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Scientists seek limits on pear pests: success could spell expansion of U.S. commercial production areas.

Homer called pears one of the "gifts of the gods," and more than 2,000 years ago, the Greek naturalist Theophrastus wrote about the differences between wild and cultivated pears.

Even in ancient times, pruning roots and girdling stems were said to hasten fruiting. But then, driving iron pegs into the trunk to "punish" the tree was also said to make the tree bear fruit earlier.

Pears are known to have been under cultivation in New England as early as 1629. The first record of a nursery selling pear trees is from Massachusetts in 1641.

"Until about the middle of the 18th century, Massachusetts was the center of the American pear industry," says Howard J. Brooks, ARS associate deputy administrator for plant sciences. "And up until the 19th century, most of the new pear varieties and nearly all of the introductions from abroad came by way of Massachusetts."

Those introductions, he says, are probably the offspring of pear seeds imported by French, Dutch, and English settlers in America.

Legend has it that the pear trees that were early landmarks along the Detroit River in Michigan were descendants of three pear seeds brought to America in the vest pocket of a French emigrant.

The establishment in 1730 of a nursery in Flushing, Long Island, New York, gave the fledgling East Coast pear industry a boost. That nursery was unique: It sold budded or grafted pear trees. Until then, pears--as well as most other fruits--were grown from seeds.

"Coincidental with this event came another of paramount importance to pear growers," Brooks says. "Fire blight appeared on the scene and became epidemic in orchards along the Hudson River in the 1790's."

For the East Coast pear industry, this was the beginning of the end, for fire blight eventually wiped out most of the eastern U.S. pear orchards.

And the demise was further aided by pear psylla, a small, aphid-sized insect first discovered in the United States in Connecticut in 1832.

Now, ARS plant pathologists, horticulturists, and entomologists at the Appalachian Fruit Research Station in Kearneysville, West Virginia, are helping to revive this long-lost East Coast fruit industry. Their research could also help ensure the continuing health of the existing industry on the West Coast.

These scientists have new ways to fight the deadly blight, caused by the bacterium Erwinia amylovora. And they also have innovative ways to combat the pernicious insect.

In addition to breeding new pear varieties that are resistant to fire blight and using a computer program to predict blight from weather conditions, they are investigating environmentally safe, biological methods to control the pear psylla. And breeding a tree that isn't bothered by the insect offers yet another possible solution.

Fire Blight--Can We Prevent It?

"Fire blight is indigenous to North America," explains Tom van der Zwet, a plant pathologist at Kearneysville. "It was well over a century after that first discovery, in the Hudson Valley, that this disease was reported in a foreign country."

Now widespread throughout Western Europe and the eastern Mediterranean region, its effects can be devastating. According to van der Zwet, 95 percent of Egypt's Le Conte pear crop was lost to the disease in 1985.

Fire blight probably first occurred on such American natives as crabapple, hawthorn, and mountain ash, he says. From these hosts, the bacterium could have spread to susceptible cultivated pears and apples planted by early American settlers.

Considered the oldest, most serious, and least understood disease of pomaceous fruit trees, fire blight attacks all parts of a tree--from roots to fruit. Affected areas look scorched and blackened, as though they have been burned. Fire blight is most destructive to pears but also damages apples, quince, and some ornamental plants. Once blight strikes, a tree can die rapidly or within a few years, for treatments such as antibiotic sprays are not wholly effective.

Since the incidence and severity of fire blight are typically very sporadic, accurate estimates of annual losses from fire blight for given localities or for the nation as a whole are difficult to obtain. However, an outbreak of the disease in southwest Michigan in 1991 caused an estimated combined loss of fruit and trees of $3.8 million.

"This disease has its place in history--it was the first plant disease ever attributed to a bacterium," van der Zwet says. "Fire blight was the beginning of plant bacteriology."

In the early 1900's, USDA began the first pear-breeding program under the direction of M.B. Waite and E.F. Smith. In the 1930's, John Magness, chief of the horticulture section of the Plant Industry Station at Beltsville, Maryland, started gathering pear germplasm from around the world.

Magness hybridized the better quality pears that showed resistance to the destructive blight. He made large pear plantings across the river from Washington, D.C., at Arlington Farms, now Arlington National Cemetery. In 1938, the program was transferred to the Agricultural Research Center in Beltsville, where it remained until 1979.

In 1960, USDA hired Brooks to continue the search for a fire-blight-resistant pear. "I spent 7 weeks in southern Russia in 1967 gathering pear germplasm," Brooks recalls, "because the literature reported a vast array of pear genetic material there."

He shared the genetic material he brought back from Russia with U.S. and Canadian scientists also involved with the research.

And to screen and evaluate the new pear seedlings produced from the germplasm, Brooks hired van der Zwet.

Grappling With Fire Blight

Brooks says that fire blight had not been a significant problem in California, Oregon, or Washington until recently. But during the last several years, it has become nearly epidemic in certain West Coast production areas. The new pear varieties that ARS will introduce from the breeding program now located at Kearneysville will also be tested in Western states. If adapted, these varieties could prove better than existing ones, he contends.

"We now ship pears for fresh market from the West to the East Coast," says Brooks. "If the new varieties grow well in the East, they could become an alternative crop there. And consumers would pay less for good-quality, freshmarket pears, since shipping would be eliminated."

In the 1960's, USDA released two fire-blight-resistant pears--Magness and Moonglow. But only Magness has been grown commercially. Although an extremely high quality pear, Magness is not very productive. Moonglow is still used for backyard production, but its fruit quality doesn't meet commercial standards.

"A new release, Potomac, may be a different story," says Richard L. Bell, Kearneysville horticulturist and pear breeder. "Besides having superior resistance to fire blight, this new pear has excellent flavor and texture. Although we plan to release this variety for home orchards, commercial growers who plan to store the fruit for less than 2 months may also be interested."

Bell says that back in 1961, Brooks made the genetic cross from which Potomac was selected. Potomac has been evaluated in Maryland, West Virginia, Arkansas, New York, Ohio, and Oregon for fruit quality, fire blight resistance, and productivity. Release was approved in May of this year, which means it should be available to growers in about 2 years.

Potomac has light-green, glossy skin and moderately fine and buttery fruit texture with very little grit. The taste is mild, with a good balance of sugar and acid.

"Even more exciting are the newer potential varieties we're working with now," Bell says. "About eight selections from crosses made since the late 1960's--all blight resistant--have been sent to cooperators around the country. All are more blight resistant than Bartlett, the most popular variety."

These varieties are being tested in Ohio, New York, Washington, Oregon, and California. Additional cooperators are being sought in Arkansas, Michigan, and other states. Bell says the varieties have good appearance and size, represent a range in flavor types and harvest dates from early August to early October, and maintain good quality late in storage.

The evaluation process will take some time, Bell says, but several of these new varieties could be released within the next 7 or 8 years.

Computers and Cultural Practices Can Help

"One way growers can fight fire blight is to predict when it will occur, so they can be ready for it," van der Zwet explains. "We've used a computer program developed jointly with the University of Maryland for more than 6 years now, and it has been nearly 100-percent accurate in predicting blight conditions each year."

Called MARYBLYT, the program was developed by Paul Steiner, of the University of Maryland, and Gary W. Lightner, an ARS computer analyst at Kearneysville.

"Weather conditions have a lot to do with the bacteria being distributed and infecting trees," van der Zwet says. "Warm, moist conditions are ideal for bacterial growth."

To operate MARYBLYT, a grower needs only a simple rain gauge, a minimum and maximum thermometer, and a sharp eye. The program operates on any IBM or compatible personal computer, including most portables, and is considered easy to run.

Tested at over 20 locations throughout the United States and Canada, the program resulted in better fire blight control and reduced the number of antibiotic sprays needed.

"Although some growers spray their pear orchards with streptomycin, it is not absorbed throughout the tree and is not effective on unopened blossoms," van der Zwet says. "Furthermore, some strains of E. amylovora are resistant to it. In these instances, copper compounds may be substituted, but these are not nearly as effective."

Since there are stringent rules governing the use of antibiotics on food crops, van der Zwet and colleagues at Kearneysville are also experimenting with what they call "horticultural manipulation" of the tree to prevent the disease from striking.

"Cultural practices such as maintaining an adequate balance of calcium in the soil, making sure proper amounts of fertilizer are applied, and using drip instead of overhead irrigation can reduce the bacterium's spread," van der Zwet notes.

They're also testing biocontrol agents against the disease.

For example, a microorganism found on the leaves of honey locust trees growing in a West Virginia forest may prove to be effective in deterring fire blight. It is a bacterial antagonist that fights E. amylovora, and van der Zwet and colleagues have identified a sugar that speeds up its growth, The sugar may help encourage this and other naturally occurring antagonists of fire blight's bacterial carrier to proliferate on the blossoms and leaves of pear and apple.

Additional ARS research on microbial biocontrol of fire blight is conducted at Wenatchee, Washington, and Corvallis, Oregon.

Pear Psylla--Can We Stop It?

A yellowish-green insect that looks like a miniature cicada--the pear psylla--is the other main reason there is no longer a pear industry on the East Coast.

A sucking insect, Cacopsylla pyricola is prevalent throughout pear-growing regions of the United States and Canada. When the insects feed on leaves, says Gary J. Puterka, an entomologist at Kearneysville, it causes severe wilting and defoliation, which reduces yields and weakens the trees.

Pear psylla also carry a mycoplasma-like organism (MLO) that causes pear decline, the premature death of pear trees.

And, "If these things weren't bad enough, the immature form of the insect secretes copious quantities of honeydew, a sugary, sticky substance that allows a black, sooty mold to grow on both fruit and leaves," Puterka says. This mold not only reduces the quality of the fruit; it also blocks sunlight from the leaves, decreasing photosynthesis.

Previously, growers controlled pear psylla with insecticides--an expensive and sometimes ineffective method, for the pear psylla has developed resistance to several common ones. Now, growers must rotate the few available pesticides that are appropriate.

So Puterka is taking several different approaches. Natural plant compounds, fungal pathogens, and different orchard ground covers all look like promising controls. And host-plant resistance-- another name for identifying pear genetic material with a natural defense against the pest--is another viable alternative.

Sugar Ester anti Fungal Pathogens

"The most successful plant compound we've tried so far has been a sugar-ester extracted from wild tobacco," Puterka says.

Mixed with water and sprayed on pear leaves, the sugar-ester gave 100-percent control of the psylla, killing most nymphs within 2 hours. Puterka reports that full control took only a day.

"Even nymphs that hatched 3 to 5 days after spraying were affected," he says. "They appeared normal as they crawled out of the eggs. But when the nymphs walked on a leaf that had been sprayed with the compound, they suddenly died."

The sugar-ester is a nontoxic, natural plant compound that was originally extracted for use against the sweetpotato whitefly by ARS chemist George Buta and entomologist John Neal at Beltsville. Scientists are now searching for a way to mass-produce it. And Ray Severson, an ARS chemist at the Phytochemical Research Laboratory in Athens, Georgia, is collaborating with Puterka on field tests.

Puterka is also testing fungal pathogens, even though none has ever been identified on pear psylla. He got the ones he's using--several naturally occurring strains of Beauveria, Verticillium, and Paecilomyces--from ARS insect pathologists at Ithaca, New York. Collected from aphids and some psylla species, all gave 100-percent control within 5 days.

These fungal pathogens have one important advantage over both the sugar-ester control and other insecticides: They have a lasting effect.

"The sugar-ester may linger on the plant for only about a week, whereas the fungus can establish itself in the ecosystem and last indefinitely," Puterka explains.

When applying the pathogenic fungi, Puterka mixes spores with either oil or water and sprays the psylla-infested leaves. Soon after the spray hits the nymphs, they become infected and die in 1 to 3 days. Within 5 days, 100-percent control can be achieved.

Nearly perfect biocontrols, the fungi are host-specific, completing their life cycle on infected insects on the plant-- and are therefore nontoxic to humans, animals, or beneficial insects. Dead pear psylla nymphs literally turn into white puffs of fungus. After killing their host, the fungi release hundreds of spores, each capable of infecting another pear psylla.

"We're testing these pathogens in the field this spring," says Puterka. "If our results are what we expect, we'll seek commercial development so the pathogens can be made available to pear growers."

Puterka thinks it would not be difficult or expensive for these fungi to be produced commercially by private industries involved in developing biological pesticides. Since growers already spray with a dormant oil in the spring, Puterka says it would be very easy for them to incorporate the fungal spores into their current spray program.

Or Perhaps Parasites or Predators?

Parasites are also being investigated as a means to control pear psylla. The parasite most common to the insect on the West Coast, Trechnites psyllae, wasn't found during a recent survey that Puterka conducted on the trees at Kearneysville.

What he did find was a parasite tentatively identified as a Psyllaephagus species. Insects infested with this parasite exhibit bizarre behavior. "Parasitized insects would just walk right off leaves and down the tree trunk and hide under cardboard we had wrapped around the tree," he says. "They were very agitated and disoriented." More research is planned on pear psylla parasites.

Since pear psylla also have several predators, planting ground covers between tree rows to attract them could provide a measure of control. While Kentucky 31 tall rescue is a standard orchard floor cover in most of the Northeast, it provides a poor habitat for psylla predators such as lady beetles, pirate bugs, and lacewings. So Puterka is experimenting with perennial crops like clovers to attract aphids and other insects that would support the predators.

"Compared to Kentucky 31, clover attracted about 10 times more aphids to feed the lady beetles, whose numbers increased significantly. As the number of predators increases and their food supply decreases, we hope the predators will then move into the pear trees to feed on pear psylla," he says.

Another season's data is needed to see how the increased number of predators affects pear psylla populations.

These methods could all be used together in an integrated pest management program, Puterka explains. The sugar-ester and fungal pathogens would control the pear psylla, while sparing its natural enemies and allowing them to become well established. "Insects have an innate ability to adapt themselves to single control measures, but they have more difficulty adjusting to the simultaneous onslaught of multiple control measures."

Strengthening Pears' Self-Defense

Some plants are naturally able to defend themselves.

Richard Bell has found this is the case with several pear varieties brought from Eastern Europe by van der Zwet during 1978-80. There is apparently an antagonistic relationship between these varieties and the pear psylla that is not well understood.

To study it further, Puterka is using an electronic feeding monitor connected to the insect to look at the differences in feeding behavior on resistant and susceptible pear varieties.

Although adult pear psylla are only about one-tenth of an inch long and fairly fragile, Puterka is able to anesthetize them and attach tiny gold wires to their backs.

When pear psylla feed on susceptible pears, a particular waveform pattern appears on a monitor connected to the wires. That waveform is nearly absent during feeding on resistant plants.

Using microsectioning, Puterka and colleagues are trying to find out which plant tissue is touched by the insect's mouthparts at the time of this particular waveform. This will also show which plant tissue is avoided and help isolate the repellent component.

"In analyzing the insect's saliva, we've found pectinase and cellulase, compounds that we know break down plant cell structures," Puterka says.

And scientists are also studying how the pear psylla affects pear tree physiology: Just how and why does the tree lose energy as the insect feeds?

An effective management system will combine host resistance with biological control. But a problem in using host-plant resistance is that the insect may eventually develop an ability to overcome the resistance.

In each new generation, naturally occurring mutant insects--members of a new race or biotype--may survive. This new biotype is able to feed and reproduce normally on the "resistant" plant, to which it somehow becomes adapted. Over time, its numbers will increase as susceptible nonmutants are killed or suppressed.

To assess this potential for adaptation, Puterka and colleagues at Kearneysville have collected psylla from Oregon, Washington, Michigan, New York, and West Virginia and are currently evaluating them on six different resistant pear cultivars.

And this isn't as easy as it may sound.

To evaluate each psylla collection against each pear cultivar, they count out 20 newly hatched pear psylla nymphs, coax them onto single-haired brushes, and gently move them onto the leaves of a 2-foot pear sapling caged within escape-proof netting.

After that, technicians do a head count every 3 days to assess how many have died and the state of development of surviving psylla.

The saplings--30 in all--represent a single test of six cultivars that have been created by grafting buds from psylla-resistant pear trees imported from Europe. In a pear orchard exposed to unchecked psylla attack, the parent trees were left undamaged. But perhaps that's not so surprising, for their fruit are small and have poor texture and flavor.

Now, the trick will be to discover what feeding deterrents or other resistance factors are in the plant tissue, which genes regulate their expression, and how to transfer these genes into commercial pear varieties.

"So far, we've found some significant variations in survival and development time among the pear psylla populations," Puterka says. "However, several resistant cultivars were effective against all psylla populations."

From a Grower Perspective

Chris Baugher has first-hand experience with the pear psylla. He grows fresh-market pears on 10 acres in Aspers, Pennsylvania.

"This pest has cost us money," he says. "Honeydew deposits from the pear psylla made some of our fruit unmarketable last year."

Controlling the insect is a real problem, Baugher says. The number of times an orchard can be sprayed is regulated by the government. And the pest seems to be able to develop resistance so quickly that growers use an insecticide for only one season, then try something else the next. This gets expensive and frustrating. Action by the Environmental Protection Agency to ban many insecticides is leaving even fewer choices for growers. "We'd be willing to try just about anything at this point," Baugher says.

ARS scientists at Kearneysville are optimistic that peat' production problems will be solved. Several approaches can act together to control both fire blight and peat' psylla while decreasing the pesticide load on the environment.

As the multifaceted research program at Kearneysville bears fruit, orchardists on the East Coast will be able to once again consider growing pears.--By Doris Stanley, ARS

Scientists in this article can be reached at the USDA-ARS Appalachian Fruit Research Laboratory, 45 Wiltshire Road, Kearneysville, WV 25430; phone (304) 725-3451, fax (304) 728-2340.
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Author:Stanley, Doris
Publication:Agricultural Research
Date:Nov 1, 1993
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