California Industry on High Alert
The Tomato Magazine
By Lisa Lieberman
T he discovery of the deadly tomato yellow leaf curl virus (TYLCV) earlier
this spring in California has the tomato industry on high alert.
TYLCV is recognized as the worst, most pathogenic tomato virus there
is, says Robert Gilbertson, a plant pathologist at the University
of California, Davis. Diseased plants were found in a Brawley
greenhouse, near the California/Mexican border. Experts believe that the
virus could have been brought in via tomato transplants from Mexico or
Texas, or that it could have been carried in by whitefl ies. Prior to
the 1990s, the TYLCV was only known to occur in the Middle East and Africa,
Gilberston says. Later, the virus was introduced into the Dominican Republic
and the Caribbean where it then spread to Florida, Georgia and Louisiana.
In 2005, it was discovered in Northern Mexico for the first time.
Wherever the virus has spread, its caused signifi cant losses
to tomato production, Gilbertson points out. The virus causes tomato
plants to become stunted and grow abnormally upright. The
leaves turn bright yellow and the fl owers fall off the plants, usually
before fruit sets. The virus is not transmitted in the seeds or by touch;
its only transmitted through the white fly, Bemisia tabaci, in particular.
The whitefly is a good vector, Gilbertson notes. Thats
how it spread so efficiently from the Caribbean to the southern United
States. As soon as the virus was discovered in Brawley, California
Department of Agriculture offi cials quarantined the greenhouse where
it was found and destroyed all infected plants.
We surveyed the area around the infected greenhouse and found nine
residential locations that had diseased plants. All were destroyed. We
havent seen any signs of the disease spreading beyond that area,
so we think thats an encouraging sign, reports Steve Lyle,
spokesman for the CDFA. Bemisia tabaci is the most common type of whitefly
in Imperial County, but doesnt do nearly as well in northern California
where temperatures are much cooler, Gilbertson explains. The vector is
limited to the southern areas of the state, although it can push itself
as far north as Kern County and into southern Fresno County.
Greenhouse growers, however, need to be on the lookout for the virus,
especially if they have received transplants from Imperial County, Gilbertson
warns. Whiteflies carrying the virus can be extremely dangerous. Just
a few infected fl ies can infect 100 percent of the plants in a fi eld
within a short period of time.
One of the big differences between TYLC and other diseases is it spreads
much faster. When whitefl ies feed off an infected plant, they pick up
the virus in just a few minutes and spread it to the next healthy plant.
One white fly can infect up to 100 plants.
This is such a serious disease that were monitoring the fi
elds with the same level of seriousness as if we were monitoring for the
Med fly, Gilbertson explains. We dont want this virus
to get established in California. The fact that we identified it so quickly
was critical, and were cautiously optimistic that weve limited
exposure. But the virus has shown it has a sneaky way of getting established
wherever it is introduced.
When the TYLCV was first introduced into the Dominican Republic in the
early 1990s, it temporarily wiped out the entire tomato industry.
They had hundreds of acres of processing tomatoes--a $10 million
industry, Gilbertson exclaims. The Dominican Republic government
was able to help put the industry back on track by imposing mandatory
tomato-free periods during the summers.
During June, July and August, growers arent allowed to do
any plowing or plant any tomatoes, he adds. The insect only
lives for one month, and the virus isnt transmitted through the
eggs, so having this three-month free period, basically cleans the virus
out of the whole system.
The problem, though, is that the virus can also infect weeds in the fi
elds, which can serve as reservoir hosts and dont always show symptoms.
By the end of the growing season, the virus comes back. But, the
delay is enough so that early maturing hybrids and resistant hybrids can
achieve good yields, Gilbertson says.
In the Dominican Republic, June, July and August arent critical
growing months for the local canneries, so the three-month tomato free
periods have worked out relatively well, the researcher notes. In
Florida, Georgia and Louisiana, where TYLC has also established a foothold,
threemonth, tomato-free periods also are being considered.
These states havent enforced the free periods because it would
take a significant effort to get everyone on board. Right now, theyre
relying on chemical controls for the whiteflies. Theyre also working
on developing varieties resistant to the virus, Gilbertson reports.
Admire, a neonicotinoid, is the commonly used chemical for controlling
whiteflies, the researcher says. So far, its been a very good material.
Its not highly toxic to animals, and while the white flies have
shown some signs of becoming resistant to it, they dont seem to
be increasing their resistance very quickly. In Florida, offi cials are
also encouraging growers to destroy the vegetation in their fields once
theyre done with the growing season. This helps with sanitation.
Some growers in the southern area of the state are also beginning to minimize
production during certain times of the growing season to
help reduce the spread of the virus.
In California tomato growing regions such as Fresno, theres a natural
tomato-free period from November to early February where there are no
tomatoes in the fields, Gilbertson says. With diseases like spotted
wilt virus, you have a wide host range for the disease, but with TYLCV,
the virus doesnt generally do well in too many other plants. So,
while its true that it can survive at low levels, the bottom line
is that with the right measures, its still possible to produce healthy
tomato crops with TYLCV, Gilbertson promises.
© 2007 Columbia
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