CropsDescription of the fungus
Plasmopara viticola is not a fungus, but a stramenopile (chromist), a microorganism classified with brown algae. It is an obligate parasite which one is currently unable to cultivate on synthetic culture medium. To conserve, multiply and study it, we have to keep it on the leaves or leaf discs of surviving vines (figure 1).
- It develops in the tissues of its host thanks to its mycelium intercellular siphoned , and forms haustoria globular which penetrate into the cells, by invagination of the membrane thereof. These structures allow it to feed at the expense of its host.
- This stramenopile multiplies and is preserved by two types of destructions which it forms on and in the affected organs, and which characterize it.
- The oospores (figures 2 to 4) materialize its sexual form (teleomorph) and ensure its winter conservation in the leaf tissues. Yellow in color, they measure between 25 and 40 µm in diameter. They germinate in the spring and give rise to a macroconidia (figure 3) forming at the end of a mycelial filament. Rather pear-shaped, this macroconidia measures 23-27 x 30-41 µm and produces several dozen zoospores piriform to ovoid (figure 4), measuring 4-5 x 6-10 µm, and mobile thanks to two flagella.
- The sporangiophores (figures 7 to 12), emerging through the stomata, are arbuscles branched perpendicularly and several hundred microns long. Their ends end in pointed sterigmas (figure 12) which carry sporangia hyaline (figures 13 and 14), ovoid to piriform, measuring 10-16 x 15-25 µm. Between 3 and 8 zoospores (6-8 x 4-5µm) (figures 6 and 15), comparable to those forming in oospores, appear at maturity inside these sporangia.
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