Downy mildew
Downy mildew is a grapevine disease caused by Plasmopara viticola, an oomycete: a fungus-like organism with a different biological ancestry from true fungi. It can damage leaves and bunches, reduce the usable crop, and weaken a vine by removing the healthy leaf area needed for photosynthesis. Warm, wet conditions make it a recurring problem in many wine regions.
Water drives the infection cycle
Resting spores carry the pathogen between seasons. When conditions permit, initial infections establish lesions on green tissue. Further spore production and infection can then amplify an outbreak. Leaf symptoms include yellowish “oilspots,” with downy growth developing on the underside under favourable conditions.
Wetness is central to this cycle. That distinguishes downy mildew from powdery mildew, whose secondary spread can occur without free water. Monitoring rainfall, temperature, and leaf wetness helps growers judge when infection is plausible, while inspecting vines reveals whether the disease has become established.
The current crop and the next season
A damaged bunch directly reduces harvestable fruit. Loss of leaves has wider consequences: less photosynthesis can compromise ripening and the vine's preparation for winter. A University of Georgia account emphasizes this connection between severe defoliation and subsequent cold injury.
The same researchers documented widespread resistance to one fungicide group in Georgia samples collected in 2017.1. Sarah Campbell and colleagues, University of Georgia, 2018 report on fungicide sensitivity in 2017 vineyard samples and associated field trials. This is resistance of the pathogen to a treatment, distinct from disease resistance bred into a grapevine. Repeatedly using a treatment with the same biological target can select pathogen populations that survive it.
Varietal choice, canopy management, observation, and appropriate protection therefore work together. A general description such as “hybrid” does not by itself establish how well a vine withstands downy mildew.
Sources
- P. A. Magarey, Managing Downy Mildew, disease-cycle presentation hosted by the Australian Wine Research Institute.
- Sarah Campbell, Phil Brannen, Clark McAllister, and Nathan Eason, “Downy mildew resistance to QoI fungicides is rampant in Georgia vineyards”, University of Georgia, 2018.