By fall, light brown swellings appear on infected twigs. The spores can germinate and infect new plants in six hours at the optimal temperature and wet conditions. The rain and wind then carry the black knot fungus spores to susceptible plants. Infected branches may wilt, not grow leaves, and can eventually kill the entire tree.Ĭauses: Through spring and summer, mature knots produce spores. Diseased twigs often bend due to knot overgrowth. Velvety, olive-green fungal growth may cover the knots. These knots appear in various areas around the tree and enlarge when the disease is left untreated. Symptoms: Black knot ( Dibotryon morbosum) appears as hard, black swellings or knots, which may stretch 1-6 inches on the tree. Black knot-resistant cherry tree varieties include East Asian cherry, North Japanese hill cherry, and Prunus maackii (Manchurian cherry or Amur chokecherry). Photo Credit: Scot Nelson / Flickr / CC0 1.0Ĭherry Trees Affected: Common among most Prunus varieties, including ornamental, edible, and native types.
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