Black knot disease, caused by the fungus Apiosporina morbosa, is a widespread fungal disease that affects Prunus trees, a genus which includes peach, apricot, cherries, and almond. The effects of black knot disease are most noticeable during the fall and winter months, after leaves have fallen off to expose branches. Black, knobby growths—called galls—congregate along tree stems and branches. If not caught in the early stages, black knot galls enlarge, and eventually decimate trees’ ability to produce fruits.
Black knot disease is characterized by thick, black abnormal growths on twigs and branches.
Black knot is a long-lasting, hardy fungal infection that becomes progressively worse year over year. Black knot first infects trees during the wet and humid spring months, when temperatures reach or exceed 60°F. In early springtime, miniscule, spongy, light brown swellings form around and behind leaves, effectively hiding the fungus from view. Without pruning away infected branches, or using chemical deterrents, the namesake black knots continue to grow and become more brittle throughout the summer months. Areas that experience high levels of humidity are especially prone to black knot, as the fungus depends on moisture to survive and grow. By fall, galls become hard, thick, and rough, and are firmly entrenched.
Over the next growing season, the knots swell larger, and eventually encircle twigs and branches, becoming up to one foot in length and up to two inches in diameter. Over the next few years, new galls continue to form on the peripheries of old galls.
Galls typically die after two or three years. However, this doesn’t end the threat to infected trees, Dead galls eventually crack open, allowing other pathogens, such as the fungus Trichothecium roseum, to invade and infect the tree. Infected galls may turn a light pink or white color.
Prunus tress do not fare well during this long infection period. The thick, rough galls are a serious strain to weaker twigs, which usually die within a year. Thicker branches can survive several years, but will inevitably die if they become fully girdled by galls. Death of the entire tree is possible if control measures are not taken.
Aided by the wind, Apiosporina morbosa quickly spreads from tree to tree in the springtime.
The spores of A. morbosa germinate on twigs and branches, where they are encased in a thin layer of protective moisture, allowing galls to grow and spread to branches and twigs. Infection occurs most readily when spores come into contact with wounded plant tissue, especially in areas where moisture has seeped in. Over the course of the first year of infection, A. morbosa produce spores which are dispersed by the wind, helping the fungus to gain a stronger foothold in infected orchards. The fungus overwinters within the hosts’ plant tissue, and then resumes the reproductive process when temperatures warm.
Spores are generated and released from the time of bud break in spring until shoot growth stops later in the season, though the vast majority of this occurs between white bud and shuck split.
It should be noted that rain is a crucial element for black knot infection. Though spores are known to germinate during relatively dry periods, rain must be present for infection to occur.
Controlling the spread of black knot disease requires careful planning, attentive pruning, and the use of chemical fungicides.
It is important to plant new trees in strategic locations to reduce the likelihood of future infection. Since the fungus overwinters in twigs and branches, and can survive in orchard spoils, it is best not to plant new trees downwind of an old orchard, especially ones which exhibited black knot infections. Wild, unkept trees pose a particular threat to new trees, as lack of pruning and other maintenance makes it more likely they carry black knot disease.
Established orchards should be regularly examined for the presence of black knot. The best time to check for black knot is during the transition from winter to spring, right around the time of bud break.
Infected twigs and branches should be quickly pruned away, and the spoils should be carefully discarded to prevent spores from spreading. It is critical to prune two to four inches below the visible symptoms of black knot, as the fungus is present beyond the visible edge of the black knot growths. Only trimming away visibly infected tissue will leave traces of the fungus behind, and infection will continue.
Treatment varies with thicker infected branches. In some cases, it may be possible to remove galls using a chisel or pruning knife, cutting down until healthy wood is visible, and with a margin of an inch or two around the periphery of the gall. More severely infected branches should be removed entirely.
Fungicide sprays are effective in preventing black knot, but the timing of such sprays is important. In areas of established black knot, fungicide spray is most effectively used at the time of bud break, through early summer. Where black knot is not established, spraying should be conducted from bud break to shuck split, because this is the period in which spores are most prevalent. Trees should also be sprayed before periods of rainy weather, as moisture accelerates the growth and spread of black knot. Importantly, fungicides alone are not capable of eliminating the disease. Careful pruning is necessary to rid individual trees of black knot. Check on trees during the winter to determine the effectiveness of your efforts—winter is when black knot swelling is most evident.