"Citrus leprosis causes yield reduction and eventual
death of the trees if its mite vectors are not controlled," says
Ochoa. "A mite carries the virus from plant to plant, but the mite
is not affected. The virus resides in its gut."
SEL is getting assistance in this effort from the Electron
Microscopy Unit of the ARS Soybean Genomics and Improvement Laboratory,
which is also based in Beltsville.
"The only known vectors of leprosis are species in
the flat mite genus Brevipalpus," says entomologist Ethan
Kane, who is working with Ochoa. "The history and cost of this
disease in South America are alarming, given the fact that other mite
species believed to be capable of spreading it are already abundant
in California, Florida, and Texasthree states that are the backbone
of the U.S. citrus industry."
Ochoa and Kane are collaborating with Florida Department
of Agriculture and Consumer Services acarologist W. Calvin Welbourn
to clarify differences among the Brevipalpus species implicated
as leprosis vectors. Ochoa says that while only one species of BrevipalpusB.
phoenicishas been experimentally confirmed to transmit citrus
leprosis virus, two closely related speciesB. californicus
and B. obovatusare suspected transmitters.
Fast Identification Is Vital
"These three mite species are extremely similar in
appearance and have historically been confused and misidentified,"
says Ochoa. "In an attempt to precisely define their identities,
we've examined specimens from outbreak sites and compared them with
reference material held in the U.S. National Mite Collection at SEL."
Adds Ochoa, "Fast identification of both the mite and symptoms
of the disease is vital. This is an adaptable mite that can quickly
increase to millions on a tree."
He says B. phoenicis is unique in that more than
1,000 plant species can be a host to it. "This is phenomenal,"
he says. "Most mites have a limited range of hosts."
SEL's role in the fight against citrus leprosis is within
its mission to develop comprehensive classification systems for insects
and mites on a world basis and to furnish taxonomic services to federal,
state, and private organizations involved in research and action programs
in agricultural, biological, and health sciences.
The laboratory's work is part of a wider project to minimize
the impact of citrus leprosis that's being funded in part by USDA's
Foreign Agricultural Service and APHIS.
This project focuses on surveying for Brevipalpus
mites and perfecting molecular and morphological diagnostic techniques
that can precisely identify the leprosis vectorsand in turn lead
to more accurate risk assessments. It is headed by renowned acarologist
Carl C. Childers, an entomology professor with the University of Florida.
Childers's team also includes molecular biologist Maria Gallo-Meagher
and plant pathologist José Carlos Rodrigues, also with the university,
and APHIS mite specialist Eric McDonald. Working with this group is
Joel Floyd of APHIS's Pest Detection and Management Program Planning
Childers says citrus leprosis was first observed in Florida
around 1860 and spread to at least 17 counties there by 1925. It was
eradicated when citrus growers planted in new locations and used sulfur
for mite control. But sulfur is not recommended for such use, because
overuse can kill crops.
Effort Extends Within and Outside ARS
Ochoa's group is using advanced light microscopy techniques
and low-temperature scanning electron micrographs provided by ARS botanist
Eric Erbe, of the Soybean Genomics and Improvement Laboratory. This
work has shed light on the subtle structural differences that separate
the three species of Brevipalpus considered capable of transmitting
"This information will be used in conjunction with
data from Childers's team to thoroughly characterize these mite species,"
According to Floyd, at this time, the only economical
way to tell whether mites are carrying the disease is to monitor for
plant damage. "The key is early detection," he says. "We
need to educate growers to spot disease symptoms. Then we have a chance
to break the virus-vector cycle."By Luis
Pons, Agricultural Research Service Information Staff.
This research is part of Plant Diseases (#303) and
Crop Protection and Quarantine (#304), two ARS National Programs described
on the World Wide Web at www.nps.ars.usda.gov.
Ochoa and Ethan C. Kane
are with the USDA-ARS Systematic
Entomology Laboratory, 10300 Baltimore Ave., Beltsville, MD 20705-2351;
phone (301) 504-7890 [Ochoa], (301) 504-7041 [Kane], fax (301) 504-6482.
Eric F. Erbe
is with the USDA-ARS Soybean
Genomics and Improvement Laboratory, 10300 Baltimore Ave., Bldg.
177B, Beltsville, MD 20705-2351; phone (301) 504-8046, fax (301) 504-8923.
"Targeting a Threat to U.S. Citrus" was published
in the March
2004 issue of Agricultural Research magazine.