A reader from Mohicanville contacted me around June 15 a few years ago to ask about
the deformed branches on her sour cherry tree.
My reader’s tree had elongated, rough black swellings or knots that developed on the woody portions of the tree. This type of knot is found mostly on small branches and twigs.
In heavily infected trees, the main scaffold branches and trunks may exhibit these knots as well as the smaller branches and twigs. The smaller twigs can become infected near the point of leaf attachment.
Initially, the knots are soft and green, but as time progresses, the branch grows brown and hard, then black. Mature knots can develop a white or pink powdery substance this time of year, which is the spore itself.
In rainy weather between April and June, a tree infected with the fungus ejects the spores.
Temperatures between 60 and 80 degrees F are ideal for spreading the disease. These wounds attract boring insects and can leave the tree weaker and susceptible to winter damage.
Black Knot or Dibotryon morbosum lives during the winter in the knots or branches in the immediate area of the infected tree. In the spring, the fungus forms these infective spore sacs at the microscopic level on the knot itself.
These spore sacs form on the surface of the knot, and when rain hits the spore sac, they are ejected into the air and blown moderate distances to the next site. Keep in mind that only the new, green, and succulent twigs are susceptible to this disease.
They can also be infected if the twig remains wet for a sufficient period. The season has been ideal for this fungus to do well.
Next year, you will see the knots. Only a few hours of rain with temperatures above 55 degrees will be enough to spread the disease, and small, round galls or balls start to grow on the infected branch.
Prevention in this case would have been the best cure if you had scouted and seen some cherry trees upwind from where you planted them and eradicated the infested trees; that would have been best.
Each knot needs to be removed two to four inches inside the tree towards the old wood. I would not attempt to recycle this pruned twig and send it to the curb after placing the cutting in a trash bag or burning it.
This fungus spreads further down the stem each year. Every pruning cut you make must be dipped in rubbing alcohol.
Untreated pruners will spread the disease. If major scaffold branches are involved, cut only half an inch beyond the infected area and treat the wound with 70% rubbing alcohol over the wound.
These are the first steps in the correction process.
Without this pruning and cutting, the game is over. Winter pruning is the best for your cherries. If you leave these knots on the tree and they continue to grow, the knots produce more spores that will continue to live as long as the tissue remains alive.
To combat this devastating disease, vigilance and timely intervention are key. Regular inspections should be conducted during the growing season, especially in the months when the temperature and rainfall create ideal conditions for the fungus to thrive.
Left unchecked, Black Knot disease can transform a healthy cherry tree into a shadow of its former self. The visual signs of infection — black swellings and spore-laden knots — are a reminder of how nature can dramatically influence the life of cultivated plants.
Beyond the immediate symptoms, the damage inflicted by the disease is insidious, weakening the tree’s structure and exposing it to further harm from pests and environmental stressors.
Fungicides are very effective in controlling this disease. This disease can spread quickly under the right conditions. You won’t find out you missed some until the following year.
Because I have seen that this disease can kill your cherry tree, here are some fungicides that will kill this fungus. Captan, Lime sulfur, Benemyl, or Benolate are applied at the rate suggested by the manufacturer and sprayed just as the green tissue appears at the first application.
The second application is to be made just before and again after the blooms come. Finally, spray every two weeks until after the terminal bud growth stops in mid-June.
Follow the directions. This should correct the disease.
Have a nice stroll through your garden this week. If you have any challenges, let me know at ericlarson546@yahoo.com.
I’ll answer them the best I can to help.
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