Economically important plants are under constant assault by pests and pathogens moved around the world by international commerce in horticulture. Here is the cautionary tale of how a very unusual bacteria threatens some of our most important plants.
In 2013, olive trees in Puglia, Italy, suddenly began dying from a disease. Puglia or Apulia is the boot-heel of Italy and a major region for olive oil production, The disease was named olive quick decline syndrome. It now threatens olive crops and olive oil production throughout Europe. By 2014, olive oil production in Puglia had declined by 35% as trees died. In 2015, infected trees are being destroyed to slow disease spread. France has declared a quarantine on import of horticultural goods from Italy. The new disease could reduce olive oil production throughout Europe and slow trade in horticultural goods.
In California, vintners have long had trouble growing grapes near riparian areas because of Pierce’s disease, which causes grapevines to wilt. In 1989, the disease began spreading outside of riparian areas and is now a major threat to grape production throughout the state. Losses to California grape production are about $100 million per year and control costs about $50 million per year in a $3.2 billion industry (Tumber et al 2014). While this may seem minor, there is no resistance to the disease in grapes and little control over its spread. Climate change could rapidly accelerate the spread of the disease, which is limited by very cold weather.
In the Bluegrass of Kentucky, I have watched pin oaks, planted as shade and street trees, over several decades as they slowly declined, with leaves turning brown and wilting in the summer. Trees rarely die immediately, but linger for a long time, losing their ornamental and shade value long before they are dead. Nearly all urban pin oaks are affected, as well as large numbers of pin oaks on our famous horse farms. Other urban trees, including elm, maple, sweetgum, sycamore and several oak species are infected from Texas to the east coast (Gould and Lashomb 2007). Fortunately, bacterial leaf scorch seems not to infect the same trees in their natural habitat, or at least does not cause serious disease problems.
A disease called citrus variegated chlorosis (CVC) has spread throughout São Paulo State of Brazil, in the most important sweet orange producing area in the world. By 2000, CVC had spread to infect 24% of all the citrus trees in the region. CVC has not been found on citrus in the United States.
Remarkably, olive quick decline, Pierce’s disease, bacterial leaf scorch of shade trees, citrus variegated chlorosis and many other tree and shrub diseases are caused by a single organism. Newton B. Pierce first described a wilt in vineyards in Northern California in 1892, later to be called Pierce’s disease, he could not figure out the cause of the disease. Finally, in 1987, John Wells of the USDA Agricultural Research Service and his colleagues identified a new organism in the xylem of infected plants and named it Xylella fastidiosa. (Wells et al 1987). Why the long delay? The bacterium is fastidious, meaning that it is difficult to grow in a Petri dish except on very specific media. This is probably why it took so long to identify. Since then, Xylella fastidiosa has been shown to cause various diseases in a huge number of economically important plants.
Xylella lives in the xylem of infected plants where it grows, reproduces and creates biofilms which gum up the xylem. The plant declines and dies because the xylem is plugged and unable to carry sufficient water from the roots to the leaves. The leaves die of water stress and eventually the entire plant succumbs. In pin oaks, death only comes after many decades, but in the olives of Puglia, death seems to come quite rapidly.
The number of tree and shrub species susceptible to Xylella is astonishing. Although caused by the same bacterium, the disease goes by different names because it was often discovered independently. Phony peach disease, coffee leaf scorch, oleander leaf scorch, plum leaf scald and other diseases all are caused by Xylella. This does not mean that Xylella causes a single disease. Recent genetic analysis of Xylella shows that, while there is only one species, X. fastidiosa, there are many genetic strains specific to certain hosts (Retchless et al 2014). Xylella can also be recovered from plants that have no symptoms.
How does this bacteria, lurking in the xylem of trees, get around? Xylella hitches rides from plant to plant in the mouthparts of sharpshooters, tiny leafhoppers that suck the xylem of leaves and small stems. In the process of feeding on a plant, the sharpshooters pick up Xylella and carry it to other plants.
In grapes, the leafhopper causing the most trouble is the glassy-winged sharpshooter (GWSS), Homalodisca vitripennis Germar. Native to the southeastern US, GWSS found its way to California, probably on ornamental plants, sometime before 1989. It was the GWSS that allowed Xylella to broaden its range throughout the vineyards of California. The range of GWSS and Xylella are limited by cold, but with climate change upon us, we can expect the range to expand northward.
There is no known resistance to Xylella in susceptible plants, though grape breeders expect to have resistant varieties in 10 years. Grape breeding takes a long time. Breeding olives for resistance is even more challenging because of the long time it takes for olive trees to reproduce. For now, control of the spread of Xylella depends on the use of strategies to control leafhoppers, such as the use of parasitic wasps, and on quarantines, such as the French quarantine on plants from Italy.
The spread of GWSS as well as Xylella and the diseases it causes are the result of international trade in horticultural plants as well as lax quarantine and inspection rules. In Italy, Xylella was probably introduced on imported ornamental plants, just as GWSS was probably brought to California in ornamentals. Novel pests and pathogens often find susceptible host plants with little or no resistance, and can ravage crops and wild plants. Experience with introduced pests and pathogens shows that quarantine – trying to control the problem after it has begun – is rarely effective. As long as there is international trade in horticultural crops and inadequate barriers to the movements of pests, new organisms will continue to appear on plants of economic importance.