In addition to host resistance, another way of disease management
is by the use of fungicides. We found that metalaxyl and
azoxystrobin fungicides were effective against P. helianthicola at
concentrations as low as 1 or 2 mg/mL, respectively. For metalaxyl-
M this was in the range as reported for sunflower downy mildew
inhibition by Oros and Viranyi (1987). The used fungicides are
thought to inhibit germination of zoospores and sporangia by
inhibiting ribosomal RNA synthesis (Davidse, 1995) and mitochondrial
respiration (Goodwin et al., 1992). Metalaxyl-M is a systemic
phenylamide fungicide which is predominantly useful
against growing mycelium of P. halstedii on the host (Mouzeyar
et al., 1995). It is used extensively as a seed dressing for controlling
sunflower downy mildew (Molinero-Ruiz et al., 2005). In the
past Viranyi et al. (1992) observed diversity in tolerance to metalaxyl
seed treatment for downy mildew isolates. In our study, we
found no variation in sensitivity to metalaxyl-M between single
spore strain SLS 1000-SS-2-2010 and the field isolate OS-1236
when tested on plants. The host independent stages did not give
hints for specific inhibitions on either of the target processes such
as rRNA synthesis (metalaxyl-M), mitochondrial respiration
(azoxystrobin) or cell wall formation (dimethomorph). The
zoospore release and encystment, which were thought not to be