No, black knot disease is not really a cancer, but it certainly acts like a cancer… and a cancer that develops metastases at that!
You can’t help but notice black knot disease in the winter. Since the tree is leafless, its galls are highly visible. They are black misshapen growths that extend along the stem. Black knot is a fungal disease (Dibotryon morbosum, syn. Apiosporina morbosa) whose spores are usually carried by the wind from wild cherry or plum species to cultivated trees and shrubs. Or were already present at the time of purchase (often the case with the purple-leaf chokecherry ‘Schubert’, for example). Once black knot does reach the tree, however, it spreads locally: other galls appear here and there and the number keeps increasing over time.
This disease affects only Prunus species, mostly cherries and plums, although it has been know to affect apricots, peaches and almonds under certain circumstances. (All are Prunus species). There is more then one strain of black knot and the one that infects cherries seems specific to cherries while the one that affects plums is apparently specific to plums, so if you have only wild cherries with the disease in your vicinity, you might be able to grow plums and vice versa.
Susceptible or Not?
Not all plums and cherries are equally susceptible to black knot, however.
Among cherries grown for their fruit, sour cherries (P. cerasus) and hardy dwarf cherries (P. cerasus x fruticosa) are rarely infected and can usually be successfully grown in home gardens, while sweet cherries (P. avium) are highly susceptible and not always a good choice if black knot is commonly seen in your neigborhood.
Most North American species of wild cherries (P. pennsylvanica, P. virginiana, etc.) are extremely susceptible to black knot as are most ornamental cherries, including bird cherry (P. padus). Almost all the very popular purple-leaf cherries, like P. virginiana ‘Schubert’, P. virginiana ‘Canada Red’ and P. padus ‘Colorata’ are highly susceptible, although purple-leaf sand cherry (P. x cistena) is only moderately susceptible and can be grown as long as it is planted well away from wild cherries.
Amur chokecherry (P. maackii) is proving itself to be an exception among ornamental cherries. It is rarely affected by the disease and the popular cultivar Goldrush® (P. maackii ‘Jefree’) is said to be highly resistant.
The situation is much worse in the case of plums, where all species, both fruiting varieties and ornamental varieties, are highly susceptible to the disease. This includes American plum (P. americana), Canadian plum (P. nigra), European plum (P. domestica), Japanese plum (P. salicina), purple-leaf plum (P. cerisifera) and their hybrids. There is one cultivar (P. domestica ‘President’) that is known to be very resistant to it.
Note that the word “resistant” does not mean “immune.” If you plant a resistant variety close to diseased specimens, it can still become infected.
What to Do?
If your tree is only slightly affected, with two or three fairly small galls, you can always try removing them. Cut the branches infected at least 4 inches (10 cm) below the gall, into healthy wood, disinfecting your pruning shears with rubbing alcohol between each cut. And burn or put in the trash all branches removed. Keep monitoring the affected tree regularly, as once it is infected, the disease is likely to return.
March is considered the best month for pruning out black knot as not only are last year’s galls very visible, but you can even distinguish emerging galls as they start to appear in March. If you look carefully (the stem will be swollen and greenish, but no yet black), you’ll be able to remove them as well. Also, the tree’s growth will soon resume and thus the wound will close faster.
In regions like mine where snowfall is heavy (there is usually a 4- to 6-foot [120-180 cm] buildup by this time of year), pruning in March has the further advantage that you can often reach the infected branches just by putting on snowshoes, thus without having to climb a ladder!
If There are Many Galls…
If there are more than 3 galls in your tree, it is unlikely that you’ll be able to stop the scourge: the tree is doomed. Removal is really the only logical option, at least in a home environment.
Of course, a tree suffering from black knot can often still live 5 to 15 years (the disease will eventually spread to the trunk, which is what finally kills it) and look reasonably good during the summer, as healthy foliage can hide the damage. Even so, I still feel early removal is the best choice. That way you can replace it with something less disease-prone that will soon grow to a respectable size rather than watching your tree die slowly. Also, if you leave a sick tree standing, it can infest other cherries or plums in the neighborhood, causing problems for your neighbors. It is, in a sense, your civic duty to remove the tree.
I am a laidback gardener, of course, so I like things to be simple. Therefore I’m not going to suggest using fungicides to treat black knot. That’s fine for commercial growers, who have the manpower and the money. They usually plant young trees without symptoms of the disease and spray several times a year (lime sulfur would be a suitable organic fungicide) to keep it away. They also usually make sure any wild plums or cherries disappear from the sector. For home gardeners, though, having to spray your plum or cherry several times a year is a major hassle, one you’re likely to give up on pretty quickly.
I recommend instead the most laidback method of dealing with black knot: not planting susceptible plums or cherries. Instead, choose a species or cultivar known for its resistance to black knot (see Susceptible or Not? above for a few suggestions). Also, remove, if possible, all nearby wild plums or cherries if you want to grow even resistant trees, as the wildings remain the main disseminators of black knot disease.