When installing new bare root trees or doing some dormant season pruning, be on the lookout for orange bulges on stems or branches. It might be burr knot.
Once considered a disease, burr knots start out as smooth orange bulges on stems or branches that develop into adventitious roots. These tumor-like bulges are actually masses of tiny roots that somehow ended up growing in the aboveground portion of a tree. This condition can be mistaken for crown gall.
Remember, most fruit and nut trees purchased these days are actually two trees that have been grafted together. The upper portion is selected for fruit or nut production and pest and disease resistance, while the rootstock is chosen for its ability to establish itself quickly and make the best use of soil resources.
Trees susceptible to burr knots
This condition is most common on apple trees. It occurs on scion cultivars, particularly Gala and Empire, and on dwarf and semi-dwarf tree rootstocks. Specifically, semi-dwarf trees grown from M.7, M.26, MM106, or MM.111 and dwarf trees with M.9 rootstock are likely to develop burr knots. Scion cultivars develop burr knots on the underside of limbs, while grafted trees tend to develop burr knots at nodes. Nodes are where leaves and stems normally emerge. Instead of developing normally, primordial roots cells begin to develop, creating a tumor-like bulge.
Conditions that encourage burr knots
In addition to being a grafted apple tree, other conditions, such as shade, increase the likelihood of burr knots occurring. High humidity and temperatures ranging from 68°F to 95°F during a tree’s first year encourage the development of tiny growths, called root initials, during its second year. These root initials can break through the bark of a tree, making room for more roots to form, increasing the bulge.
Problems associated with burr knots
As roots push their way through the bark, they create entry points for pests, such as plum borers, apple clearwing moths, and wooly apple aphids, and diseases. These diseases include fireblight and wood-rotting fungi. Limbs can become structurally weak and more likely to break. Several burr knots on the same tree can also interfere with nutrient movement through the phloem, causing stunting. These weakened areas are more prone to frost damage in winter.
Preventing burr knots
First, be sure to select tree varieties that are suitable for your microclimate. Next, be sure to install your tree at the proper depth. Improper planting leads to several problems and can ultimately kill your tree. Keep weeds away from your young tree and make sure that tree supports are used properly and only for as long as they are needed. If burr knots are seen, they can be cut out with a knife or filed out with a rasp.
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