Pruning out Disease to Keep Trees Healthy
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Mary Robson (Ret.) Area Extension Agent
Regional Garden Column November 11, 20001
Fall and winter brings time for pruning trees, to maintain their health and to correct occasional damage from storms. Gardeners get lots of information on how to prune but less on what to do if the tree shows some disease problems. Must we handle tools with disinfectants? What about keeping the wood? This article by the Master Gardeners in King County uses information gathered by Dr. Warren Copes, Plant Pathologist, Washington State University, Puyallup.
One suggestion often made is to dip pruning tools in a disinfectant between each cut as the surest way to prevent the spread of disease. It makes sense that if you cut through a section of stem where a disease organism is present, then structures of that pathogen spread to the next ten or more cuts made with that tool. The easiest disinfectant to use is alcohol in a spray bottle, but this will not necessarily get rid of all disease if it's present.
But if the cut is made where no pathogen is present, disinfecting the tool is a waste of time and a genuine nuisance that can slow down the task. How can you decide? Deciding not to disinfect requires a correct diagnosis, plus knowledge of how a specific pathogen spreads. Learning to distinguish disease patterns takes time. If you are in doubt, disinfect the tool after cuts.
Here are some specific problems that can cause branches to die. (This is, of course, conveniently called "dieback".)
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1. You see a specific spot or blotch on a branch, where the pathogen is localized in a lesion or canker. This can result in the entire branch dying back. Phomopsis blight on juniper (caused by Phomopsis juniperovosa) is an example of this type of disease. The pathogen can cause a lesion, which you can compare to a wound on an animal, and girdle a stem at one point. When stems are completely encircled by a disease, the plant can no longer get water through to keep tissues alive and the branch dies above the lesion.
The pathogen will not have spread far, possibly 2 inches or less out of the lesion. In this case, a cut can be made in healthy wood that is 4 inches or more away from (usually below) the edge of discolored wood. Be sure to cut back to healthy wood and bag up the dead and diseased piece.
Many of the diseases in this category are more likely spread in water that washes fungal spores or bacterial cells down a limb than on pruning shears. If you stay below the disease, you would not need to disinfect tools.
- Plants exhibit dieback or blight and the pathogen is present in the discolored wood and in healthy looking wood well beyond but still associated with the discolored wood. Diseases such as bacterial blight on lilac, maple, and cherry (caused by Pseudomonas syringae pv. syringae). The pathogen, whether bacterial or fungal, will advance in to healthy looking wood and be present 3 to 10 or more inches from the dieback. The pathogens often move down the water conducting tissue (xylem).
Pruning cuts should be made just above a bud or branch in healthy-looking wood that is at least 12 inches away from the edge of discolored wood. You will probably get the pruners contaminated while exploring the edges of the affected wood.
The main purpose of pruning far enough away from dieback is to completely remove the pathogen so the pathogen will not continue to kill that limb or be spread to healthy limbs. If you have encountered any discolored wood, disinfect before pruning into healthy wood.
- We've been describing disease that is localized and then one that's less localized. Sometimes the entire plant can be affected. Plants exhibit dieback or blight and the pathogen is distributed considerable distances from the dieback and possibly throughout the vascular system of the plant.
- This category includes a range of bacterial and fungal diseases such as leaf scorch of elm, maple, oak, and sycamore (caused by Xyella fastidious), Verticillium wilt of maple (caused by Verticillium spp.), and Dutch elm disease (caused by Ophiostoma spp.).
Symptoms often do not develop until after the pathogen has become established and potentially spread throughout the plant. Although these diseases are not easily transmitted on pruning tools, it can happen. Diseases in this category usually cause decline and death of a tree. Because of the seriousness of these diseases dipping pruning tools in disinfectant after every cut would be justified. Do not chip or recycle the wood from these trees.
- Plants exhibit dieback or blight and the pathogen is present on the roots or collar of the plant. Phytophthora spp., Armillaria spp., and various root pathogens can kill a portion or all of a root system, which limits water and nutrient uptake and causes limb dieback. The pathogens are not present in the limbs and it is not necessary to disinfect pruning tools.
- Plants that exhibit a wide array of possible growth abnormalities, such as yellow patterns on leaves or stunted growth, and the pathogen is spread throughout the vascular system or through numerous types of plant tissue.
Viruses and viroids primarily cause the types of diseases in this category. Although transmission on pruning tools may not have been demonstrated for many viruses and may range from common, unlikely, to impossible. If you suspect a virus, a good general rule would be to either prune those plants last or disinfect pruning tools between each plant, and definitely disinfect tools between
Learning all this takes some time but this provides a guide to use when working with and observing problems during tree pruning.
Hortsense: Managing plant problems with Integrated Pest Management
