Saturday, April 08, 2017 1:00 am
Black knot difficult to get rid of
Q. My ornamental cherry tree is covered with these strange reddish growths on the branches and trunk. Should I be concerned?
A. It appears as if your cherry has black knot; a fungal disease that is usually fatal.
According to the University of Minnesota Extension, black knot affects several species of trees including American plum, ornamental cherry and plum, apricot, chokecherry and flowering almond trees. Outbreaks are common in both landscapes and natural areas. This disease can result in leaf wilt, leaf, shoot and branch death, and even death of susceptible trees
The disease is characterized by knobby swollen black or crimson growths (called galls) that grow parallel along the length of stems and branches. In early summer, young galls or new areas of growth are covered with velvety olive green spores. Galls may also occur on the main trunk of the tree and may ooze sticky liquid.
Infected branches may distort and bend due to the one-sided growth happening within the gall. When the galls completely girdle a branch, leaves on infected branches do not emerge or wilt and die in early summer.
During wet periods in the spring, spores are expelled and windblown to infect young green shoots or wounded branches.
It is not uncommon for galls to completely encircle and girdle a branch. When this happens, the leaves beyond the gall wilt and die.
Controlling black knot is very difficult. Heavily infested trees should be removed and destroyed.
The black knot fungus does not systemically infect the tree, but only infects the branch at and around the galls. Black knot galls can be removed from infected trees through pruning. It may take two years of pruning to completely remove all existing infections as young galls are often overlooked.
Remove at least 4 inches of healthy wood (towards the interior of the tree) before the black knot gall. It is vital to sterilize your pruning tools after each cut is made to avoid spreading the disease. Use a 10 percent bleach solution or other disinfectant on the pruners. If you can, prune the galls in late summer or early fall to avoid spreading the fungus.
Fungicides can be used to help protect young or highly susceptible trees from infection. Sprays must be applied in early spring to protect young green shoots. Begin fungicide treatment when flower buds are just beginning to open. Repeat sprays according to label instructions until shoots mature or weather is consistently warm and dry.
For large trees, high-pressure spraying equipment is needed in order to get complete coverage; therefore hire a professional arborist who can safely operate all necessary equipment.
For smaller trees, use a fungicide containing Captan, Chlorothalonil (Daconil), Thiophanate- methyl or lime sulfur.
Black knot is a very serious disease of cherries and plums in our area. Proper treatment and care are needed for succeptible trees to survive.
The Plant Medic, written by Ricky Kemery, appears every other Saturday. Kemery is the extension educator for horticulture at the Allen County branch of the Purdue Extension Service.