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Certain
Landscape Trees May be Naturally Doomed Oftentimes trees were introduced to cities and towns because they were considered well suited to the local environment. They were used exclusively in large plantings such as streets lined with a single species of shade tree. This practice is unlike nature, where several species are found together and natural selection guarantees the survival of the strongest while the weakest may fail and die. Other times masses of trees were used in cities and towns where the local environment was quite different from their natural habitats. This practice requires tender loving care of trees when conditions become harsh. Both of these scenarios provided ideal conditions for the evolution of species-specific insect pests and diseases. Mass plantings of a single species supplied a large, almost continuous food supply. One of the most dramatic and often-cited examples of a disease problem caused by over planting a species is Dutch Elm Disease on American, English and Scotch Elms. This disease has eradicated much of the elm population in the eastern United States and it now infects some areas of California. The Chinese Elm, which is widely planted here, is not susceptible. Modesto Ash, once thought to be indestructible, was planted in large quantities as street trees 30-40 years ago. It has since been debilitated by Ash Anthracnose. Unfortunately Modesto Ash is the most susceptible of all the ash species. Raywood Ash, a fairly drought tolerant cultivar brought here in the early 1960s from South Australia, became an instant success. It was trouble-free for a few years until an occasional partial canopy death became a serious concern to landscape managers. This problem did not begin to appear until hundreds of thousands had been planted over several states. Aristocrat Pear seemed to be the perfect tree with fast growth, a symmetrical habit, spectacular fall color, heavy bloom in spring, and consistent success in most landscape situations. After they were planted throughout the nation, Fireblight (a bacterial disease) found a good ornamental host. Another potentially devastating disease, a fungal canker, seemed to kill major limbs within two years of the arrival of balled-and-burlap trees from Oregon nurseries. Aleppo Pine is naturally found in areas of high ocean influence and sandy soils in Mediterranean countries. We have used this species in great quantities in every conceivable climate and soil and, as a result, Western Pine Rust has become a serious problem that sometimes kills trees. Leyland Cypress produced by crossing cypress species native to coastal climates, but used extensively as hedges in interior climates, is attacked by the disease, Cypress Canker. California Pepper Tree, actually from Peruvian stream sides, is common throughout California. In the early 1980s the Pepper Tree Psyllid began to infest these trees. The psyllid had been in the United States before that, but the use of large quantities of these trees near the coast and in severely drought-stressed micro-climates has resulted in almost ever-present damage to pepper trees. In the worst cases, most of the leaves drop by mid-summer and the tree gradually starves. Italian Alder, since the mid-1980s, has been hit by a disease which kills the tissue at the base of the trunk when irrigation water is sprayed on the root collar. Eventually it kills the tree. White Alder, when used in poorly drained clay soil or in turf with frequent shallow irrigations, becomes drought stressed. This sets the stage for Alder Borers to lay eggs in the branches. Under severe conditions the tree dies in three to six years. European Olive and Chinese Pistache are affected by the soil borne fungus, Verticillium albo-atrum. Eastern Dogwood gets Dogwood Anthracnose disease. Coast Redwood, which thrives in cool summer temperatures is used extensively in the hot, dry interior of California suffers from this environment and is also attacked by the fungus, Botryospheria ribis. Stressed Tasmanian Blue Gum and other eucalyptus species are attacked by the Eucalyptus Longhorn Borer. Lombardy Poplar, genetically identical to those selected several centuries ago in Italy, has been used inappropriately in arid areas, and is now attacked by poplar canker and crown gall. No tree is perfect for all situations and all trees have their
weaknesses. When choosing your trees understand that it's not
easy to predict what nature holds for their future.
August 28, 2003 |