Burr Knots

When installing new bare root trees, or doing dormant season pruning, look for orange bulges on stems or branches. It might be burr knots.

Burr knot on apple tree (Ondrej Spanel) CC BY-SA 3.0

Once considered a disease, burr knots start as smooth orange bulges on stems or branches that develop into adventitious roots. These tumor-like bulges are masses of tiny roots that somehow ended up in the aboveground portion of a tree. This condition can be mistaken for crown gall.

Grafted trees

Remember, most fruit and nut trees purchased these days are two trees grafted together. The upper portions produce more fruit and have good pest and disease resistance. Rootstocks establish themselves quickly and make the best use of soil resources.

Trees susceptible to burr knots

This condition is most common in apple trees. It occurs on scion cultivars, particularly 'Gala' and 'Empire', and 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 tend to develop burr knots on the underside of limbs, while grafted trees develop burr knots at nodes. Nodes are where  leaves and stems emerge. Instead of developing normally, these primordial root cells grow, creating a tumor-like bulge.

Conditions that encourage burr knots

Too much shade can increase the likelihood of burr knots occurring. High humidity and temperatures ranging from 68°F to 95°F during the first year also 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 through the bark, they create entry points for pests, such as plum borers, apple clearwing moths, 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 the nutrient movement through the phloem, causing stunting. These weakened areas are more prone to frost damage winter.

Preventing burr knots

These tips can help your tree stay healthy and free of burr knots:

• Select tree varieties that are suitable for your microclimate. 
• Install trees at the proper depth. 
• Keep weeds away.
• Use tree supports properly and only for as long as needed. 
• Cut out any burr knots seen.