The Other Guignardia...

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Joe Boggs wrote a great article “Guignardia Leaf Blotch Running Rampant” the other day, which caused me to reflect on another Guignardia fungus often ignored, but very important.  The other Guignardia, Guignardia bidwellii, is one responsible for causing the disease Black Rot of Grape.


Black Rot of Grape- Guignardia bidwellii
Black Rot of Grapes Guignardia bidwellii fungal lesions on grape leaves


This fungus causes the green, unripe grapes to first show a tiny round, black spot on the grape. This little round black spot suddenly becomes a small, round, light-brownish spot to form on the fruit.  The infected brown tissue in the spot begins to soften and collapse.  The brown spot enlarges quickly, encompassing the entire berry in just a few days.  The diseased fruit shrivels up, looking very much like a raisin attached to the cluster.  These raisins are hard, black, wrinkled and really called mummies.  Tiny black pycnidia can also form on the fruit mummies.


Black Rot of Grape fungus lesions on foliage
Black Rot of Grapes fungal lesions on grape leaf


While the loss of grapes is often catastrophic for most planning to flood the world with their homemade wine, the symptoms of Guignardia bidwellii often go unnoticed on the leaves of grapes.  The infections on the leaves start out as small, yellow spots, which then enlarge and form a dark border around the margin of these lesions.  The centers of the lesions turn reddish-brown and when lesions reach 1/8-1/4 inch in diameter, minute black dots appear inside of the lesion.  The black dots are fungal fruiting bodies (pycnidia) and contain thousands of summer spores (conidia).  These spores are released to continue to infect the fruit and the cycle just continues on to make grape growers… wine in disgust!