Burr knots (burrs or burls) are masses of small root initials, plant cells that are dividing and transforming into root tips, which appears on the above-ground portion of a rootstock, or on the trunk and limbs of apple trees. This is an inherited disorder where root-producing structures develop on aerial parts of certain apple trees. It’s not a disease, but it can be a problem!
Certain rootstocks, such as the dwarf M.9 rootstock and semi-dwarf M.7, M.26, MM 106, or MM.111 rootstocks are genetically predisposed to developing burr knots, as are some scion (grafted) apple cultivars, such as Gala and Empire, which can produce burr knots on the undersides of their limbs.
Why use them then? These rootstocks were actively selected for in early rootstock breeding programs and chosen because they root readily, making them easier to propagate from cuttings or by layering. But this propensity to produce root easily sometimes also leads to a tendency to produce adventitious roots and burr knots.
What Are Adventitious Roots?
Adventitious roots are plant roots that form from any non-root plant tissue, such as stems, leaves, and other underground plant structures, such as corms, rhizomes, and tubers. They can be produced during normal development, such as the nodal roots on strawberries, or in response to environmental stressors (stress conditions), such as flooding, nutrient deprivation, and wounding.
Which Conditions Favor Burr Knot Development?
Burr knots are often triggered by specific environmental conditions that favour their development, which include shaded portions of the trees/low light, high humidity, and temperatures between 20-35°C (68-95°F).
These conditions favour the development of root initials during the first year of planting. In the second year, during the growing season, the root initials can rupture through the epidermal surface (bark) of the tree to form adventitious roots.
Over time, as the tree ages, more roots form, and the burr knots increase in size. These roots do not grow in length very much, remaining stubby, producing a knobbly appearance in the affected area. The older burr knots eventually harden, and may develop an irregular covering of bark.
Are Burr Knots a Problem in Apple Trees?
The formation of adventitious roots is a survival mechanism that plants have evolved to allow them to adapt to environmental stress, which humans also utilise for the vegetative propagation of plants. Burr knots permit wild trees to layer (root into the ground when a branch or stem makes contact with the soil) and produce new trees, especially when growing on hillsides, or if they are blown over by the wind.
Apples are normally propagated by grafting a known variety onto a specific rootstock, but some older apple cultivars (cultivated varieties) can be propagated by rooting pieces of branch with burr knots, known as ‘pitchers’. This was once common propagation practice in south-west England (particularly Cornwall) and south Wales. Apple cultivars that produce a lot of burr knots include ‘Ben’s Red’, ‘Burr Knot’, ‘Cornish Aromatic’, ‘Lord Burghley’, ‘Small’s Admirable’(syn ‘Captain Broad’), and ‘Winter Banana’.
Despite the benefit of easy propagation, burr knots are a problem for the following reasons.
- Large burr knots, or the merging of several knots can create structural weakness in tree, which can cause it to break during windy conditions or when bearing a heavy crop of apples.
- They provide an entry for damaging pest insects such as woolly apple aphids, as well as dogwood and plum borers.
- They provide an entry point for infection by fire blight (Erwinia amylovora) bacteria and wood-rotting fungi.
- When several burr knots join together, they can stunt tree growth and also limit the functioning phloem, which restricts the transport of sugars from the leaves to the roots in the affected part of the tree.
- The adventitious root of burr knots are more susceptible to low winter temperatures than the adjacent plant tissue.
Can Burr Knots Be Prevented?
There are several preventive measures that can be used to minimize the development of burr knot in apples.
Selection apple trees grafted onto rootstock that is not susceptible to burr knots is obviously the best method of control, but not always possible when a dwarf tree is needed.
During the growing season, don’t allow weeds and thick vegetation to grow under the tree and close to the trunk, maintain a vegetation-free area to allow good air circulation, and rapid drying of the trunk after rain
Avoid the use of tree guards that fit around the trunk (especially opaque ones that block out the light) unless absolutely necessary, as they provide shelter for insects, and also create shade and increase humidity, which can promote burr knot growth.
Some sources suggest planting the tree in the ground so that the graft union is just above the soil surface. I would advise against this idea, because if the graft union gets covered in soil, the scion above the rootstock may root into the ground and the dwarfing effect of the rootstock is lost, resulting in the tree growing to its full size. Furthermore, it is common practice to transplant all plants and trees at the exact height that they were in the pot or in the ground – planting them deeper can lead to collar rot, which will ringbark and kill them, while shallower planting can lead to exposed roots.
There are no chemicals available to gardeners for the prevention or eradication of burr knots.
With that in mind, another questionable recommendation is painting Gallex on burr knots to aid in callus formation, or healing of the tissue. Gallex is an agricultural chemical sold in the US for prevention and eradication of the disease crown gall, which may be difficult to obtain for small plantings of trees. It contains a mix of creosote-base chemicals, it’s pretty toxic and not something you would want contaminating the soil. For anyone interested, the ingredient list is as follows – Meta-cresol (0.466%), 2,4-xylenol (0.463%), Kerosene (57.67%), Water (25.90%), proprietary emulsifier A (8.25%), proprietary emulsifier B (3.25%), Diphenyl methane (2.00%), Dimethyl naphthalene (2.00%).
The use of this product is questionable at best for treating crown gall, which is different to burr knot – to quote the NSW government, Department of Primary Industries in Australia “As there is no cure for infected plants, prevention of infection is essential.” They suggest a biological control “Nursery plants and transplants can be protected from crown gall by treating the seeds, seedlings or cuttings with a commercial biological control agent. This agent was developed in Australia and is now used by many nursery workers and orchardists. The agent is a live culture of a bacterium closely related to the crown gall bacterium.” Even University of California recommend it – “The K-84 strain of Agrobacterium tumefaciens (formerly A. radiobacter), which is available for use in preventing infection by the crown gall pathogen, is an excellent biological control agent.“
Removal and Treatment of Burr Knots
The root initials (adventitious roots) of burr knots can be cut out manually using a knife, file, or rasp. This is feasible if the affected area is small, but the damage to the tree still leaves a potential entry point for diseases such as fire blight/apple canker.
When large burr knots reduces or stops the growth of a branch, it may be necessary to prune off the section of the branch with the burr knot, to promote the growth of new shoots to produce a replacement branch.
What Is the Difference Between Burr Knots and Crown Gall?
Apple trees are also susceptible to another condition which produces similar looking outgrowths, which is caused by a bacterial disease infection.
Crown gall is a bacterial disease caused by Agrobacterium tumefaciens (syn. Rhizobium radiobacter), which produces tumour-like swellings on the bark of trees and plants. It can infect a wide range of dicotyledonous (broad-leaved) plants, particularly members of the Rosaceae (rose) family such as roses, raspberries, almonds, cherries, peaches, pears and apples.
These disease-causing bacteria live in the soil and only enter fresh tree wounds (less than 24 hours old), which are often caused by insect and frost damage, mechanical damage from mowers and lime trimmers, pruning, or transplanting damage. That’s why it’s important to sterilise pruning tools when going from one tree to another!
Crown galls are genetically mutated and infected plant tissue that changes appearance over time. In the early stages they tend to be more wart-like, starting off light-coloured, soft, and spongy, then becoming rough and corky with an irregular surface and a hard woody interior. As they mature, they turn dark brown and woody, and may will disintegrate over time, while others may remain for the life of the plant.
A crown gall infection may kill a very young apple tree by girdling the stem, while a mature apple tree may be able to tolerate it and remain productive.
For more information see the article Apple Tree Diseases – Crown Gall.
By comparison, apple burr knots have a fairly uniform knobbly appearance, and the presence of many stubby adventitious roots, with some of them growing into narrow, protruding root tips.
- eXtension Apples Community of Practice, Burr Knots on Apple Trees, August 22, 2019 <https://apples.extension.org/burr-knots-on-apple-trees/>
- University of Missouri, Integrated Pest Management, Getting to the Root of Burr Knots on Apple Trees, December 16, 2015 <https://ipm.missouri.edu/MEG/2015/12/Getting-to-the-Root-of-Burr-Knots-on-Apple-Trees/>
- Royal Horticultural Society, Burr knots on apple trees <https://www.rhs.org.uk/problems/burr-knots-on-apple-trees>
- Steffens B, Rasmussen A. The Physiology of Adventitious Roots. Plant Physiol. 2016 Feb;170(2):603-17. doi: 10.1104/pp.15.01360. Epub 2015 Dec 23. PMID: 26697895; PMCID: PMC4734560.
- NSW government, Department of Primary Industries, Australia, Crown gall of plants, Series: Agfact H1.AB.3 , Edition: Second edition Last updated: 26 Mar 2012 <https://www.dpi.nsw.gov.au/agriculture/horticulture/pests-diseases-hort/information-for-multiple-crops/crown-gall-of-plants>
- University of California, UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries, UC ANR Publication 3392 https://www2.ipm.ucanr.edu/agriculture/floriculture-and-ornamental-nurseries/Crown-gall/