New Method Tests Severity of Key Citrus Virus
December 24, 2009
A new rapid way to test severity of
the devastating citrus tristeza virus (CTV) in citrus trees has been developed
by Agricultural Research Service (ARS)
scientists in Parlier, Calif. The results of this study were presented at the
Second International Citrus Biotechnology Symposium in Catania, Italy, earlier
CTV is the cause of the most economically damaging virus disease in citrus.
CTV has led to the death of millions of citrus trees on sour orange rootstock
worldwide, and virulent stem-pitting strains can drastically reduce fruit size,
quality and production of millions of other citrus trees on tolerant or
resistant rootstocks. Sour orange is generally considered the best overall
rootstock (or tree stump) for citrus and is still widely used in Texas, Mexico,
Central America, and Mediterranean countries.
In central California, typical CTV strains are mild, remaining symptomless
on tolerant or resistant rootstock. This has led to the spread of symptomless
CTV in recent years. Over time, if undetected, a virulent strain may be
introduced or become predominant in mixed infections with mild strains. There
are no high throughput diagnostic methods that distinguish between strains,
which presents a real problem in detecting and eradicating the virulent
At the ARS
Joaquin Valley Agricultural Sciences Center in Parlier, plant pathologist
Yokomi has developed a system to screen local CTV isolates for severity,
using a technique called real-time polymerase chain reaction (PCR). Tree
samples were tested in Parlier for CTV infection by serology, using the
antibody MCA 13, which reacts to nearly all severe CTV strains, but also to
some mild strains.
CTV severity testing was placed into three categories in trees grown on
resistant rootstock: mild (no physical symptoms on tolerant or resistant
rootstock), moderate (decline of tree growth on sour orange rootstock), and
severe (stem-pitting and seedling yellows). CTV isolates associated with the
so-called T30 genotypes of the virus cause mild to moderate symptoms, while the
T3 and VT genotypes cause severe symptoms.
This reliable and sensitive method to identify isolates that have potential
economic impact in commercial citrus groves supports the
U.S. Department of Agriculture priority of
promoting international food security.
ARS is USDAs principal intramural scientific research agency.