Researches on Methodologies to Verify Reduced Sensitivities of <i>Venturia inaequalis</i> in Field to Difenoconazole and First Indications of a Survey in Italy

Abstract

Resistance of Venturia inaequalis to the first IBS triazoles occurred in the 80’s in Italy and other countries. Successively number of applications sensibly decreased and problems of resistance to IBS generally disappeared. In 2010 suspects of resistance to the newer IBS difenoconazole, the actually most used IBS, appeared in several countries. In vitro tests the results confirmed the previous researches: the monoconidia from populations which were poorly controlled showed moderate reduced sensitivity respect to well controlled and wild-types. New in vivo tests were realized with preventive and curative applications on seedlings. No different activity among the three types of populations (poorly, well-controlled and wild-types) was found in preventive applications. On the contrary, in curative applications, wild-types and well controlled populations showed a visible and statistically significant activity respect to the poorly controlled. A survey of sensitivity to difenoconazole was carried out in two Italian apple areas in 2013-2015 and showed the presence of populations resistant to difenoconazole. Probably the cause of the resistance spread was the increase of the number of applications of this IBS in the last years. However the problem appears more complex because nowadays resistance regards about all fungicides and with a higher severity respect previous decades. An increase of virulence of V. inaequalis strains appear s to be the cause of this situation also for the high presence of cultivars with high or moderate susceptibility to scab. Foreseeable remedies consist in a differentiate and appropriate setting of each chemical groups of fungicides in apple scab control, but in long terms new strategies in apple cultivation should be necessary.