After discussing the overall situation with various subspecies of Xylella fastidiosa and the impact of Xylella on grapevine, this blog focuses on other food/fruit plants from the graphics bellow – this time on 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 and through grafting. CVC causes trees to have reduced vigor and growth. Symptoms include yellowing and mottling patterns on the leaves indicative of nutrient deficiency (mainly zinc) 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 first 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 resided in replacing sour orange by alternative rootstocks giving tolerant combinations with scions such as sweet orange.
However, another disease struck: In 1987, São Paulo reported 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 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 whereas grapefruit, mandarins, mandarin hybrids, and limes show less severe symptoms. CVC tolerant varieties are Rangpur lime, lemon, citron, and pummelo.
Tree death is not common from CVC, but production of fruits is greatly reduced – yield of severely affected plants may be reduced by 70-90 %. The size of fruits on infected trees is diminished up to 65%, with hard thin rinds, coloring early but not reaching ripeness. Once a plant is infected, 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 favorable 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. Especially the U.S. is quite cautious: In Southern U.S., the harvested citrus acreage has averaged about one million acres in the pas years with citrus production estimated at above 3 billion UDS p.a.. One of the main reasons cited 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, it is recommended to remove all scion on the infected plant and to graft the newly sprouted shoots of ‘Rangpur’ lime (Citrus limonia Osbeck) or ‘Cleopatra’ (Citrus reshni Hort. ex Tan.) rootstocks with healthy buds allowing production of fast-growing and productive new scions free of CVC.