With the beginning of the pruning season, disorders on apple tree trunks and limbs are often more apparent than when trees are in full leaf during the growing season. Burr knots are an inherited disorder that appear tumor-like, but are actually masses of small roots initials found on the aboveground portion of a rootstock or on limbs of apple trees. Dwarf trees with M.9 rootstock and semi-dwarf trees with M.7, M.26, MM 106, or MM.111 rootstocks can develop burr knots. Scion cultivars, such as Gala and Empire, can also produce burr knots on the underside of limbs.
A burr knot is often initiated from a root primordium at a node. Shaded portion of the trees, along with high humidity and temperatures ranging from 68°F to 95°F favor the development of root initials during the year of planting. In the second growing season, root initials can rupture the epidermal surface of the tree. As the tree ages, additional roots form and a burr knot continues to increase in size.
Burr knots are problematic for several reasons. First, they create a structural weakness, causing the tree to break when loaded with a heavy crop of apples and windy conditions occur. When several burr knots coalesce, they can stunt the tree or also limit the functioning phloem, which restricts sugar transport in the affected portion of the tree. Burr knots also provide an entry for damaging insects such as dogwood and plum borers or wooly apple aphids. They also provide an infection site for fire blight bacteria and wood-rotting fungi. Root initials of burr knots are also more susceptible to low winter temperatures than adjacent tissue.
There are a few preventive measures that can be used to minimize burr knot development. At planting, place the tree in the ground so that the graft union is just above the soil surface and water the tree. Make sure that after the tree settles the graft union remains above the soil surface to prevent scion rooting. If the union becomes buried in soil and scion rooting occurs, the dwarfing effect of the rootstock is lost. During the growing season, maintain a vegetation-free area beneath the tree to increase air movement and promote rapid drying of the trunk. Also, avoid the use of tree guards that fit around the trunk as they provide shade and increase humidity, which promote burr knot growth.
When burr knots are visible, the root initials can be cut out using a knife, file, or rasp if only a few apple trees are affected. Alternatively, burr knots can be treated with naphthalene acetic acid (NAA) which is also used in the spring for apple fruit thinning and is available from orchard suppliers.
REVISED: March 15, 2016