Bacterial and fungal diseases inflict Yoshino cherry trees (Prunus x yedoensis), diminishing their foliage health and potentially causing branch die-back and reduced springtime flowering. This hybrid species of ornamental cherry from Japan suffers less from diseases as most other cherry species, however. Severe infections of diseases kill cherries.
With cankers, bacteria typically enter the cracks in bark of branches in autumn and then prosper the following spring. A swollen area on the branch oozes gum, and if the canker encircles a branch, it kills all tissues above it, affecting leaves and flowers at branch tips.
Also the result of a bacterium, crown gall enters a Yoshino cherry only through trunk wounds near the root flare, or the transition area from the trunk to the underground root system. Once infected, small frosty brown balls form in the branches, eventually becoming black and hard. No treatment exists for this disease, and prevention is the cure. Avoid mechanical injuries to the root flare from lawn mowers and weed whackers.
With leaf spot, bacteria and fungi cause small red to purplish black spots to occur on foliage first, eventually progressing to cause entire leaf blades to brown and rot out in their centers. Typically cool, wet springtime weather favors the infection of leaf spot.
You first see black knot in fall or winter once foliage drop and you see thick masses of black along sections of tree branches. The masses feel hard and brittle, like dried knots of tissue. This fungal disease, native to and most prevalent across North America, inflicts most harm to plums and apricots, but harms cherry species, too.
Witches broom results from a fungus that causes irregular, small clusters of twigs to form a "broom" on normal tree branches. This conditions fails to harm the tree, but results in untimely, earlier flowering and leafing out on the Yoshino cherry, detracting from its visual ornamentation. Branches also dwarf and become irregular.
Powdery mildew fungus causes a white bloom to cover foliage, shading sunlight and reducing the food produced by the Yoshino cherry tree. Warm, humid, sultry summertime conditions lead to powdery mildew, as does the presence of aphids and other insects' honeydew residue on shaded foliage.