Branch symptoms
- Black knot galls are most noticeable during fall and winter after all the leaves have fallen.
- Knobby, swollen black growths called galls grow along the length of stems and branches.
- In early summer, young galls or new areas of growth on the edges of older galls are covered with velvety, olive-green spores.
- These galls turn black and hard by the end of the summer.
- Infected branches may bend to one-sided due to growth of the gall.
- There can be anywhere from a few galls to hundreds of galls within the tree canopy.
Leaf symptoms
- Leaves remain healthy and green even on branches with galls in black knot tolerant trees.
- Leaves wilt, turn brown and die on branches with galls in trees that are highly susceptible to black knot.
- Brown, wilted leaves at the end of branches are often scattered throughout the tree on highly susceptible trees.
Trunk symptoms
- Large areas of rough black swollen bark form on the main tree trunk.
- Black knot galls on trunks are often cracked and may ooze sticky liquid.
- Wood decay fungi may enter the trunk through cracks caused by black knot galls and cause wood rot.
Quick facts
- Black knot is a common fungal disease of Prunus trees including ornamental, edible, and native plum and cherry trees.
- Hard swollen black galls (tumor like growths) form on branches and occasionally on trunks.
- Many Prunus trees tolerate black knot. Tolerant trees have many galls throughout the tree with few negative effects on the health of the tree.
- Some Prunus trees are more severely affected by black knot. In these trees, leaves and shoots wilt and die on branches with galls.
- Management will vary depending on how severely the tree is affected by black knot.
How to Control
- Inspect your trees carefully for first signs of the disease. This is best done in winter, when leaves are absent, but should be continued as well throughout the growing season. Look for cracks, discoloration, swelling, or other first signs of infection. Check carefully around twig and leaf axils.
- Remove any knots that are found. This is best done during winter when spore production is down. Cut well-past the galls, four to eight inches, to ensure all the infection and its spores are removed. Larger branches with established knots should be removed entirely. Use a pruning knife or chisel to remove galls on trunks and large branches, cutting down to the wood and out to at least an inch beyond the infection.
- Continue to inspect for and remove galls as the season progresses.
- Take care not to spread spores when pruning trees with black knot. Don’t allow twigs or other cuttings to fall to the ground where the spores could survive.
- Dispose of infected stems and branches by burying or, where allowed, burning. Small cuttings can be stuffed in trash bags and hauled away. Do not compost any infected cuttings unless your heap has an internal temperature of 160 degrees (not many do).
- Clean pruning tools as you use them with a solution of 1/2 cup bleach to a gallon of water. Wipe tools between cuts and leave your pruning blades in the solution for three to six minutes when finished. Or use a safe, commercial fungicide cleaner such as Physan 20.
- Fungicides can offer significant protection against black knot, but are unlikely to be effective if pruning and sanitation are ignored. Organic gardeners will want to avoid all but OMRI listed fungicides.
- Spraying trees with NEEM oil, a natural fungicide that controls leaf spot, rust, scab, and other tree fungus, can help inhibit the spread of black knot (it will not kill fungus that is already present). Spray trees per instructions just ahead of leaf and blossom emergence and, if possible, ahead of rain. Continue on a 7-10-day cycle until weather dries. Use of other fungicides can also discourage spores from germination. But few are specifically indicated for use on already infected trees.
- Spraying lime sulfur on trees during the dormant period is said to prevent the production of spores. Copper sprays applied during dormancy may also inhibit spore production.
- Take out wild cherry and plum trees around your property. They harbor the disease and release spores that are easily carried to your susceptible nursery trees.
- When planting new trees, place them away and upwind from established or wild prune and cherry trees.
Source:
https://extension.umn.edu/plant-diseases/black-knot
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