1 - There are several possible causes for foamy sap on the trunk of trees.

Yard and Garden October 12, 2013

Q.

I have a 21 year old globe willow. It is leaking so much foamy liquid that the flies are all over it. What is causing this problem? My home was in the zone of that suburban fire 3 years ago, my home was spared. I only lost my barn.

Cibola Co.

A.

It is not uncommon to find globe willows leaking foamy sap. This could be due to several factors. Some possible causes include borers, sun damage to the bark resulting in cracking, or other physical damage. It may also be due to slime flux (wet wood) disease.

All of these possible problems cause the sap, full of sugars to leak onto the bark of the tree where it attracts insects who feed on it. On the surface of the bark, it will be exposed to yeasts and bacteria on the surface and begin fermenting and foaming. When it ferments, it can create foul smells. Washing the tree trunk with a garden hose can help reduce the quantity of sap on the surface of the tree trunk. This will reduce its ability to attract insects. In extreme cases, people have scrubbed the area with a 10 percent chlorine bleach solution to temporarily surface sterilize the trunk and reduce fermentation. The yeast and bacteria will soon return, but this is a temporary solution if foul smells are a problem.

Close observation of the tree trunk to look for bore emergence holes or cracks in the bark as a source of the leaking sap may help determine further action. If it is borers, emergence holes may indicate that the insects have left the tree to deposit new eggs on the same or other trees. The fire may have stressed this and other nearby trees, making them more susceptible to insect attack. However, globe willows are often attacked with no apparent stress leading to the attack. Look for signs of borers in the globe willow and other important landscape trees. If bore attack is determined, you should contact your local NMSU Extension Service Agent.

Sun damage often appears on the southwest side of the tree and is called southwest injury. This occurs where the bark on the southwest side of the tree trunk warms during winter days and freezes at night. The warming can cause that region of bark to break dormancy and lose cold hardiness too early. This injury may also be aggravated by drought so that the tree has little water to help moderate temperatures just under the bark. As the bark dies, it cracks and adjacent living, tissues leak sap through the crack in the bark.

Wet wood disease is a bacterial disease of the wood inside the trunk. It causes a buildup of gas pressure that causes cracks to form and sap to be expelled through the cracks. There is no cure for wet wood disease, but it rarely causes a tree to die quickly. This disease is common in some deciduous trees in New Mexico. The bacteria change the pH of the sap making it more alkaline. This may cause yellowing of leaves above the infected wood tissue. This yellowing results from interference with the movement of iron in the sap to the leaves. The high pH sap may also drip to the ground under the tree and cause death of grass.

It may be hard to distinguish between southwest injury and wet wood disease because they are similar. It is also very possible that wet wood disease may have entered through bark injured by southwest injury or insect attack. It also enters through pruning cuts and wind broken branches. More than one of these causes may be responsible for the symptoms in the tree you have described.

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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