Research on Dithianon to Evaluate Suspects of Reduced Sensitivity of Venturia inaequalis with Different Methodologies in Italian Apple Areas

Dithianon is a multi-site fungicide and has never been object of suspects and reports of reduced sensitivity and activity. Italian IFP technicians had the suspect of reductions of activity by this fungicide on Venturia inaequalis. Methodologies, in vitro and in vivo were carried out to verify this suspect. Populations poorly controlled with suspects on dithianon and sensible ones were utilized. The tests in vitro permitted to evidence light and non-significant reductions of sensitivity of poorly controlled populations with respect to sensible ones. In vivo tests on seedlings were non-reliable for a general low activity of dithianon. On the contrary, the original in vivo methodology on grafted apple plants showed several reductions of activity, with moderate levels and a spot distribution in orchards. The cause was probably due to the increased treatments with dithianon caused by problems on other groups of fungicides and by a high infective pressure in some years. It is discussed if this reduction can be considered a resistance phenomenon or a temporary modification of the interactions of plant-fungus-fungicide.