NEW INFORMATION ON TREE DISEASES
I recently received a bulletin from John Karlik, Environmental Horticulture/Environmental Science, UC Cooperative Extension in Bakersfield. The title was “Tree Problems in the Bakersfield Area” and although we are not in the San Juaquin Valley, west of the Sierra Nevada Mountains, nevertheless disease problems from that area will eventually, if not already have, come our way.
The trees in question are Sweet Gum (Liquidambar styraciflua), Purple Leaf Plum (Prunus cerasifera ‘Atropurpurea’) and Raywood Ash (Fraxinus oxycarpa ‘Raywood’). I wrote a while back on Oleander Scorch, a bacterial disease that attacks Oleanders carried by sharpshooter insects. It is well known that this disease infects grapes, an important agricultural product in California. Now the disease has been found on more species in western states. The trees mentioned above are very common in our area, even though we are east of the Sierra Nevadas, thus the importance of bringing attention to this problem. We need to worry about the plants that grow in our area and in our neighborhood.
What to look for is dieback at the top outer branches extending downward. The disease takes several years for the plants to succumb. The bacteria are carried by an insect that feeds on leaves from a sick tree then transfers it to a healthy one. There is no cure.
The bacterium is called Xylella fastidiosa which, like many diseases, has subspecies that infect citrus, coffee, almond trees, olives, some oaks and maples, elms, plane trees, sycamores, plus the Raywood ashes, liquidambars, and purple leaf plums. In fact, these various subspecies of X. fastidiosa infect more than 100 species of plants worldwide.
The bacteria infect a plant through the mouthparts of an insect, the sharpshooter, that carries the disease. It gets into the xylem or water conducting tissue and plugs it up. The plant looks like it is water stressed. The leaves begin to dry out, drop to the ground and the stems slowly die back.
If trees susceptible to this disease are getting adequate water but show symptoms of dieback, this could be the problem. If there are other healthy oleanders, Raywood Ash trees, liquidambars, or purple leaf plums in your neighborhood, the best procedure is to get rid of the diseased plants before those other trees get infected.
Replacement plants are the next choice. So far, mulberry trees have no disease threat but they are water guzzlers. Pistachio trees have beautiful fall color. These trees require a lot of water to stay healthy to keep from stressing out.
Honey Locust and Flowering Locust require less water. Since we are in the desert, there are several choices for trees with less water usage. Mesquite trees, Palo Verde trees, and Desert Willows are good choices. There are a few pine species that do well in desert conditions, such as Aleppo Pine, Afghan Pine, and Italian Stone Pine.
Shade is important to frame the house and keep it cool. It is important to maintain healthy trees. Be on the lookout for problems, keeping in mind that this disease spreads and can be a serious pest in your neighborhood and beyond.