Spreading Disease by Pruning

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent

What’s your favorite type of pruning tool... a pair of ratchet hand pruners, a lightweight pair of loppers, or a handy folding pruning saw?  Tools like these are standard equipment for gardeners who have a lot of trees and shrubs in their landscape.  They’re needed to prune off ill-placed branches, remove older wood, and eliminate any dead or diseased portions of a plant.  Removal of these infected tissues is very important in preventing the spread of disease.  However, many of us home gardeners may actually be spreading disease if we’re not disinfecting our pruning tools between cuts.

How can pruning tools spread disease?  According to WSU Plant Pathologist, Dr. Warren Copes, if you cut through a section of stem where a pathogen, such as a virus, bacteria, or fungus is present, then the parts of the pathogen may adhere to your pruning tool. When certain diseases are suspected, it’s a good idea to disinfect your pruning tools after every cut.

Copes also points out that not every dead branch is the result of an infection by a disease.  There are also cultural and environmental problems that can lead to dieback.  Let’s look at some common diseases that cause problems in our area and the “protocol” suggested for disease management through pruning and disinfecting the tools we use.

When we have cool, wet springs, a common problem in this area is sycamore anthracnose, also known as sycamore “blight.” True to its name it attacks sycamore trees.  This fungus disease causes cankers or lesions, which eventually girdle a stem and lead to the death of tissues beyond the canker. Over time, repeated infections create sycamores with unsightly crowns.

Where practical, it’s recommended to prune out the cankers caused by the anthracnose fungus.  This improves a tree’s appearance and more importantly removes a source of future infections.  Cuts should be made four inches below any discolored wood.  The disease is spread primarily by wind and rain, not by pruning equipment.  With this disease, you don’t need to disinfect pruning tools after cutting off infected tissue.

One fungus disease that we’ve seen quite a bit of this year is verticillium wilt fungus.  Unlike sycamore anthracnose, verticillium wilt is distributed within a plant at considerable distances from the area of obvious dieback.  It usually enters the plant by way of the root system and moves through the vascular system to plant tissues.  Dieback of branches and limbs is caused by impairment of the root system and the plugging of the vascular system... the system that transports water and nutrients within the tree.  In our region, verticillium wilt is a common problem on maple, ash, sumac, redbud, catalpa, Russian olive, and smoketree. However, there are many other types of trees, shrubs, and garden plants susceptible to verticillium wilt.

While the verticillium wilt fungus can travel throughout a tree, it’s usually not evenly distributed in the plant.  Copes points out, “Since the pathogen isn’t evenly distributed, it’s difficult to identify where the pathogen would be located and also why the fungus is not always spread on pruning tools.”  Dead portions of trees infected with verticillium should be pruned out, but we must keep in mind that this doesn’t remove the pathogen which is located in the roots.  Pruning tools used on trees that have been diagnosed with verticillium wilt or are suspected to have it, should be disinfected between trees and when you are done pruning.

Some “blights” or diseases are caused by a bacterium rather than a fungus.  The most common one that can cause problems in local landscapes is fireblight,  Erwinia amylovora.  Fireblight attacks many members of the rose family including apple, flowering crabapple, pear, pyracantha, hawthorn, cotoneaster, quince, and mountain ash.

Symptoms of a fireblight infection usually start in the spring with infected flowers appearing water soaked.  The flowers may turn brown to black and fail to fall from the tree or shrub.  The bacteria moves down the infected blossom into the twigs, resulting in the sudden death of other flowers and fruits on the same twig.  Often, the twigs will develop a hooked tip or  "shepherd’s crook."  Twigs and leaves look almost black, as if scorched by fire.  As the bacteria moves down a twig into a branch, the tissues darken and die. If the outer bark is cut or peeled away, the inner tissues will appear red and water‑soaked.

Fireblight infections commonly occur in the spring when bacterial strands from dead infected tissues are splashed to flowers by rainfall or irrigation water.  The bacteria may also be spread to the flowers by insects that have visited infected plants.  Fireblight often enters a plant through natural openings in the floral parts.

Infection is favored by wet weather and temperatures between 65 and 86 degrees Fahrenheit.  Because of the relatively low rainfall conditions in this region, fireblight is not often a problem in home landscapes.  However, in some years the spring moisture and temperature conditions are just right.  This past spring’s weather provided perfect conditions for fireblight infections in a number of home landscapes.

Once an infection is noted in a plant, it’s important to prune it out immediately.  Pruning cuts should be made at least 12 inches below any discolored wood.  This is because discoloration is a chemical response by the infected plant.  It takes a while before the discoloration forms in infected wood. 

This rule of pruning at least 12 inches below discolored wood can cause a dilemma.  Copes notes that, “Sometimes, a blighted twig or spur may be less than 12 inches away from a larger limb and such removal would disfigure the tree.  In this case, there is no easy answer. Removing the scaffold limb may be necessary.”  If you don’t remove the limb, you should monitor the tree to see if the disease is still present.  While 12 inches is the rule, the disease doesn’t always follow the rules.  It may be present further down, especially if there is a length of time from the first appearance of symptoms and the removal of infected wood.