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Downy mildew caused by Plasmopara viticola is a devastating disease of grapes. Four formae speciales have been identified in the pathogen, with P. viticola f. sp. aestivalis and P. viticola f. sp. riparia responsible for epidemics in eastern Canada. The objective of this study was to evaluate the effect of temperature on zoospore release from sporangia and the aggressiveness of these two formae speciales. The effect of temperature on zoospore release was determined for incubation time by estimating the per cent empty sporangia. The effect of temperature on aggressiveness was assessed on the grape variety 'Vidal' using a leaf disc assay; incidence, latency, severity, sporulation efficiency during infection, penetration and sporulation processes were evaluated. The optimum temperature for zoospore release was 20°C. A significant interaction between temperature and forma specialis for zoospore release was observed; however, zoospore release did not differ between formae speciales. During infection, penetration and sporulation, the optimum temperature for aggressiveness of the two formae speciales was between 17 and 21°C. Indices of aggressiveness for P. viticola f. sp. aestivalis were higher (P < 0.001) than for P. viticola f. sp. riparia. At the optimum temperature, P. viticola f. sp. aestivalis had a 14.3% higher incidence, 13.3% higher severity, produced 2.3-times more sporangia, and had an infection/sporulation cycle that was one day shorter than P. viticola f. sp. riparia. These results suggest that the two formae speciales have different epidemiological profiles, and that decision-support tools should be adjusted to take into account this differential response to temperature. [ABSTRACT FROM AUTHOR]

Quantifying genes in soil is important to relate the abundance of soil bacteria to biogeochemical cycles. Quantitative real-time PCR is widely used for quantification, but its use with environmental samples is limited by poor reaction efficiencies or by PCR inhibition through co-purified soil substances. Droplet digital PCR (ddPCR) is a technology for absolute, sensitive quantification of genes. This study optimized eight ddPCR assays to quantify total bacteria and archaea as well as the nitrification (bacterial and archaeal amoA ) and denitrification ( nirS , nirK , nosZI , nosZII ) genes involved in the generation or reduction of the greenhouse gas nitrous oxide. Detection and quantification thresholds were compared with those of quantitative real-time PCR and were equal to, or improved, in ddPCR. To validate the assays using environmental samples, soil DNA was isolated from two vineyards in the Okanagan valley in British Columbia, Canada, over the 2017 growing season. Soil properties related to the observed gene abundances were determined. Total bacteria, nirK , and nosZII increased with time and the soil C/N ratio and NH 4 + -N concentration affected total archaea and archaeal amoA negatively. The results, compared with those of other studies, showed that ddPCR is a valid alternative to qPCR to quantify genes involved in nitrification or denitrification.