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

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 appears 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.

Persistence of <i>Venturia inaequalis</i>Populations Resistant to Strobilurins in the Field and in the Glasshouse

The research was focused on the persistence of resistance of V. inaequalis to strobilurins from resistant populations occurring in the field. It was studied with two types of experiments: the first in apple orchards where resistance occurred in 2005 and employ of these fungicides was suspended from 2006 to 2011, realizing every year sensitivity/resistance assays on strobilurins. In the second was verified the sensitivity/resistance of the same 2015 populations, repeatedly multiplied (ten inoculations) in glasshouse on apple seedlings. Results showed that the resistance survived in orchards all the years of tests (six years), despite sensitivity fluctuations occurred on infected leaves due to sexual crosses, competition with sensible strains and soil management. In glasshouse, the level of resistance of all populations increased quickly in first inoculations and values remained high (EC50 > 10 mg·L-1) in all the following inoculations, until the last, the 10th. V. inaequalis resistant strains seemed to show high fitness: relevant persistence in the field, and a capacity of prevail on sensible ones in absence of factors of field variability.