Most of California's rural economy relies on agricultural production, an $18
billion industry annually. One in 10 California residents is employed in
agriculture, from farm to market.
Income from tree fruits contributes substantially to many communities. Just
onepeachesyields $100 million in annual income and accounts for
more than half the total U.S. annual productionsome 1,700 million pounds,
out of 2,600 million.
Plant diseases can raise havoc with tree fruit crops. Yield and grower
income would be significantly reduced if it werent for research by
Agricultural Research Service scientists.
ARS plant pathologist Jerry K. Uyemoto and coworkers at the University of
California-Davis, are helping the cling peach industry to ensure that new peach
trees are virus tested and used to establish new orchards. Under ideal
circumstances, these trees will continue to be high producers for 18 to 25
years.
Peach ringspot disease, caused by Prunus necrotic ringspot virus
(PNRSV), has probably been a problem ever since peach trees were first planted
in California, around the turn of the century. The virus is easily spread by
using infected seedlings as rootstock, by grafting buds from infected trees,
and even by infected pollen. PNRSV is hard to detect visually because the
leaves usually look normal, unlike some other viral diseases. Its presence is
reflected in decreased yields.
Between the mid-1950's and early 1960's, methods were developed to help
growers avoid the virusalthough it wasn't easy. Most relied on heat
therapy to eliminate the virus from propagated material.
Potted trees were heated to 103° F for 10 to 30 days. That allowed new
shoots to grow away from virus-infected portions. The new shoots were clipped
and grown, then retested. Those that proved to be virus-free were used as new
plantings.
Despite the expense of the procedure and up to 8 weeks to complete it, just
about every major peach grower in California planted these certified trees.
However, because of difficult economic times of the late 1970's and early
1980's, many growers sought cheaper, noncertified tree stock. And, because some
growers had diseased trees that seemed to be economical, the industry pretty
much ignored the problem.
Then, Uyemoto was hired by ARS in October 1986 to investigate virus diseases
of the Prunus species, which includes almond, peach, cherry, and prune.
He developed diagnostic tools such as one using ELISA (enzyme-linked
immunosorbent assay) to spot infections.
"These tools are quick and very sensitive to the numerous virus strains
that cause mild to severe damage to trees," says Uyemoto, who is in the
ARS Crops Pathology and Genetics Research Unit at Davis.
"While older identification techniques could only be done during the
summer and took at least 30 days to complete, ELISA takes only a day and can be
performed in spring, before buds are collected for grafting," he says.
In 1988, the California Department of Food and Agriculture began a screening
program using this technology. Samples from some 20,000 Prunus trees
that nurseries use as sources of budwood and seed are tested each year for
PNRSV and another virus called prune dwarf virus (PDV).
"The ELISA testing program has provided a very valuable tool to
nurseries," says Robert Woolley, chairman of the California Fruit Tree,
Nut Tree, and Grapevine Improvement Advisory Board.
"Nurseries can efficiently screen hundreds of trees early in the
growing season, before collecting bud-wood or seed, making it easier to prevent
propagation of virus-infected trees," he adds. Woolley is owner of Dave
Wilson Nursery in Hickman, California, a small community about 20 miles east of
Modesto, in the San Joaquin Valley.
But not all growers are buying certified virus-free stock.
"They should be," says Uyemoto, who conducted a survey in 1988 to
learn the extent of the disease. It showed that, depending on peach tree age,
the incidence of PNRSV ranged from about 11 to 49 percent.
"Based on an average yield of 20 tons of peaches per acre, growers lost
more than 3-½ tons per acre. That translates into a loss of $720 per acre,
assuming the crop sells for $200 a ton," says Uyemoto.
He adds that the loss is actually more than that, since growers have fixed
production costs per acre. They must spend the same on water, fertilizer,
equipment, labor, and mortgage per acreregardless of yield.
"A previous survey showed that many newly planted trees were infected
with ringspot virus. Although these viruses can be carried by pollen, the new
trees were presumably infected before planting," says Uyemoto.
Losses mount when ringspot is present along with prune dwarf. The resulting
disease complex is known as peach stunt, and yield losses can be two to three
times those from ringspot alone. There is no cure for an infected orchard tree.
While it's impossible to prevent necrotic ringspot virus from moving from a
diseased orchard to a healthy one, it's more likely to spread from infected
trees within an orchard. That's why it's so important to detect infected trees
as early as possible, so they can be removed.
"The ELISA program has reduced the incidence of viruses in recent
plantings of peaches," says Uyemoto.
"A survey in April 1992 showed that among ELISA-tested nursery stock,
only 2 peach trees tested positive to PDV and 3 to ringspotout of 210
tested. This is quite a contrast to the 25 percent we found in 1987," he
says.
"Certain varieties that we growespecially the Carson cling
peachare more susceptible than others, and we use certified stock,"
says John Starn. He cultivates 200 acres of peaches near Hughson, California,
just south of Modesto.
Starn says that the ARS research has benefited everyone; nurseries have
happier customers, and growers have healthier trees. "It increased
everyone's awareness of the problem," he adds.
Cherry Management Improved
Cherry production brings in from $20 to $60 million annually to California
growers.
Uyemoto's research is also helping the cherry industry.
Buckskin disease of sweet cherry trees cannot be eradicated but can be
controlled to keep growers in business. The disease is so named because
cherries are small and turn buckskin color rather than their normal deep
purple.
"It's a fickle disease. Some growers experience catastrophic losses;
their whole orchards die within 2 or 3 years, while other growers are
unaffected," says Uyemoto. "Now, all orchards can be protected.
"Buckskin disease, also called X-disease, is caused by a microorganism
that is carried from infected trees to new victims by mountain leafhoppers,
Colladonus montanus, and by Flor's leafhoppers, Fieberiella
florii."
Uyemoto was a member of a team that developed several steps over the past 20
years that growers can take to keep their orchards in peak production. The
steps focus on close insect monitoring and control, along with prompt removal
of infected trees.
The management program is cost effective too, according to Joseph A. Grant,
farm adviser for San Joaquin County in Stockton, California. He calculates
direct loss to his countys growers would run more than $2 million each
year. But with insect control and tree removal on about 80 percent of the
countys 7,100 acres of cherry trees, that loss is reduced by two-thirds,
for an annual benefit of almost $1.4 million. Insect control costs run about
$70 per acre.
A test called PCRpolymerase chain reactionis more
accurate and may put the finger on additional plants that serve as reservoirs
for the X-disease organism.
In collaboration with colleague Bruce Kirkpatrick at the University of
California at Davis, Uyemotos latest research focuses on developing more
sensitive tests to better identify the disease causing organism.
A test called PCRpolymerase chain reactionis more accurate and
may put the finger on additional plants that serve as reservoirs for the
X-disease organism.
In developing cherry tree recommendations, ARS also worked closely with the
California Cherry Advisory Board. ARS research on peaches was supported by the
California Cling Peach Advisory Board. -- By Dennis Senft, ARS.
Jerry K.
Uyemoto is in the USDA-ARS Crops Pathology and Genetics Research Unit,
University of California, Davis, CA 95616; phone (530) 752-0309.