Observations on the Behaviour of Different Populations of Plasmopara viticola Resistant to QoI Fungicides in Italian Vineyards

Abstract

Grapevine downy mildew, caused by Plasmopara viticola, is probably the most damaging fungal disease
of grapevine world-wide. Among the fungicides recently developed for downy mildew control is the QoI class of
fungicides, which inhibits mitochondrial respiration. Since 1999, selected P. viticola populations in northern Italy
have been monitored for resistance to QoI fungicides. Detached leaf discs and whole potted plants were used under
controlled conditions to test the sampled populations. QoI-resistant populations of P. viticola were found in all the
vineyards sampled in 2001 and 2002 in Trentino Alto Adige and Friuli Venezia Giulia, where failure in QoI control
was reported. Many of the populations had minimum inhibition concentration (MIC) values 3– 30 times higher than
those of sensitive reference populations. Populations of P. viticola sampled from vineyards in Piedmont, where no QoI
fungicides had previously been used, showed MIC values equal to, or lower than those of the reference populations.
Most of the P. viticola populations collected in Trentino Alto Adige in 2001 showed high virulence in leaf disc test and
were not controlled by QoI fungicides, applied both at field and double field rates in the whole plant test. Most of
these populations retained their virulence in the subsequent leaf disc test in water.