What is that sap from my mulberry tree?
Ihave had several people asking about dark sap dripping from their mulberry trees — either from old pruning cuts or just from where the branches separate from each other.
The problem is a disease called slime flux. Almost every mulberry in the Antelope Valley has the disease. Slime flux is also found in all of the Siberian elm trees, cottonwoods and poplars in the Antelope Valley.
When the temperature rises, or the trees become stressed, the symptoms become more noticeable. With the high temperatures, drought and change in water patterns, it is becoming much more noticeable. When you stop watering your lawn, your tree is affected. If you took out your lawn, you have disturbed the roots and are not watering the tree the same.
Many trees are going to suffer in the Valley, and whatever diseases are common to the tree are going to become more noticeable.
Chitalpa trees seem to show more problems with dripping sap. It is probably a bacterium that clogs the xylem of the tree. Xylem is the tissue that conducts water in plants. This bacterium clogs the xylem and causes leaves to fall off, water staining on the tree and other problems.
Ash trees are showing limb death, which is probably a physiological problem, where the xylem collapses. The water is being pulled through the xylem and if there is not enough water in the soil, it collapses. It’s kind of like trying to drink a thick milkshake through a straw and the straw collapses.
Back to slime flux: The disease is easy to identify in a tree because of the dripping dark sap. In other trees, the wood bark either turns black or looks water-stained, which is the reason the disease is also called wetwood.
The tree has been infected with bacteria. The bacteria can enter the tree through wounds, pruning or even insects. The bacteria can live inside the tree for years without any outside evidence.
After a few years, the waste gasses produced by the bacteria causes the pressure to build up inside the tree tissue, increasing the sap pressure. With high sap pressure, sap starts to drip out of openings in the bark. Most commonly, these openings are pruning cuts from the last few years where the bark has not healed completely. It also commonly drips from the crotch of two branches or from natural bark splits.
Another bacteria or fungus will then start growing in the oozing sap, turning it black. In elm trees, the sap becomes light brown and is less noticeable. If you have a white, foamy material coming out of your trees, especially willow, the problem is borers tunneling through the branches.
There is no cure for slime flux. At one time, it was recommended to drill small holes into the tree around the base and insert straws to allow for drainage of the sap under pressure, just like a pressure relief valve. However, this caused more damage to the tree than good. Drilling a hole into the tree can increase the spread of the bacteria and allows other pathogens to enter.
Preventing a healthy mulberry, cottonwood or Siberian elm tree from being infected is impossible. The best prevention practice is proper pruning, so the cuts will heal quickly. Do not use pruning seals; they will slow down healing.
Try not to disturb the soil, this causes damage to the roots, which can allow entry of the bacteria. This even includes your lawn mower hitting tree roots growing up in your lawn.
The best prevention is keeping your tree happy and healthy with enough water and fertilizer. When plants are stress-free, they are less likely to get infected or an infected tree will not show the problem.
Slime flux normally does not kill mulberry, cottonwood or Siberian elm trees; however, they will ooze sap all over the ground, car, or sidewalk. Sometimes the sap will start to smell really bad. The dripping sap is only going to become worse over the next few years.
If the sap really bothers, you should remove the tree. Mulberry, cottonwood or Siberian elm trees are always on my bad list due, partly, to this disease and because they are very thirsty trees requiring large amounts of water.