Black Knot Fungus: Preventing Fungal Galls

The black knot fungus, Apiosporina morbosa, is a plant illness that impacts species from the Prunus genus of timber and scrubs. This illness creates giant black galls and cracks on timber – inflicting important financial injury to stone fruit crops, decorative crops, and the timber trade. The injuries additionally create alternatives for a lot of different ailments to enter the plant and trigger secondary infections. 

This wooden decay fungi is indigenous to North America and customary all through Canada, the U.S., and Mexico. There was one reported case of black knot in Taiwan within the Nineteen Seventies and this illness is taken into account an EPPO A1 quarantine pest internationally. 

Black knot seems on the woody components of timber resembling on younger inexperienced shoots, twigs, limbs, and trunks. It is likely one of the most seen of the stone fruit ailments as contaminated timber will exhibit giant black knot galls that may ooze sticky liquid and utterly encircle contaminated branches. Severely contaminated timber are eliminated to restrict the unfold of fungal spores to close by wholesome timber. Like different fungal ailments, as soon as established, this illness is difficult to eradicate. Monitoring timber and steadily pruning diseased branches are key methods to handle black knot in plum and cherry timber. 

What Is Black Knot Fungus?

Black knot, the frequent title of this fungus, is the symptom of the illness. The black knot inflicting fungus Apiosporina morbosa is within the Venturiaceae household and intently associated to the fungus that causes apple scab. 

Life Cycle of Black Knot

The fungus, Apiosporina morbosa (previously Dibotryon morbosum), overwinters on the floor of host crops as fruiting constructions. These constructions on black knot galls launch spores in early spring, triggered by moist situations and delicate temperatures (55-75 levels F). Transported by wind or rain, black knot spores will infect younger inexperienced shoots and twigs ranging from bud break by when terminal shoots cease rising in June. Wounded branches are additionally susceptible to new infections. Just like different fungal plant ailments like brown rot, Apiosporina morbosa additionally instructs plant tissue to provide additional plant cells which disrupts regular progress in contaminated branches. 

The fungus varieties fruiting constructions on older black knot galls which proceed to provide and launch spores yearly to extend the illness cycle. Like different systemic plant ailments, this one may also journey all through the plant, inflicting new fungal progress a long way away from the preliminary website of an infection. It is likely one of the most typical plum tree ailments, and likewise one of the frequent cherry tree ailments, amongst different species.

Signs Of Black Knot

The telltale signal of black knot galls seem late within the rising season, many months and even years after preliminary an infection within the early spring. Usually, the black knot galls develop into seen after leaves drop within the fall.  Contaminated branches could present some preliminary swelling that may simply go unnoticed. After a few 12 months of an infection, the swelling can tackle a velvety olive inexperienced masking and start transitioning into mild brown swellings. The older knots enlarge over time because the fungus grows contained in the plant tissue. Black knot galls can range in dimension from beneath an inch to over a foot lengthy. Over time, wholesome wooden will remodel into giant laborious black knots, which could trigger leaves and branches to die. Open wounds and cracks attributable to the illness on contaminated timber invite new infections resembling different wooden decay fungi.

What Crops Does Black Knot Fungus Impact?

Many Prunus timber are inclined to the black knot illness together with round 25 wild and cultivated Prunus species. Black cherry (Prunus serotina) is a prized timber species native to North America. Injury and wooden rot attributable to bugs, wounds, and ailments considerably affect the market worth of this hardwood which makes the black knot fungus a very troubling illness for the timber trade. Many decorative and fruiting cherry and plum timber are inclined as properly, together with the American plum (P. americana), chokecherry (P. virginiana), and European “Stanley” plum (P. domestica), to call just a few. 

Controlling Black Knot

As a result of outdated knots are seen on timber, eradicating contaminated twigs or branches is a key management technique. Perform this course of within the winter as timber are dormant then and the dearth of leaves makes black knot galls simple to identify. Cuts must be made 4-6 inches from woody galls on twigs. When engaged on bigger branches, be certain that to go away at the very least one inch of wholesome plant tissue from the place you will have eliminated the prevailing galls and fungal tissue. Use clear pruning instruments and finest practices to advertise wholesome regrowth over the pruning wound. Sterilize instruments utilizing an answer of at the very least 70% rubbing alcohol. It’s possible you’ll must seek the advice of an arborist to hold out extra in depth removals. 

If left uncontrolled, the knots will increase to cowl your complete tree and the entire tree would possibly must be reduce down. This may be a multi-season mission since any seen black knot signifies that an an infection is at the very least a 12 months outdated. 

Stopping Black Knot

Business growers could use fungicide sprays containing chlorothalonil beginning in early spring and routinely spray inclined timber each two weeks all through the rising season. Chlorothalonil can also be efficient in opposition to brown rot, one other fungal illness that impacts Prunus timber. 

Copper fungicides may also be used to guard timber and younger crops from late fall into late winter (roughly Thanksgiving, Christmas, and Valentine’s Day remedy occasions). A preventative spray of lime sulfur may also be utilized when the timber are in dormancy. Don’t use these sprays after flowers bloom or earlier than the rain. 

Some Prunus species are much less inclined to black knot illness. For cherry timber, bitter cherry (P. cerasus) and Nanking cherry (P. tomentosa) are just a few examples of cultivars which might be much less affected. Flowering almond (P. triloba) and apricot (P. armeniaca) are additionally hardly ever affected. There are even black knot-resistant plum and cherry timber out there in the marketplace resembling the attractive Flowering cherry (Accolade) and President plum. Think about rising one in all these varieties, as bitter cherry black knots or Nanking cherry black knots are much less frequent and that slight enchancment in susceptibility can provide important safety.

Wild plum and native plum and cherry Prunus timber can host the black knot fungus and act as a reservoir the place the fungus overwinters. Think about eradicating these wild crops or preserve a detailed eye on them to mitigate in opposition to the unfold of the black knot illness to different timber. 

Continuously Requested Questions

Q: Can black knot fungus be cured?

A: Black knot fungus is a illness that may be managed however could be very troublesome to treatment as soon as an an infection has taken maintain. Severely diseased timber must be eliminated. 

Q: Can I burn black knot fungus?

A: Sure, it is suggested to burn the plant materials after pruning for the reason that fungus can stay dormant on particles or within the soil. 

Q: Does black knot have an effect on fruit?

A: Black knot impacts woody tissue in timber. It won’t immediately have an effect on fruit, however can lower the general vigor and well being of the tree and have an effect on fruit manufacturing and high quality.