A Pox on That Virus!
Attempts To Eradicate
Plum Pox Virus
Plum pox virus is spread by aphids.
Here, Fred Gildow (left), of
Pennsylvania State University,
and ARS plant pathologist Vernon
Damsteegt set up an experiment
to screen aphids for their ability
to transmit the destructive virus.
|Plum pox potyvirus (PPV) is a
devastating disease of wild and cultivated Prunus species, which include
plums, peaches, apricots, nectarines, and many other stone fruits. The disease
manifests as yellow rings and blotches on leaves or fruit. Although not a
danger to consumers, PPV can ruin the fruit's marketability by increasing
acidity and causing deformities.
In Europe, 100 million trees are infected, and PPV has made a small but
worrisome foray into the United States, limited so far to several orchards in
and around Adams County, Pennsylvania.
But plant scientists haven't been completely caught off guard. Since 1991, four
USDA scientistsVernon Damsteegt, Laurene Levy, Ann Callahan, and Ralph
Scorzahave conducted basic research in anticipation of just such an
invasion, including development of a transgenic plum tree that resists the
virus (see story on page 11).
An alate (winged) green peach
aphid (Myzus persicae), an
important vector of plum pox virus.
|When and how PPV entered the United
States remains speculative. But shortly after its presence was announced in
October 1999, the team's work took on new urgency, says Damsteegt, a plant
pathologist with ARS' Foreign
Disease-Weed Science Laboratory at Fort Detrick, Maryland, about a 45-minute
drive from the infected Pennsylvania orchards.
Cooperative ties were soon forged with research teams at Pennsylvania State
University (PSU), the Pennsylvania Department of Agriculture (PDA), and groups
Levy, one of the first to confirm the disease, is a plant virologist with
USDA's chief regulatory arm, the Animal and Plant Health Inspection Service
(APHIS), in Beltsville, Maryland. Scorza, a horticulturist, and Callahan, a
geneticist, are both assigned to another ARS lab.
When PPV emerged in Pennsylvania, APHIS quarantined all stone fruit trees,
seed, budwood, and nursery stock in a 4-square-mile Adams County area to keep
PPV from escaping via plant material bound for other U.S. stone fruit regions.
Over 1,000 acres of stone fruit trees have been destroyed so far, with grower
indemnities totaling $12 million, reports Stephen Poe, an APHIS domestic and
emergency operations officer.
The yellow spots on this
leaf are evidence of infection
by the Pennsylvania isolate
of plum pox virus. The leaf
is from Chenopodium foetidum,
an important indicator species
of plum pox disease.
| Plotting Against Plum Pox
The PPV research focuses on six fronts: epidemiology, herbaceous weed host
identification, wild and ornamental Prunus species susceptibility,
insect transmission, molecular detection and characterization, and disease
resistance in cultivated Prunus crops.
Eradicating PPV from the Adams County area is the ultimate goal, "one that
we should be able to achieve in the next few years," says Poe. Even then,
scientists aren't taking any chances. They're building a body of knowledge that
U.S. stone fruit growers can draw on to cope with the disease if it recurs.
"If we could eradicate the pathogen, it would be a historic
achievement," says Frederick Gildow, a professor at PSU's Plant Pathology
Department who assists with the collection, rearing, and study of insects known
to spread PPV. But "it's likely that there could be future invasions of
this virus. By doing the research now, we'll all be better prepared for the
next time around."
Culprit and Accomplice
Their target is strain Done of four PPV serogroups (D, M, C, and EA) that
plague stone fruits and other Prunus species in Europe, the
Mediterranean, Balkan States, Middle East, India, Chile, Canada, and elsewhere.
Strain D is less aggressive than the others and is the only PPV strain reported
in North America, notes Levy. She and Vessyla Mavrodieva, a postdoctoral
scientist from Bulgaria, have sequenced the genomes of two strain D specimens
isolated from an Adams County peach and plum tree. Andrew Stone, a biologist at
ARS' Fort Detrick lab, is sequencing a third isolate from the same area for
PPV is transmitted by aphidssmall, teardrop-shaped insects with a sharp
mouthpart called a stylet for sucking out plant sap. About 20 aphid species
worldwide are known to transmit PPV to Prunus plants. Since spring 2000,
Pennsylvania entomologists Jim Stimmel and Greg Krawczyk have conducted
extensive surveys of orchards in Adams and neighboring counties, identifying
the aphid species that live there. Gildow and Damsteegt have begun studies to
examine these insects' ability to transmit Pennsylvania strain D. One Adams
County accomplice they've fingered is the green peach aphid, Myzus
Inside the ARS quarantine facility at Fort Detrick, where live strain D
specimens can be safely studied, hundreds of hungry aphids are mixed with
PPV-infected leaves. After 10 to 30 minutes the aphids are placed in cages over
healthy test seedlings for 24 hours and then killed. Scientists monitor the
seedlings for 30 days for evidence of PPV infection. The researchers also use
immunological assays to confirm infection.
A Deadly Duo
The plum pox virus itself is little more than a filament of RNA surrounded by a
protein coat. Once transmitted, it uses the host plant's cells to replicate and
Aphids transmit PPV while test-probing the plant's epidermal layer, not while
mining the phloem for sap. Though transmission takes just a few seconds, the
degree to which plants become infected varies by season, host crop, variety,
age, aphid species, and other factors.
PSU researcher John Halbrendt coordinates surveys of weed species that may
harbor PPV and are fed on by aphids. Says Damsteegt, "We follow up on
those studies by looking at 'clean' plants of those reservoir species and
inoculating them with the virus to find out whether it will make them
Likewise, Scorza and research technicians Kevin Webb, Mark Demuth, and Phaedra
O'Connell are inoculating native and ornamental Prunus species with
strain D to evaluate their susceptibility. "This information is important
to monitoring any spread of the virus to woodland and garden trees," says
By compiling and analyzing such information, the team hopes to understand
strain D's cycle of disease, starting with the aphids' overwintering habitat,
seasonal migration, and host plant feeding.
"We can show in the lab that a plant can be a host, but in the field it
has to be something that's frequented by aphids and plays a part in their life
cycle," says Damsteegt. "The crux of the matter will be determining
what aphids there are, which of these actually transmit the disease, and where
they go from there."
This understanding will help officials and commercial growers take steps to
deprive PPV of its avenues of attack and escape from Adams County. But spraying
orchards with insecticide may not be the solution, since chemicals don't kill
the aphids on contact, which gives them time to transmit PPV.
"The key is to stop them before they take flight," Damsteegt notes,
"or try to disrupt their life cycle." Around orchards, for example,
this could mean depriving aphids of overwintering sites or hiding places by
killing host weeds.
PPV eradication efforts hinge on continual surveying of aphid and weed
populations. To expedite this, Damsteegt and Stone are working to combine two
common diagnostic tools: serological kits and genetic fingerprinting. The kits
use antibodies to bind to and help detect a pathogen by the specific antigens
it produces, such as a protein. The fingerprinting technology is based on
revealing amplified sequences of a pathogen's unique nucleic acid coding. The
idea is to combine the serological kit's ease of use with genetic
fingerprinting's sensitivity and reproducibility to furnish real-time results
during field surveys.
Our goal is to develop faster, more sensitive detection methods that can be
used with large numbers of samples, says Damsteegt. In the war on plum pox,
such capabilities will prove ever more important in rooting out its hiding
Suszkiw, Agricultural Research Service Information Staff.
This research is part of Plant Diseases, an ARS National Program (#303)
described on the World Wide Web at http://www.nps.ars.usda.gov.
To reach scientists mentioned in this article, contact
Jan Suszkiw or
Judy McBride, USDA-ARS
Information Staff, 5601 Sunnyside
Ave., Beltsville, MD 20705-5129; phone (301) 504-1630 [Suszkiw], (301) 504-1658
[McBride], fax (301) 504-1641.
"A Pox on That Virus!" was published in
2001 issue of Agricultural Research magazine.