Plasmopara viticola, the causal agent of grapevine downy mildew, is a heterothallic oomycete that overwinters as oospores in leaf litter and soil. In the spring, oospores germinate to produce macrosporangia, which under wet condition release zoospores. Zoospores are splashed by rain into the canopy, where they swim to and infect through stomata. After 7–10 days, yellow lesions appear on foliage. During favorable weather, the lesions sporulate and new secondary infections occur.
Plasmopara viticola, also known as grape downy mildew is considered to be the most devastating disease of grapevines in climates with relatively warm and humid summers. It was first observed in 1834 by Schweinitz on Vitis aestivalis in the southeastern of the United States. Shortly after this first observation, the pathogen was introduced to European countries where it played a devastating role in the yield and production of their grapes, and consequently their wine. France was among the first of the European countries to gain experience in dealing with the pathogen. Within just a few years of the pathogen's introduction the French attempted to graft American root stock to their own vines in order to produce a more resistant strain of grape. Depending on the year, production of grapes in France has been estimated to have been reduced by as much as 50%. Because of numbers and results like these, downy mildew has been considered the most devastating disease of a filamentous pathogen to affect European vineyards.
Symptoms cover a fairly large range depending mostly on the host. Common symptoms include necrosis of the stem or shoot, discoloration including brown spotting (lesions) and yellowish-green tips of the leaves. Grapes may exhibit sporangia and sporangiophores, appearing as white to gray coat on the outer surface.
Downy mildew has a specific set of environmental conditions to reproduce and infect. A warm, moist, and humid environment is required. Studies in Sicily have shown optimum time for oospore germination is between the end of February and the middle of March. With this understanding, if fungicides are used just before optimum conditions occur, they have proven to be an effective control method of the pathogen. Other control methods include proper watering, and a good location where the plant can receive continual sunlight.
The hosts of the Plasmopara viticola pathogen are the European Vitis vinifera and the American vitis varieties. The European cultivar is most susceptible to the pathogen, as it lacks evolutionary resistance that the American varieties have. This is because the pathogen originated in the Americas and was later transmitted into Europe. Regardless of cultivar, once this oomycete lands on a susceptible host, the disease cycle begins. The first symptom to occur includes irregular, yellow spots on the surface of the leaves. Over time, if one follows these spots through a cross-section of the leaf to the bottom, bunches of sporangia will be found. Sporangia will form the “white downy” appearance of the leaves, giving the pathogen it’s common name. As the disease progresses, the yellow surface spots turn brown due to necrosis, death, of the plant tissue. Furthermore, the underside of the leaves progress from a white color to grey. The ultimate result is premature defoliation, when the infected leaves detach from the stem. Moving from symptoms of the leaves to that of the grapes, when the pathogen is present, the growing fruit become covered with the sporangiophores. This results possible thickening of the fruit “skin” or distortion of shape. A color change from purple to a reddish brown can be noted as well as a mosaic pattern when the grapes are infected.
The disease development of grape downy mildew is known to be heavily reliant on the efficiency of the asexual propagation cycles. Kiefer et al. (2002) demonstrated that the early development of Plasmopara viticola is regulated specifically and coordinately by unknown factors originating from the host grapevine plant Vitis vinifera. The host factors influence the pathogen development in three ways: (i) accelerating the release of zoospores from mature sporangia, (ii) coordinating the morphogenesis of the germ tube through the reorientation of the polarity of the zoospores during the attachment to the host cell, and (iii) targeting the zoospores to the stomata by active chemotaxis from the open substomatal cavity. The expression of pathogenesis-related (PR) genes by the host plant V.vinifera during the infection of P.viticola has been investigated over the years. The expressions of PR-2, PR-3 and PR-4 genes are induced in the grapevine host during pathogen infection, which encode for cell wall-degrading enzymes B-1,3-glucanase (PR-2) and chitinases (PR-3 and PR-4). It was previously understood that oomycetes differ from true fungi by the presence of cellulose in the oomycetes’ cell walls as opposed to chitin in true fungi. However, it was found that chitin synthesis is regulated during in planta growth and asexual propagation of P.viticola and this is further demonstrated by the presence of chitin on the cell walls of the hyphae, sporangiophores and sporangia of the grape downy mildew pathogen. Hence, both the cell wall-degrading enzymes are synthesized by the host grapevine plant specifically to target and degrade the cell walls of the oomycete pathogen. In addition, the upregulation of the PR-9 gene that encodes for peroxidase, which is a reactive oxygen species is associated with the systemic acquired defense of the grapevine host. The roles of other constitutively expressed PR genes during P.viticola infection such as PR-5, PR-1 and PR-10 genes remain ambiguous. PR-5 is involved in the synthesis of thaumatin-like proteins and osmotins, which are believed to inhibit the spore germination and germ tube growth of Plasmopara viticola by creating transmembrane pores.
The history of downy mildew control began with an accidental discovery in 1882. In order to prevent passersby from eating the grapevines close to the road, Pierre-Marie-Alexis Millardet sprayed those vines with a mixture of copper sulfate and lime, which was both seeable and awful-tasting. He then noticed that the treated grapevines did not show any symptoms of downy mildew, whereas the rest of the vineyard was infected by the disease. After further studies, Millardet published the recommended treatment of the grapevines against the downy mildew in 1885 in which he proposed the use of 8:15:100 of copper sulfate: hydrated lime: water mixture in the treatment (later named as Bordeaux mixture after the Bordeaux region where Millardet conducted the research). The treatment of Bordeaux mixture against the downy mildew was well-received globally due to its strong adhesion to the leaves, its long perseverance in the vineyard, and its color, which allows for observable distribution of the treatment.
Copper-based control methods are still commonly used today. As science became more precise, the amounts of copper-sulfate used in solution were optimized for best control of the Higher concentrations of copper-sulfate (3-4%) are recommended for high risk conditions, while low concentrations (1-1.5%) are for low risk conditions. The risk of susceptibility for the plant is highly correlated with the season. Most of the ineffectiveness in fungicide use is correlated with spraying at inappropriate times. It is necessary to spray right before budding in the spring. Furthermore, coating the entire leaf with a film of the fungicide is necessary for control; thinning the foliage makes achieving an entire coat possible.
Unlike in Vitis vinifera, there is a wide range of susceptibility to downy mildew among Vitis interspecific hybrids. For example, among North American grapevine species, Vitis aestivalis and Vitis labrusca are moderately susceptible while Vitis cordifolia, Vitis rupestris and Vitis rotundifolia are relatively resistant. Certain Vitis interspecific hybrid cultivars display organ-specific resistance. For example, leaves of Aurore and Delaware cultivars are moderate to highly susceptible, whereas their fruit are highly resistant to downy mildew. Another example is the cultivar Chancellor in which the leaves exhibit moderate levels of resistance to downy mildew, while the clusters, tendrils and shoot tips are highly susceptible to the disease. Several new European cultivars such as Regent have been developed from progeny of crosses between V.vinifera and resistant North American species in an effort to incorporate the most desirable qualities of both parental branches.