Take a walk in the woods, and you tend to come across gnarly old trees with strange swollen growths at the base and branches.
If you spot this on a wild cherry tree, it’s most likely a serious case of black knot disease. Unfortunately, this can just as easily affect your young, newly planted cherry tree too.
Cherry tree black knot disease is an infection caused by the fungus Apiosporina morbosa and is commonly seen on plum and cherry trees. It occurs during humid, wet spring weather in which spores are carried on the wind, germinating on shoots and branches before growing into large black galls.
This unsightly growth will become tougher to manage if left untreated, so the best way to control black knot is to spot signs of the disease before it has a chance to become well-established.
To give your tree the best chance, familiarize yourself with the symptoms and causes of black knot as well as how to remove it, how to distinguish it from similar infections, and more.
Black knot disease is just one of many possible issues with cherry trees. I explain the most common problems and provide solutions and prevention tips for each in my comprehensive article, Cherry Blossom Tree Diseases and Pests.
Understanding Cherry Tree Black Knot Disease
Black knot disease can be present for many months on your cherry tree before you start to notice something is wrong.
Before we look at ways to manage and prevent it, let’s first understand what the disease is, its contributing factors, and how to identify it.
Cause and Transmission
Black knot disease begins its cycle during wet and humid spring weather, normally when temperatures are between 55-75°F.
Existing black knot galls in the vicinity send out spores from the tiny sacs in the knot’s surface that are then spread by the rain and wind.
These spores are carried onto a new tree host where they overwinter and germinate on stem and branch growth just near the leaf axil.
Symptoms and Identification
During the fall period after the infection, the fungal disease begins to form light-brown, spongy swellings before slowly developing into olive-colored corky lumps. These may remain well hidden behind the leaves at this stage.
By the following spring, black knot lives up to its name as the green swellings develop into large black tumor-like galls with a hard and uneven brittle texture.
Galls can vary from an inch in length to almost a foot, encircling entire twigs and branches.
Factors Contributing to Black Knot Disease
The two significant factors linked to black knot, according to research at the Connecticut Agricultural Station, are temperature and humidity.
The release of spores is dependent on rainfall levels and temperatures during early spring with new shoot growth being especially vulnerable to the infection. Studies reveal that most infections occur before bloom or after petal fall.
Professor of Plant Pathology at Ohio State University Michael A. Ellis notes that spores are most likely to germinate and infect twigs that have remained wet for a sufficient length of time.
Susceptible Tree Species and Cultivars
Wild cherry trees in forests and woodlots, especially wild black cherry, are known to be highly susceptible to black knot disease.
Besides native cherry trees, most edible and ornamental species tend to be vulnerable to black knot.
Tart cherry trees are believed to be less susceptible to the fungus compared with sweet varieties, and susceptibility can vary depending on climate zones, meaning that susceptible types may be less so when planted in dryer, cooler climates compared to humid southern areas.
Long-Term Prognosis of Tree
Without removing or treating the infected growth, black knot fungus will grow thicker and become firmly ingrained on the tree, girdling entire branches and trunks if allowed to spread.
Older dead galls from previous seasons can also crack open, inviting other fungal infections into the tree. If no measures are taken to control developing galls, the fungus will lead to the death of the entire tree.
Distinguishing Black Knot Disease From Similar Disorders
Black knot disease can often be confused with the bacterial disease crown gall, which is caused by the soil bacterium Agrobacterium tumefaciens and resembles overgrown warts.
Crown gall warts grow similar in size and shape to black knot, resembling inch-to-foot-long tumor-like growths, but the galls remain light-colored (the same color as the bark) even when large.
Black knot galls, meanwhile, visibly darken and have a much rougher texture compared with crown gall.
Black Knot Disease Prevention and Management Strategies
Now that you can identify black knot disease and have a better understanding of what causes it, make sure this fungal nasty doesn’t take over your beautiful cherry tree. Here’s what you can do to manage and prevent the disease.
Pruning and Sanitation Practices
In the winter when spore production is low, use a sharp pruning knife to remove galls, making cuts down to the wood an inch beyond the infection.
Larger growths that have completely encircled branches may need to be pruned away entirely using heavy-duty shears or loppers.
When using pruning tools, always make sure to disinfect them before and after use to avoid spreading the disease further.
Wipe tools between cuts and let blades soak in a solution of ½ cup of bleach and a gallon of water for several minutes.
For best results, aim to apply an organic OMRI-listed fungicide, like this one, ahead of leaf and blossom emergence and ideally before springtime rain sets in.
When used on a 7-10 day cycle, the fungicide should be able to help slow the spread of black knot disease but will not be able to kill any present fungus.
This is why pruning and care practices (discussed below) are so crucial in preventing the spread at an early stage.
There are a number of steps you can take to minimize and control the spread of disease in your tree’s immediate environment, including:
- Immediately removing fallen twigs or other cuttings to prevent the survival of spores on the ground.
- Disposing of infected stems by either burying or burning (avoid composting diseased cuttings unless your compost heap has an internal temperature of 160°F).
- Spray lime sulfur onto the tree during its dormant period to help inhibit spore production.
- Monitor trees for black knot in early winter. This is the best time to monitor since there are no leaves for the fungus to hide behind, so carry out a careful inspection of branch and shoot tips in this period.
To ultimately prevent black knot disease from taking hold anywhere near your cherry tree, it’s important to take safety measures well before planting.
This can mean removing any wild cherry or plum trees (the only other fruit tree species that is highly susceptible to black knot) from your property.
If tree removal isn’t possible for whatever reason, at least aim to plant your cherry tree sapling upwind and spaced well away from any wild cherry or plum trees.
Secondly, when purchasing from a local garden center or online tree nursery, be sure to ask a tree expert about disease-resistant cultivars for the best chance of success.
Disease-Resistant Ornamental Cherry Trees
- East Asian cherry
- North Japanese hill cherry
- Manchurian or Amur chokecherry
Does Black Knot Spread to Other Cherry Trees?
Yes, the spores of the fungal disease can be spread by the wind during spring, where they can easily land and germinate on another cherry tree.
Young new shoots or branches become infected, and the fungus develops over months with no visible symptoms.
Can a Tree With Black Knot Be Saved?
Trees infected with black knot can be saved by pruning diseased growth before applying a fungicide in dry weather (ideally when temperatures are above 60°F).
Be sure to disinfect pruning tools before and after use and between cuts to prevent further disease spread. If untreated, black knot will spread and kill the trees.
In summary, cherry tree black knot disease thrives in humid and rainy spring weather and typically hides its developing growths behind leaves.
The trick to managing the infection is to catch the growth before it becomes large enough to engulf twigs and branches, and even then, whole branches may be pruned off entirely to help save your tree.
You can also do your bit to prevent the survival of fungal spores by keeping a clear and ventilated area beneath your cherry tree, planting disease-resistant varieties, and applying organic fungicide before leaf and blossom bud to stay one step ahead of the disease.
The more knowledge you have in regard to dealing with cherry tree problems, the better equipped you’ll be to handle issues when they arise. Learn about these problems next: