Click on any
image for a more
detailed view

Pathogen Biology

The pathogen, Plasmopara viticola, produces asexual, biflagellate zoospores and sexual oospores. Its mycelium is aseptate. It is an oomycete in the order Peronosporales. Other important downy mildew pathogens that belong to this group include species within the genera Bremia, Peronospora and Sclerospora. Most taxonomists no longer include oomycetes within phylogenetic groupings containing ascomycetes or basidiomycetes, and have placed them instead in the kingdom Chromista. However, phylogenetic considerations aside, oomycetes share many biological, ecological, and epidemiological characteristics with fungal plant pathogens.

Plasmopara viticola is an obligate parasite, and it absorbs its nutrients from the living host tissue via globose haustoria. The hyphae are largely internal in the host. Sexual reproduction occurs through the fusion of antheridia and oogonia within the host tissue. Plasmopara viticola has only recently been shown to be heterothallic. The resulting sexual spore is an oospore, which is the survival and resting stage of the pathogen.

Oospores represent the primary inoculum, and may overwinter in leaf litter or may be released into the soil as leaves decay or are buried by detritivores. They generally begin to germinate in significant numbers shortly after bud break of grape, and populations of oospores may continue to germinate for the entire growing season in some growing regions. Oospores form a single germ tube terminating in a sporangium. Zoospores form within the sporangia and are then released. Zoospores germinate and penetrate the plant only through functioning stomata, i.e., only on green host tissue. Sporangia and zoospores are easily desiccated. They die within 2 to 3 hr of exposure to low humidity and sunlight, so most infection occurs soon after their release. However, they may survive on leaf surfaces for more than 24 hrs under cool humid conditions.

Sporangia (Figure 8) also serve as a means of secondary spread of the pathogen. Treelike sporangiophores, bearing white, lemon-shaped sporangia, are produced from a mycelial mat within the host tissue and emerge through stomata. Sporangia are disseminated by wind and rain splash. Zoospores released from the sporangia swim in free water on the grapevine surface, and encyst near a stoma. Zoospores then germinate and penetrate through a stoma by the means of a germ tube. Oospores are produced in infected host material towards the end of the season. The resulting oospores are thick-walled and serve not only as survival spores during the intercrop period, but are also a source of genetic variation.

Figure 8

RETURN TO TOP