on Vf resistant apple varieties. From a practical viewpoint these early season fungicide applications are also necessary for the control of powdery mildew, as the main Vf- resistant apple cultivars appear to be relatively susceptible to powdery mildew.
For private gardens apple varieties with field resistance to apple scab should be advised, not Vf resistant varieties.