If you are considering planting a tree, you might want to eliminate southern magnolia (Magnolia grandiflora) and jacaranda from your list of choices. There is a bacterial disease that is infecting these trees, and liquidambars and sycamores, too, especially in the San Fernando Valley, but throughout the greater Los Angeles area as well.
I would also be reluctant to plant an olive tree. Over the last five years in Puglia which, geographically speaking, is the heel of Italy’s boot, eleven million olive trees – some of them 500 years old – have been killed by the same bacteria that has been proliferating among our local trees.
The bacteria involved, Xylella fastidiosa, happens to be the bacteria responsible for oleander leaf scorch. At one time, oleander was the preferred selection for hedges throughout Southern California. It flowered most of the year in white, pink, red, or salmon and, once established, did not require summer irrigation. Today, it is seldom seen on account of the deadly bacteria mentioned above.
There are several strains of this bacteria; each strain has a distinct group of species that it infects. The strain that infects oleanders also infects jacarandas, southern magnolias, and daylilies.
The history of this pestiferous bacteria began in the 1500’s in the colony of Florida. European grapevines were brought there by the first Spanish settlers. After living a short time, the vines died, although wild grapes indigenous to Florida were unaffected. The signs of disease on those vines as described by the settlers of the time strongly resemble the symptoms we now associate with Xylella fastidiosa infection.
This same grapevine blight, only to a more devastating effect, was encountered in the 19th century in our own backyard. California’s wine industry had its origins in Anaheim where, starting in the 1850’s, 10,000 acres of grapes were eventually planted. (Disneyland’s 160 acres were formerly an orange grove but, prior to that, they had been planted to these grapes.) As the gold rush frenzy abated, Germans who had come to Northern California to prospect for instant wealth got together and decided to move to Southern California where they would become shareholders in the Los Angeles Vineyard Society. Each member of the society would receive 20 acres of land, including 8 acres of grapes, and share in the profits. The Society thrived and Anaheim grapes achieved a notable reputation, referred to in reports of the time as “the sweetest and best grapes in the state,” with the wines themselves described by a discerning oenophile as “unusually pleasant and light.”
Alas, in the 1880’s, the Anaheim wine industry, which numbered 50 wineries (including a kosher one), came to an end when all the vines died. Newton Pierce, a plant pathologist from the USDA, was called upon to investigate what had happened and hypothesized that a bacteria was involved. In honor of his investigations, the grapevine blight became known as Pierce’s disease. This disease, which has no cure, continues to ravage grapevines throughout California although, due to breeding programs, vines resistant to the disease are now available.
Although grapevines are infected by one bacterial strain and oleander, magnolia, and jacaranda by another, all are transmitted by an insect known as glassy-winged sharpshooter. The insect has a vague resemblance to a small grasshopper, with wings resting tent-like on its back.
Control of this insect has been greatly assisted by the introduction of parasitoid chalcid wasps. These wasps lay eggs in leafhopper larvae. As the wasp eggs hatch out, the developing wasp larvae cannibalize the leafhopper larvae.
Xylella fastidiosa is carried in the gut of both leafhoppers and spittle bugs, the latter noticed as globs of what looks like spit, which protect the insects cloistered inside it, on a wide variety of plant species. Leafhoppers and spittle bugs have piercing-sucking, syringe-like mouth parts which enable them to inject the bacteria they carry into the xylem, or water conducting vessels, of plants. The bacteria stimulate growth of compounds that plug the vessels, disrupting passage of water and leading to scorched and wilted leaves.
The number of species that host this bacteria in their sap without showing symptoms of the disease is alarming. Although not indigenous to California, the bacteria have been found in native coast live oaks and California sycamores, as well as in almond, plum, peach, and mulberry trees. Ginkgo, hackberry, crepe myrtle, maple, and eucalyptus trees may also harbor the bacteria, as well as Photinia shrubs, rosemary, and even roses.
Tip of the Week:
Flowering vines can be a mixed blessing. Enamored with their beauty, you may loathe the thought of pruning them, but letting them just grow can spell trouble for plants in the vicinity. Before you know it – as occurred in my own neighborhood — ‘Cecile Brunner’ roses planted at the base of jacaranda trees (installed along the property line between two front yards) will be competing with the trees for air space and sun exposure. Don’t get me wrong; ‘Cecile Brunner’ is a magnificent, sweet-smelling climbing rose. However, it is best suited to growing up a pergola or archway or even against the façade of a house, but not in proximity to tall shrubs or trees.
A similar scenario presents itself in the case of a lavender trumpet vine down the street. Originally meant to hide a chain link fence, it found its way to an adjacent Eugenia hedge where it has now covered one end of the hedge and, left unpruned, will obliterate the hedge completely. Maybe this is not such a bad thing, as long as you have made a conscious choice to let your former hedge be a support for your flowering vine, which will still serve quite admirably, upon the skeleton of your dead hedge, as a tall barrier or screen.