What is a key characteristic symptom of black knot disease?

A key characteristic symptom of black knot disease is the presence of black, knot-like galls on the branches and twigs of affected plants, particularly in species like black cherry, chokecherry, and plum trees. These galls are a direct result of the fungal pathogen, Apiosporina morbosum, which causes the tissue to swell and form distinctive, dark, irregular growths. Over time, these galls can hinder plant health by restricting growth and can even lead to branch dieback if left untreated. This symptom is particularly noticeable during the growing season as the galls can become quite large and impact the visual appeal of the tree.

In contrast, other choices, like yellow leaves, wilting flowers, and brown spots on fruit, are associated with a variety of other plant diseases or conditions but do not specifically indicate black knot disease. Each of these symptoms would point to different issues that might affect plant health, such as nutrient deficiencies, environmental stress, or other pathogens, but they do not reflect the signature appearance that is definitively linked to black knot disease.