Click on any
image for a more
detailed view

Significance of the Disease

History and Etiology

In 1892, Newton B. Pierce (Figure 21), state plant pathologist of California, examined grapevines (Vitis vinifera) with a scorch and decline of unknown cause (Table 6). At that time, the problem was known as California vine disease. Although the disease was eventually named for him, Pierce could neither isolate nor culture the causal agent, and he ended his career suspecting that a “minute microorganism” was involved. About the same period, peach growers in Georgia observed that many trees were stunted (“pony trees”) and the fruit they produced was undersized and unmarketable (in a disease charmingly called “phony peach disease”). It would be many years before scientists could prove that the causal agent of these two seemingly unrelated maladies was related.

Figure 21

Clues to the etiology of what would become known as “Xylella-associated diseases” began to emerge in 1936 when both Pierce’s disease and alfalfa dwarf were transmitted through root grafts and by insects. Because of these properties, the causal agent was considered to be viral in origin until 1971, when antibiotics applied to grapevines with Pierce’s disease suppressed symptom development. Since viruses are not subject to control by antibiotics, the organism was reclassified as a mycoplasma-like organism (or MLO). In 1973, electron microscopy revealed an organism in the xylem vessels of infected grapevine leaves and peach roots that had a rippled cell wall and could not be cultured on standard culture media. This “rickettsia-like” bacterium (RLB) was subsequently found in many hosts, and the list of these “Xylella-associated diseases” grew.

Study of these diseases was significantly advanced in 1978 when a culture medium sufficient to isolate and grow the bacterium in vitro was developed. In 1987 this organism was properly described as a bacterium closely related to the bacterial plant pathogen Xanthomonas and was given the name Xylella fastidiosa (“Xylella” because it grows in the xylem; “fastidiosa” because the organism is fastidious and difficult to grow in culture). X. fastidiosa is one of several “xylem-limited bacteria.” More information on this type of pathogen can be found at: http://www.apsnet.org/education/IntroPlantPath/PathogenGroups/fastidious/

Today, diseases caused by X. fastidiosa continue to increase in importance as entire crops such as grapes, citrus, and coffee, in certain locations are threatened. Since the late 1800s, wine grapes in California were protected from Pierce’s disease by planting grapevines at least 100 m from bodies of water where alternative host vegetation predominates. This strategy worked for over 100 years because the vectors of this disease were unable to traverse this distance from alternative host vegetation to the grape plantings. Recently, however, the glassy winged sharpshooter (H. coagulata), common in peach and plum orchards in the southeastern United States, was introduced to the grape-growing regions of California. This voracious feeder is a strong flier and has the potential to easily spread Pierce’s disease to valuable grape plantings. Many resources are now allocated for research to help prevent the potential devastation to the California wine industry from Pierce’s disease.

X. fastidiosa is of particular concern in South America, where plantings of citrus and coffee are affected by closely related, if not identical, strains. Coffee leaf scorch (or Requeima do Café) causes leaf scorch (as the name suggests) as well as premature leaf drop and shoot dieback. Symptoms of citrus variegated chlorosis (CVC) include small fruit with hard rinds that cannot be marketed. CVC has been of such importance that this was the first of various X. fastidiosa strains to have its genome completely sequenced.

The incidence of BLS in shade trees is equally important from both economic and aesthetic points of view. For example, in some communities in New Jersey (Figure 22), BLS affects as many as 35% of street and landscape oaks. Current loss of value plus replacement costs for older trees affected by this disease is estimated at $8,000 per tree. Landowners and tree care professionals in these locations must plan for the loss of property values and high costs of replacement as shade trees in landscapes, wood lots, and golf courses affected by BLS decline and must be removed.

Figure 22

RETURN TO TOP