DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Downy mildew infection. Light micrograph of a section through a grapevine (Vitis sp.) leaf infected with downy mildew (Plasmopara viticola). Each downy mildew spore germinates and forms a tube that penetrates the leaf's stomata. Together the tubes form a matrix of mycelium that penetrates the cells of the leaf and extracts the nutrients. Hyphae then produce sporangiophores (blue, branching) which protrude through the stomata to the outside of the leaf. They bear sporangia (red-purple) that contain spores and form white, grey or violet patches on the leaves. Magnification: x70 when printed 10 centimetres wide.
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