After discussing the overall situation with various subspecies of Xylella fastidiosa and the impact of Xylella on grapevines, the third part in this blog series focuses on other food/fruit plants from the graphics bellow – in particular, citrus species.
Citrus species
Citrus variegated chlorosis (CVC) is a bacterial disease caused by a subspecies pauca of the Xylella fastidiosa bacterium, which lives in the xylem of the plant and limits the function of its vascular system. This bacterium is transmitted plant-to-plant by several species of large leafhopper insects called sharpshooters. It can also be transmitted mechanically through the process of grafting. CVC causes trees to have reduced vigour and growth. Symptoms include yellowing and mottling patterns on the leaves, typically consistent with nutrient deficiency (mainly zinc), and with corresponding brown, gummy lesions on the underside of leaves.
Spread, recurrence and persistence of CVC is a major economic concern above all in warm regions of South America.
The Brazilian state of São Paulo is the number one sweet orange growing region in the world. The citrus industry there has been, and still is, under constant attack from various diseases. In the 1940s, tristeza-quick decline (TQD) was responsible for the death of 9 million trees in the state of state São Paulo. The virus was efficiently spread by insect vectors, and killed most of the trees grafted on sour orange rootstock. Control of the disease was achieved by replacing sour orange by alternative rootstocks giving tolerant combinations with scions such as sweet orange.
However, then another disease struck: in 1987, São Paulo reported the first occurrence of citrus variegated chlorosis, which affected approximately 100 million sweet orange trees (variety pera) in the region during the next 20 years. Fortunately, even if the disease has not been eradicated yet, today the São Paulo State’s CVC epidemics is relatively under control.
In the north-eastern Bahia State, the second most important citrus-growing region in Brazil, CVC has been present since 1997.
CVC is currently found also in Argentina, Costa Rica, and Paraguay. Sweet orange cultivars – such as those planted predominantly in Brazil – are highly susceptible to CVC. Additionally, grapefruit, mandarins, mandarin hybrids, and limes can be affected, albeit with less severe symptoms. CVC tolerant varieties include: rangpur lime, lemon, citron, and pomelo.
CVC does not typically cause tree death, but production of fruits is greatly reduced. The yield of severely affected plants may be reduced by 70-90 %. The size of fruits on infected trees is also diminished by up to 65%, with fruits producing hard, thin rinds. The fruit also colour early but do not reach ripeness.
Once a plant is infected, the typical treatment is to remove infected branches or whole trees to try to prevent the spread.
There are no known occurrences of CVC outside of South America; though, should the pathogen get transmitted to other regions with favourable climatic and management practices (practice of orange propagation by grafting), it could spread rapidly and aggressively into and through the citrus orchards in that region. The U.S. is especially cautious: in the Southern U.S., the harvested citrus acreage has averaged about one million acres in recent years. The value of this citrus production is estimated at over USD$3 billion per annum.
One of the main reasons cited for why CVC has not been established in the U.S. is that both Florida and California both have budwood (grafting) certification programs, which limit the legal introduction and dispersal of citrus propagative materials.
In the EU, Xyllela fastidiosa is on the European Plant Protection Organization’s A1 list of regulated quarantine agents.
As said, there is currently no cure known for CVC. To curtail the problem, the removal of all scion on the infected plant and the grafting of newly sprouted shoots of ‘Rangpur’ lime (Citrus limonia Osbeck) or ‘Cleopatra’ (Citrus reshni Hort. ex Tan.) rootstocks with healthy buds is recommended. This allows the production of fast-growing and productive new scions free of CVC.
