A Yoshino cherry is a rapid-growing tree treasured for its scented flowers and attractive lumber. Although, like many fruit bearing trees it is host to a variety of diseases including black knot, a common fungal disease found on wild plums, prunes and cherries. It is a parasitic disease that can leave a Yoshino cherry tree drained of nutrients and unable to support itself.
Black knot fungus is a common disease that attacks fruit trees. The disease is often identified by black swellings that can appear on the underside of branches. It can take as long as a year before the symptoms of the infection occur. This is a spreading disease that is likely to contaminate all nearby susceptible trees.
The thick, black swellings caused by black knot often go unnoticed until the winter season, when the leaves no longer obscure the tree. Often the fungal disease is present long before the swellings appear. Eventually its presence causes tumor-like growths to appear by disrupting the normal cell development of the tree. As the knots mature, they go throw a variety of colors, eventually appearing white or pink due to the fungal parasite.
During ideal conditions, such as temperatures of 60 to 80 F and rainfall, spores are released from the fungus. The ascospores are spread by rain and wind, and eventually find a host in wounded tissues of the tree or plant. This can occur for two to three weeks at a time, spreading from tree to tree. It is also possible for germinating spores to penetrate fully healed surfaces of green shoots directly.
If left alone, it is possible for black knot to kill the host tree. As time passes the infection spreads throughout the tree, disrupting growth, draining nutrients and allowing for an assortment of other contagions to gain entrance into the tree. It is common for trees with black knot to lose its ability to produce leaves and eventually fruit.
To cure a Yoshino cherry tree of black knot, it is important to first prune away all infected branches. Sanitize the cutting instrument between cuts to prevent spreading the disease by applying a solution of one part bleach to six parts water. Carve out any visible signs of infection from the trunk of the tree at least half an inch deep, continuing to sanitize. Lastly, apply a fungicide like lime sulfur to the surface of the tree before leaves bloom. Be sure to cure all trees in any designated area to prevent reinfection.