Black Knot is caused by the fungus Apiosporina morbosa. The black knot fungus overwinters in the galls on branches and trunks. Spores are released during wet periods in the spring. The wind carries these spores to trees where they infect young green shoots or wounded branches.
Black knot disease is easy to diagnose because of the distinctive black gall on the stems and branches of plum and cherry trees. The warty-looking gall often completely encircles the stem and can be anywhere from an inch to almost a foot (2.5 to 30.5 cm.) in length. Older knots may become infested with a pinkish-white mold that covers the black gall.
Black knot fungus (Apiosporina morbosa) is primarily a disease of plum and cherry trees, although it can also infect other stone fruit, such as apricots and peaches, as well as ornamental Prunus species. Black knot disease spreads in spring. On rainy days, the fungus releases spores which are carried on wind currents. If the spores happen to land on the new spring growth of a susceptible tree, and especially if the tree is damp, the spores germinate and infect the tree. The source of the disease is usually wild, abandoned, or neglected trees, and finding and removing the source is an important part of controlling black knot tree disease. Fungicide sprays also help treat the black knot disease, but you may find that the black knot keeps coming back if you don’t use a combination of fungicide and pruning to remove the knots.
Black Knot Disease -
If you have ever seen a spruce with healthy green needles at the tips of the branches, but no needles further down the branch, welcome to needlecast. This fungus attaches itself to spruce trees, it has a strong preference for Blue Spruce trees and can attack the white spruce and other conifers, like pine and fir, that are also susceptible. The Norway spruce is resistance to the disease.
Needlecast takes time, usually about four seasons, to completely take hold of a tree but once the first signs of the disease begin to show on your tree it will become an obvious eyesore on your property. This fungus hits trees that are already stressed, either from drought, poor nutrients in the soil or too many trees planted together. Needlecast diseases of evergreens are caused by several fungi, but all the needlecast fungi form small structures on the infected needle in which thousands of spores form. These "fruiting structures" may be black, orange-red, or tan, depending upon the fungus.
Black knot is a serious disease of plum and cherry trees (Prunus species) throughout the United States. Black knot is a disease that gets progressively worse each year unless controlled, and it will eventually stunt or kill the tree.
As seen through the lens with a magnifying glass, there will be rows of small black dots. These dots are the fruiting bodies of the fungus, and they are a characteristic of the disease. This disease is spread by rainwater splashing the spores from infected needles to newly emerging needles in the spring. Pycnidia, the fungal fruiting bodies containing spores, begin forming in rows on infected needles during spring when moisture is high.
Black Knot Treatment The first step in treatment is to cut away branches and stems that have knots. If possible, do this in winter while the tree is dormant. The black knot fungus may extend further inside the tissue than the visible width of the gall, so make the cuts 2 to 4 inches (5 to 10 cm.) below the gall to be sure you are cutting back to disease-free wood. Burn or bury the diseased branches to prevent the spread of the fungus.
The second part of an effective black knot treatment program is to treat the tree with a suitable fungicide. Fungicides vary in their effectiveness from region to region, contacting the tree professionals at Advanced Tree Care to find out which product works best in your area. Read the label and follow the instructions exactly for best results. Timing is important, and you will have to spray the tree several times at carefully timed intervals. Cautions: Fungicides are toxic. Store them in their original container and out of the reach of children. Avoid spraying on windy days.