Downy mildew of Grapes

Plasmopara viticola

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 detatch 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.

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