Sap dripping from pruned mulberry

I have several questions about pruning mulberry trees. One of my mulberries was pruned during the warm season. Some years later, it is still "bleeding" sap from the cuts. This appears to be weakening the tree. I haven't been able to come up with anything that will stop it. What do you recommend?

My other 3 mulberries need pruning. If I prune them while dormant this winter, will the wounds bleed in the spring? If so, how do I treat the wounds to avoid this? Since I want to be sure the trees are dormant, what month is the best for pruning mulberries?

Minda S.
Socorro

I suspect the continued "weeping" from the old pruning cuts on the first mulberry tree is due to a bacterial infection of the heart wood called "wet wood disease." This is a relatively minor problem for a healthy tree. The disease causes a buildup of pressure inside the wood and forces sap out of the tree through old pruning cuts or cracks in the bark. The pH of the sap becomes somewhat caustic, so it prevents normal wound closure (the growth of callus tissue over the pruning wound). This allows for continued weeping of sap. There is nothing to apply to cure this disease. The good news is that it is a minor problem and does not quickly kill the tree. In fact, by altering the pH of the sap, the disease inhibits infection by fungi which could be more damaging to the tree. The wet wood disease does weaken the tree and, with other stresses (drought or other problems), it can speed the demise of the tree. The best treatment is to provide adequate irrigation and to prune away dead and dying branches as needed.

Pruning in the dormant season (November through February) may reduce the chance for the other trees to develop this problem but does not guarantee that they won't also be dripping sap in the spring. Disease organisms are less active in the winter, and the pruning cut has more time to dry. Once dried, the cut is less likely to become infected.

It is important to avoid spreading the disease when pruning. Prune the tree known to be infected last. Disinfect the pruning tools with diluted chlorine bleach (9 parts water to 1 part bleach), alcohol, or other disinfectant. After the chlorine bleach treatment, wipe the pruning blade with an oily rag to prevent rust. Perform this disinfection process when going from one tree to another or between pruning cuts on the same tree.

Regarding application of pruning sealers after pruning, they are not necessary if the pruning cut is properly made. The tree has mechanisms to protect itself by compartmentalizing the wounded tissue and preventing the spread of disease across the wound boundary. There is some evidence that the pruning sealer, by retaining moisture in the cut, encourages growth of disease organisms under the sealer. If you feel you must cover the wound, use a thin, light-colored (white), latex (non-oil based) paint.

You can minimize the disease problem by not spreading disease when you prune, but it is impossible to prevent the spread of disease. Natural wounds (branches broken by wind and branches rubbing against each other) allow entry of disease organisms. Proper pruning techniques and quick removal of naturally damaged branches are your best method of reducing disease problems.

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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

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