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Photograph by Kathy De La Torre
Sometimes mistaken for bamboo, giant reeds are easy to grow, but hard to get rid of.
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Clues emerging in mystery of oak tree 'sudden death'
By TONY TOMEO
No one has done more to promote investigation of the current epidemic of "sudden death" of oaks than the highly respected horticulturist, Peter Moerdyke. When the disease annihilated several hundred coast live oaks and tanbark oaks on his rhododendron farm near Scotts Valley, Moerdyke was dismayed to find that not only was very little known about the disease, but that very little was being done to find the cause--or more importantly--a cure.
He immediately began to publicize the importance of more extensive investigation, as well as the need for treatment of infected trees. After many articles in nearly every local newspaper and news broadcast, entomologists, botanists and pathologists from all over the continent began to converge on the farm and, eventually, obtained federal and state funding for continued investigation.
The two main pathogens associated with the disease have been the western oak bark beetle and the oak ambrosia beetle. In a normal ecosystem, the western oak bark beetle attacks stressed trees. This additional stress attracts oak ambrosia beetles, which in turn introduce ambrosia fungus as they bore into main trunks near the ground. The fungus, which is the only sustenance for the beetles, quickly clogs the vascular system of the infected trees, causing sudden death.
Since the beginning of the epidemic, however, both species of beetles have been attacking seemingly healthy trees. Because the beetles are considered to be secondary pathogens, it was necessary to isolate a primary pathogen, although any symptoms of infestation were only perceptible to the beetles.
Only recently has the primary pathogen been identified as a strain of the fungus Phytophthera. This discovery was made from a sample taken from one of Moerdyke's trees in which an earlier infestation of beetles had been eradicated. Although this discovery is of major importance, there is a disturbing irony associated with it. A similar fungus was also isolated from a rhododendron.
Before I continue, I should explain that it is not yet known if the two samples of Phytophthera are of the same strain. It is also important to know that rhododendrons that have not been infected do not transmit the disease. The likelihood of infection of rhododendrons in the landscape, relative to oaks, is not yet known, but treatment of infected plants is much more manageable.
Infection of rhododendrons only occurs on foliage during rainy periods and only near the ground. Spores of the fungus are airborne, but seem to complete part of their life cycle in wet soil before being splashed onto low foliage. Infection causes leaves to turn black and rot. It easily spreads to nearby leaves, into petioles and eventually into small stems.
Rhododendrons should be periodically inspected during and after prolonged wet weather. Although leaves may discolor for a variety of reasons, it is best to remove any that appear damaged or infected, regardless of the symptoms, especially those close to the ground. Any stems that die should be pruned back to healthy stems. Leaf litter below plants should be removed.
Unfortunately, the effects of Phytophthera in oaks is not so obvious. Although both species of beetles may be controlled with insecticides, long-term tolerance of the primary fungal infection is not yet known.
Perennial of the Week: Giant reed
Giant reed has several common names, including the species (Latin) name, Arundo donax. It is often labeled as bamboo. When studying it in college, we were told that it was once used as a packing material. As boats were unloaded in Sacramento, it was often dumped into the river and eventually naturalized. It is very rare in nurseries, but those who want it have no difficulty growing it from clumps taken from established plants.
A variegated cultivar is striped with yellow or white, but often produces green seedlings. Like bamboo, it is a species that garden enthusiasts are either very fond of or detest. It is very useful in poorly drained areas and near water. The cane stems are from 10 to 20 feet tall. Leaves are about two feet long and about three inches wide. Severe pruning does not harm mature plants. Giant reed can become very invasive and should only be planted where it can be controlled.
Horticulturist Tony Tomeo can be contacted at 408.358.2574.
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Cupertino, California Since 1947
