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Burr knots result from adventitious root development on aboveground portions of trees. Development is favored by low light, high humidity, and warm temperatures. Some apple varieties (e.g. ‘Lodi’) and rootstocks (e.g. M.7 and MM.111) are particularly prone to this problem. Burr knots provide areas for wooly apple aphids, borers, fire blight bacteria, and wood rot fungi to colonize trees. Large numbers of burr knots weaken trees, and resulting wood rot infections may girdle limbs. 

Burr knot (Strang, UKY)
Burr knot.

(Photo: John Strang, University of Kentucky)

Burr knot girdling limb (Strang, UKY)
Burr knot girdling limb.

(Photo: John Strang, University of Kentucky)

     

Management:

  • Treat galls with Gallex
  • Select rootstocks and varieties that are less prone to this problem

   

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