What Are Burrknot Borers: Symptoms And Causes Of Burrknot In Trees

By: Kristi Waterworth

Grafted trees are prone to many strange tricks, sometimes sending out angry-looking spines or armies of water sprouts like tiny soldiers emerging from the bottom of the tree. Burrknot of trees occurs when these rootstocks push out incomplete clusters of aerial roots, forming a rough, roundish area below the graft. Generally speaking, these burrknots aren’t harmful, unless burrknot borers are in the area.

Symptoms of Burrknot Borers

Burrknot borers, more commonly known as dogwood borers, are the larval form of a clearwing moth. Females lay eggs that will hatch in a little over a week in burrknots on trees. When the tiny larvae emerge, they bore into the burrknot, pushing out a rust-colored frass as they go. This discoloration on the surface of the burrknot may be the first and only sign of infestation.

Sites that have been used for breeding over many generations may allow the tree to be girdled over time, as larvae dig deeper and deeper through the burrknot, into healthy tissues. Chronically infested trees may slowly decline and, if they’re fruiting species, gradually show a drop in their production as the infestation expands.

Causes of Burrknot

Burrknots commonly appear on grafted trees, no rootstock appears to be immune. High humidity and shading of the graft union tend to encourage formation of these structures. Many growers mound a wide cone of soil around the exposed portion of the rootstock to encourage these burrknots to develop fully into roots, reducing their risk of harboring borers.

Borer-Infested Burrknot Treatment

Treatment for burrknot borers can be difficult since they spend most of their lives inside of host trees, but pheromone traps can help detect adults on the move. Place these about four feet above the ground early in the season so you’ll be ready when it’s time to spray. A single application of chlorpyrifos directly on and around the burrknots after the first dogwood borer appears in your trap should be sufficient for the rest of the season.

You can prevent dogwood borers from infesting burrknots by applying a white coat of latex paint to the rootstock of any trees at risk and providing them with excellent care. Like other borers, dogwood borers prefer trees that are stressed and will seek them out above all others.

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Conditions associated with an unusually high occurrence of burrknots in two young commercial apple orchards at two locations, Těšetice and Stošíkovice, South Moravia, the Czech Republic, were analysed. In the first orchard, planted in spring 2003, the occurrence of burrknot on trees of cvs. Early Smith, Jonagold and Gala on M.9 rootstock was evaluated. In the second orchard, planted in autumn 2002, cv. Gala on M.9 rootstock was assessed. Planting material used at each location was obtained from the same commercial propagator and had been established from the same propagation stock materials. Of 60 trees per cultivar surveyed in the first orchard, incidence of burrknots in cvs. Early Smith, Jonagold and Gala trees was 98, 97 and 92%, respectively. The burrknot severity (mean number of burrknots on above portion of rootstock) was significantly higher on Jonagold trees, i.e. 3.65, than on the other two cultivars. Of 60 Gala trees in the second orchard, symptoms of burrknot appeared on 73.3% of trees planted on a slight slope and 70.0% of trees planted on a plane. The burrknot severity was significantly higher on the Gala trees planted in the Těšetice orchard than in the Stošíkovice location. Burrknot incidence and incidence of root-suckers were the highest on Jonagold trees at Těšetice. However, correlations between burrknot number and number of root-suckers were not statistically significant. Five years after the tree planting, increased dying of Jonagold trees was recorded at Těšetice. Of 290 trees examined, 5.5% had died. On the rootstock portion of trunk, each dead tree exhibited burrknots associated with bark cankers that more or less girdled the trunks. Only sporadic occurrence of canker symptoms and no premature dying of young trees were observed at Stošíkovice. Attempts at isolation of the fire blight bacterium, Erwinia amylovora, and oomycete Phytophthora spp. from necrotic tissue surrounding burrknots on rootstocks were not successful. The stem associated apple tree viruses Apple stem pitting virus (ASPV) and Apple stem grooving virus (ASGV) were detected frequently in the rootstock and scion parts of cvs. Jonagold and Early Smith and less frequently in Gala cultivar. The virus positive trees included individuals both with various burrknot severity and without symptoms of burrknots. There were no correlations between the incidence of burrknots and the presence of ASPV and ASGV.

burrknot apple tree M.9 rootstock root-suckers trunk canker ASPV ASGV

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Plant Protection Science
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Bur oak blight

Bur oak blight (BOB) is a leaf blight caused by the fungus Tubakia iowensis . It is a disease of concern to anyone who has bur oak trees on their property. BOB is most prevalent on Quercus macrocarpa var. oliviformis , an upland variety of bur oak characterized by its relatively small, olive-shaped acorns. Susceptibility of bur oaks to BOB is highly variable, and trees of all sizes are at risk. Occasionally, BOB can infect swamp white oak ( Q. bicolor ) it is unknown whether hybrids of bur oaks are susceptible. Repetitive defoliation of trees by BOB increases susceptibility to secondary problems such as the two-lined chestnut borer ( Agrilus bilineatus ) and Armillaria root rot, which ultimately cause tree decline and death.


Symptoms of BOB become visible during mid-July with the presence of small circular leaf spots displaying colors of reds, purples, and browns. Next, the major leaf veins start dying and display similar colors (Fig. 1). Small, black fruiting bodies, which contain fungal spores become visible along the undersides of these veins.

As the disease progresses, wedge-shaped chlorotic-dead areas (Fig. 2) of the leaf become prominent and the bases of the petioles die and turn brown. In late August to early September, the dying petioles turn entirely brown, become swollen at the base, and develop black pimple-like fruiting bodies on the surface that are visible with the naked eye (Fig. 3 and 4). The presence of these fruiting bodies on the petiole is essential to the confirmation of BOB. Dead leaves and/or dead petioles will remain attached to the stems throughout the winter.

The disease starts in the lower, inner sections of the canopy and spreads upward and outward as it progresses.


Fruiting bodies on dead petioles from the previous season release spores during the spring and this is considered the primary infection. The spores are spread by splashing rain and infect the newly-emerged foliage. The intensity of the disease within a season is dependent on spring moisture. Although infection occurs in spring, leaves will appear healthy until mid-to-late July. The foliar symptoms progress throughout the month of August and early September. Secondary infections will occur throughout the season from spores developed under leaf veins during wet periods. Fruiting bodies are produced on dying petioles in late summer. Entire leaves, or just petioles, will remain attached to the stems until the following spring and repeat the cycle.


Cultural – There are no promising cultural methods to manage BOB. Removing leaf litter under the tree will not reduce the infection rate the following year. Plucking off petioles is likely not an effective method because the fungus can survive within the stems and it is not practical on large trees.

Chemical control – A fungicide treatment in spring after full leaf expansion is the most effective method for managing BOB. Results from studies conducted at Iowa State University showed injections of propiconazole (Alamo formulation) reduced symptoms in the fall and in the following year. Repeat applications are not recommended until the infection becomes severe in later years. Fall applications of fungicides are not recommended for BOB.

Apple tree disease

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  • That kind of growth is called a "burl". They can be caused by bacteria, fungi, insects, mechanical damage, freezing, genetics, graft incompatibility, etc, etc, etc. If the tree grows fine with them, then there doesn't seem to be any reason to try to treat them.

    posted 3 years ago

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  • That is burr knot. It is the tree creating a place from which it can grow roots if needed. It is uncommon in selected apple varieties, but there are some varieties that are prone to it. Many rootstocks get it because they are selected partly for their rooting ability. One of the goals of breeding apple rootstock is to select varieties that root easily, but do not tend to form arial burr knots. They can be a problem. They tend to get larger, not smaller and can disrupt flow of nutrients up the tree, causing stunting or weakness. They can be cut out, but that is best done when they are very small. I have a video showing cutting them out of an already established tree, where it seemed like they would be enough of a problem that it was worth opening the trunk to probable infection to get rid of them. I haven't looked at it in a while, so I'm not sure how it turned out. Removing burr knots that size is rather traumatic though. It might be better just to leave them. Another common problem is that they can create a good place for borers to get in. What is the variety, or is it possibly a rootstock or seedling?

    Cultural Treatments

    Fruit trees should be planted in full sun in well-drained soil. Wet or compacted soil weakens the trees and makes them more susceptible to diseases. The trees should be planted the recommended distance apart to allow for good air circulation, which helps prevent fungal diseases.

    • The Botryosphaeria fungus causes White Rot.
    • It stunts the growth of the affected tree and can girdles the trunk.

    Infected limbs and branches should be pruned out and destroyed. Leaves, rotten fruit and other debris under diseased fruit trees should be removed and destroyed. If diseased plant material is added to a compost pile, it could spread the disease further.

    Advice about star jasmine, border plants and mysterious growths on trees: Ask an expert

    Advice from OSU Extension Service and Master Gardeners

    As you bask in the warmth of the home hearth, look back on the gardening season and consider what you might want to do come spring. Got a question? Get answers from Ask an Expert, an online question-and-answer tool from Oregon State University's Extension Service. OSU Extension faculty and Master Gardeners reply to queries within two business days, usually less. To ask a question, simply go to the OSU Extension website and type in a question and the county where you live. Here are some questions asked by other gardeners. What's yours?

    Stephanie Yao Long/staff

    Star jasmine for privacy?

    Q: What are plusses and minuses about installing star jasmine as a vine on wire above a fence for privacy purposes? Is it invasive?

    A: I know many people who use star jasmine as a privacy plant, all with varying degrees of success. As long as you plant and maintain the vine per the directions of the grower, you should be fine. There will be times that you will have to prune the vines back, since it is a vigorous grower. However, the pros outweigh the cons of having this sweet-smelling vine. Watch for the hardiness of the plant in your area, especially if you live in higher elevations. They don't tolerate continuous, deep freezes. You may want to inquire further with the nursery you will be purchasing from, as there are different varieties.

    – Maggie Matoba, OSU Extension Master Gardener

    Courtesy of OSU Extension Service

    Q: My Fuji apple tree is about 4 years old. It has four to five quarter-sized areas with raised, round bumps. There's no oozing.

    Watch the video: Removing Burr Knots From Apple Trees

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