Pruning to Remove Eastern Filbert Blight

Jay W. Pscheidt

Once a filbert tree is infected with Eastern Filbert Blight (EFB), the infection can only be removed by pruning. This will also help reduce the number of spores available to infect other healthy trees and branches. Foliar applied fungicides are unable to kill or slow the spread of established cankers within a tree. How much wood to prune depends on the amount of disease in your orchard, where your orchard is located and your overall control strategy. No one recommendation can be made for every situation. 

Once EFB has been found in an orchard, there are several factors to consider before deciding on a pruning strategy. Is the infection confined to one or two trees or is it scattered throughout the orchard? How susceptible are the cultivars in your orchard and what is your pollinizer arrangement? Are there infections on small twigs high in the tree or on main scaffold branches? Do neighboring orchards have EFB?

There are different pruning strategies, all of which represent a continuum of pruning severity from removal of a few branches to removal of the entire tree. The various terms used are detail pruning, dehorning, hedging (or slabbing), and tree removal.

Detail Pruning

Detail pruning involves the removal of branches one to three feet below an EFB canker. This could mean the removal of a few branches and/or some scaffolding, depending on how severely a tree is infected. Equipment needed can include pruning shears, loppers, hand saws, pole saws, chain saws, ladders and pruning towers (tree squirrels). The advantages of this method include continued nut and/or pollen production, removal of cankers, which can reinfect healthy branches and trees, and it provides a good understanding of the scope of the problem in your orchard. The disadvantages include missing unseen cankers, the large time requirement per tree and the need to continue the program for several years since symptomless infections from last spring will show up during the coming summer. Another problem occurs if pruning cuts are not made low enough below the canker to remove the entire infection. New cankers will continue to develop the following summer. 

Dehorning

Dehorning involves removal of the entire tree canopy at a height of five or six feet above the ground. This means major cuts to the scaffold branches of the tree using a chainsaw. Precuts need to be made on the underside of a limb to prevent bark tearing down the trunk and to help direct the fall of large limbs. Advantages of this technique include removal of unseen cankers, removal of wood that may be infected but not yet showing symptoms, reduced tree height for easier inspection, improved fungicide coverage and, initially, a relatively small time requirement per tree. 

The disadvantages of dehorning are numerous: 1) The risk of injury is high due to the chainsaw itself, cutting at or above your head, and heavy falling limbs. (Never use a ladder as that increases your risk of injury.) 2) Large open cuts leave the tree susceptible to wood and heart rot fungi. 3) Vigorous regrowth does not always occur the following year. 4) Sunburn may occur on unshaded wood on dehorned as well as surrounding trees. 5) Dehorned trees require extensive, time consuming retraining in the first and future years. 6) There is no pollen or nut production in the year following dehorning. 7) Total orchard nut or pollen production is reduced in the first two to three years after dehorning.

Dehorned trees also may be more susceptible to EFB, given that all other factors are equal. Research has shown that infection occurs on young expanding shoots. Field observations indicate that vigorously growing shoots, such as suckers or shoots from dehorned trees have larger, longer cankers. These factors make dehorning impractical without the added protection of fungicides. Due to all the disadvantages, dehorning is not recommended for orchards without EFB.

Hedging

Hedging is an intermediate step between detail pruning and dehorning. This technique evenly cuts the tree canopy down from a third to half its previous height. Hedging can be done by hand or mechanically using rotary or rapidly moving blade(s). Hedging is generally used in filberts to promote new wood growth and thus increase yields. Hedging is a less time consuming method to remove all the fine top growth where EFB infections are likely to occur. It also reduces tree height for more thorough fungicide coverage. Hedging will not be able to remove infections in the lower portion of the tree and the vigorous regrowth may be more susceptible to infection.

Tree Removal

Tree removal is recommended for severely infected trees, especially for susceptible cultivars such as "Ennis" or "Daviana". This includes trees with many cankers or cankers that have extended into the main scaffold limbs. The advantage is a quick way to remove all known and future cankers. The disadvantage is trying to replace or regrow that tree, particularly in a mature orchard. A new, uninfected sucker could be grown to fill in the gap. Also, there are several pollinizers that are immune to EFB. In addition, Hall's Giant is relatively resistant, very vigorous, and compatible with Barcelona as a pollinizer. Replacement techniques include grafting, transplanting three or four year old trees or dehorning mature trees around the area to be replaced. Each of these techniques has advantages and disadvantages that will not be discussed here.

Integration of Techniques

A combination of pruning, tree replacement and fungicide spraying will be needed in most orchards with EFB. No one method will completely remove all infections and guarantee no reinfections. Severely infected trees must be removed. Surrounding trees infected with EFB will require some level of wood removal. The choice of pruning method is up to the grower who must consider all advantages and disadvantages of each technique. The best time for pruning and removal of infected wood is during the winter anytime after harvest. However, the infected wood must be destroyed before the next bud break when trees are susceptible to new infections. Finally, all pruned and healthy trees must be protected in the spring with approved fungicides.