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Larry Sagers Horticultural Specialist Utah State University Extension Service Thanksgiving Point Office © All Rights Reserved
The scenario is all to common. You make a leisurely inspection of the back yard orchard and there it is. Tiny blobs of clear amber or brown ooze emerging as a plague to strike your trees. It’s there, sliding down the trunks, erupting from the branches and oozing from the twigs. The mere sight of this ominous gunk is enough to panic even the most toughened and unsentimental gardener.
Take a few moments to gather your wits about you and get the facts straight. Diagnosing the problems takes a little investigation of what has happened to the tree and what the gumming really means. Gumming in and of itself is not a disease nor is the gum itself a problem. Gum is a symptom that may be roughly described as a plant protection or scab produced when the plant is injured. It may have many different causes. Just as a runny nose can be cause by allergies, colds or other problems, gumming can be caused by multiple problems.
Sleuthing out the problem is not always easy. The gum is obvious. The causes are not. The investigative process starts by narrowing down the field and trying to find the problem. To start you can rule out the apples and the pears. The stone fruits are the victims here. Peaches, plums, apricots, nectarines and cherries will produce copious amounts of gum under the right or wrong situation depending on your perspective.
Most gardeners automatically assume that the gum comes from borers. While borers are a common cause of gumming, they are not the only problem. Start by looking at where the damage occurs. There are several borers and they usually attack different sites on the tree. Mechanical damage is also a serious problem. Trunk damage often results in gumming and is a result of lawnmowers or string trimmers damaging the trunk. The bark does not have to be removed to be damaged. Scratching cats, chewing dogs, destructive humans all add to the mechanical damage.
Several common diseases add to gumming problems. Cytospera is the most common fungus that attacks the stone fruits. While it does not directly create this symptom, the damage to wood results in excessive gumming. The affected plant parts will eventually die, so prune and remove them to prevent spreading the disease. While there is no cure for this disease, healthy plants can resist the problems. Besides pruning out damaged wood keep the plants vigorous by applying adequate fertilizer and watering as needed.
Gum production is very obvious in the crotch areas of the branches. This results from winter kill. Narrow angles between the branches do not allow the wood to knit together properly. This means the tissues continue to grow late in the season to complete this process. It does not acclimate and is killed even during relatively mild winters. The best control is proper pruning to develop wide branch angle that allow the wood to grow together.
Psuedomonous syringea is a bacterial disease that is almost certain to cause gumming. In fact the common name of this disease is gummosis. With this disease the gumming comes on all woody parts of the plant including small twigs, large branches, and the trunks. When it first comes out it is clear and jelly-like but it quickly hardens and turns dark. It is not a common disease in our area but does show up if we have extended periods of cool wet weather. If you have been living underground for the past few months we have had ideal weather for disease development. There are no controls for this disease now. In more humid areas preventive copper fungicides are applied in the fall. Spraying now is of no value as the infection has already occurred.
Coryneum blight is a fungal disease that causes the death of many buds in the top of the trees. This fungus also infects the plants in the fall. Preventive sprays are the same as those outlined for gummosis. As the infection progresses the buds start gumming and eventually die. The twigs beyond the dead buds eventually killed.