Influence of environmental factors on germination of <em>Plasmopara viticola</em> sporangia sourced from mediterranean Western Australia

Abstract

Direct germination via the production of a germ tube was demonstrated in the absence of the host for Plasmopara viticola (causal agent of grape downy mildew) sporangia sourced from a major grape production area in mediterranean Western Australia (WA). In general, direct germination was favoured by environmental conditions considered less than optimal for infection by P. viticola. Most notable however was that sporangia had the capacity to germinate in the absence of free water, a factor known to be essential for the production and release of zoospores. The frequency of direct germination was low with only seven among 108 000 sporangia observed producing a germ tube. This is the first study to i) examine the influence of environmental factors viz: temperature, relative humidity (RH) and light, on the direct germination of P. viticola sporangia and ii) establish the frequency of this event. The infectivity of directly germinated P. viticola sporangia however remains unknown. Although rare, the capacity of sporangia to germinate directly and potentially infect the host, most likely when conditions are not conducive for zoospore production or survival, may provide an explanation for the source of the disease during the predominantly hot dry summer months in WA and other climatically similar viticultural areas.