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. Click the image for more information about it.
HoneySweet Plum Trees:
A Transgenic Answer to the Plum Pox Problem
Frequently Asked Questions
What is plum pox?
Plum pox is a plant disease that infects stone fruit trees including peach,
nectarine, plum, apricot and cherries. The disease, which is also called Sharka
(the Slavic name for plum pox) disease, is caused by the plum pox virus (PPV).
It is considered the most serious virus disease of stone fruit trees. Plum pox
is spread from tree to tree by aphids and through infected budwood used for
grafting, which is the normal method of propagating stone fruit trees. Symptoms
of plum pox infection include leaf and fruit yellowing, fruit deformation, and
premature fruit drop. A tree suffering from plum pox can go into serious
decline, especially if the tree also becomes infected with other viruses.
What does plum pox mean to me as a consumer?
Presently, plum pox is not an issue for consumers. In the U.S., PPV has only
been found in a few places in Pennsylvania and is being eradicated by
destroying the infected and surrounding trees. Pennsylvania is not a major
plum-producing area, so the consumer market for fresh and dried plums in the
United States has not really been affected at this time.
The concern is that if plum pox becomes widespread in the U.S., the disease
could cause a major disruption in the availability of plums, prunes and other
stone fruits if no solution to the plum pox problem is available. So to ensure
that the U.S. consumer continues to have access to the widest variety of fruit,
the Agricultural Research Service began a research program to develop plum
trees with resistance to PPV. More information about plum pox outbreaks in the U.S.
What is being done to eradicate the PPV infection in the U.S.?
In the U.S., eradication of PPV is done by eliminating infected trees plus all
potentially susceptible trees within a buffer zone around infected areas. Each
buffer zone extends from 500 meters in a circle beyond the perimeter of an
infected area. In a larger zone that extends from 500 to 1,000 meters beyond
the perimeters of the first zone is area, 100 percent of the stone fruit trees
also are checked for the presence of the 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. Click the image for more information about
What is the U.S. Department of Agriculture (USDA) doing about plum pox?
The Animal and Plant Health Inspection Service (APHIS), an agency of USDA, is responsible
for preventing the introduction of plant pathogens into the U.S. All fruit
nursery stock for importation is tested for a range of known fruit tree
pathogens, especially those that are not known to occur in the United States
(exotic pathogens). APHIS plum pox program
What is the Agricultural Research Service (ARS) doing about plum pox?
ARS has an active research program to
develop a better understanding of the virus and how mechanisms of resistance
may work in trees. The primary goal of the research program is to produce
Because developing a PPV-resistant plum tree is not a simple or quick
project, ARS has not waited until plum pox has a major presence in the U.S. to
begin research. Rather, ARS has taken the proactive step of developing a
PPV-resistant tree and doing the testing required to allow genetically
engineered trees to become available, before plum pox precipitates a crisis in
this country. (Link: Researchers
involved in developing and testing HoneySweet)
How was the PPV-resistant plum developed?
ARS researchers have developed a PPV-resistant tree through genetic
engineering. The gene for the PPV coat protein was separated from the PPV and
inserted into a piece of carrier DNA. This new piece of DNA was inserted into a
bacteriumAgrobacterium tumefaciensthat was used to infect
cells extracted from plum seeds. Seed cells that were found to have
incorporated the new gene into the plum DNA were then regenerated and grown
into complete plum trees. These trees have the new gene in their DNA and are
resistant to PPV.
The new PPV-resistant variety is named HoneySweet.
Symptoms of plum pox virus on apricot fruit and
leaves. Click the image for more information about it.
Do you know exactly what new genetic material is in HoneySweet?
Yes, we know all of the genes that have been added to HoneySweet.
Are the added genes in the fruit? Will I be eating foreign genes?
The new DNA is in the fruit. But genes are broken by digestive enzymes in the
stomach. We have analyzed the fruit from HoneySweet and compared it with fruit
from other plum trees, and there is no significant difference in composition in
terms of nutrients usually measured in plums (sugars, acids, vitamins, fiber,
Can the virus DNA harm people?
Plant viruses do not infect animals or people. Thousands of infected trees grow
in Europe and the fruit from these trees is eaten by people. There has been no
instance of harm from eating the fruit. Also, a portion of all produce, whether
from commercial growers or the home garden, is infected by various viruses.
Such fruit contains coat protein and coat protein genes from these viruses, and
people have always eaten them. In that sense, HoneySweet is no different than
many other fruits that contain coat protein genes.
Is HoneySweet safe to eat? How do I know that?
HoneySweet will only be released for sale or consumption when its safety is
determined by all three agencies that regulate genetic engineering of crops in
the United StatesAPHIS, the U.S. Food and Drug Adminstration (FDA) and the U.S. Environmental Protection
Agency (EPA). (More information about the genes added to
How did you show that the HoneySweet trees are resistant to plum
Trees were tested in a greenhouse for five years. Plant tissue infected with
PPV was grafted onto the new trees, but none of the trees developed plum pox
disease. Trees were also grown in Spain, Poland and Romania, where PPV is
indigenous, and none of the engineered trees have ever been infected by
transmission through aphids, which naturally carry the PPV virus from tree to
These transgenic plums contain a gene that makes
them highly resistant to plum pox virus. Click the image for more
information about it.
Have HoneySweet trees been grown in orchards in the United States?
Only one experimental planting has been grown in the U.S. Permits for this
planting were obtained from APHIS, which put very strict restrictions on how
many trees could be grown, where they could be grown, how far the trees had to
be from other susceptible trees, what had to done when the trees were in
blossom, and what had to done with cuttings from the treeto ensure that
test trees were kept under very tight control.
After seven years under strict control, APHIS allowed the trees to blossom
and be pollinated so fruit could be produced. This permission was granted with
the proviso that there were no other stone fruit or other Prunus species
in the area that could be pollinated by HoneySweet.
In addition to the ARS research location where HoneySweet was developed, the
trees have been grown in Spain, Poland and Romania under restrictions specific
to each country.
Could HoneySweet pollinate other trees?
HoneySweet will cross with other domestic plum trees.
To find out how far HoneySweet plum pollen will travel to other plum trees,
almost 3,000 seeds were evaluated over six years at distances from 230 to more
than 3,500 feet from the HoneySweet planting. Over the six years of testing,
only two seeds were found to have received HoneySweet pollen. These were at a
distance of about 1,700 feet from the HoneySweet trees. (More information about pollination and
ARS horticulturist Ralph Scorza. Click the
image for more information about it.
Have APHIS, EPA and FDA approved the release of HoneySweet?
APHIS has "deregulated" HoneySweet, which means APHIS has made a
determination that the organism is not a plant pest and found that it will have
no significant impact on other plants. Deregulated products have an established
history of safe use in U.S. agriculture, and APHIS has the authority to bring
any deregulated item back under regulation if new information becomes available
that demonstrates unanticipated effects or risks to plant health. APHIS has
been safely regulating GE organisms since 1986 and has overseen the
deregulation of over 70 GE plants, including corn, cotton, rapeseed (canola),
soybean, flax, sugar beet, squash, and papaya. Soybeans, corn, cotton, and
canola that are herbicide tolerant or insect resistant are the most frequently
planted deregulated crops.
With deregulated status, HoneySweet and its progeny can be freely moved and
planted without the need for permits or other regulatory oversight by APHIS.
HoneySweet is the second tree to be deregulated by APHIS. In September 1996, a
genetically engineered papaya became the first tree to be deregulated.
FDA has reviewed HoneySweet and had no questions about the fruit's safety
An application for registration has been filed with EPA for HoneySweet.
Am I likely to see HoneySweet plums in my grocery store soon?
People are not likely to see these plums and prunes in stores anytime in the
near future. Plum pox is still being contained and eradicated at this time. But
as with most foreign diseases, there is the potential that plum pox will become
established in the U.S., and all of our current plum tree varieties are
susceptible. Since it takes years to develop and multiply new varieties, we
can't wait until the problem becomes endemic to do the research to have
resistant trees. USDA needs to be ready before the problem becomes a crisis.
(More information about
how HoneySweet is expected to be used)
Is ARS consulting with plum growers and the nursery industry about the
planned release of a genetically engineered (GE) plum?
Research horticulturalist Ralph Scorza, who has been heading ARS's research
program, spoke about his research on this plum and the planned deregulation at
the International Fruit Tree Fruit Association (IFTA) meeting in February 2006 in Hershey,
Pa.. He will be consulting with grower and industry groups in the near future
as HoneySweet nears release. More information for the plum orchard industry
Where can I get more information about HoneySweet
development and ARS plum pox research?
Contact Kim Kaplan, ARS Information Staff, 301, 504-1637,
Links to information about plum pox from
Links to information about agricultural genetic engineering