The topics for “Gardening Corner” columns arise from a variety of sources; many derive from questions asked by friends, neighbors, and readers concerning the culture of a particular species, while others result from observations made by other gardeners related to events occurring in their gardens. These questions and comments provide the best topics, because they reflect real problems, and not just subjects I pick because I like a certain plant, or have read about problems with a specific garden pest or battled a specific plant disease, although those topics find their way into columns, too. A recent text message from my neighbor, Judith, alerted me to a common tree disease I had not encountered in the past. Black knot, a disfiguring and potentially fatal disease, is caused by the fungus Apiosporina morbosa. This fungal disease affects wild and cultivated, edible and ornamental trees of the genus Prunus, members of the Rose family that include plums, cherries, peaches, apricots, and al...