Printer Friendly

Safeguarding winter's food supply.

The frost is on the pumpkin and the fodder's in the shock. Late October sunshine adds a soft glow to brilliant patches of color splashed by oaks, maples, and beeches across the landscape. Autumn has come and it's harvest time.

But for farmers in the Sunshine State, it's planting time. More than half of the fresh fruits and vegetables eaten by Americans during winter months are grown in Florida.

To produce this bounty, Florida growers plant more than 730,000 acres of citrus and over 10 million acres of other crops and pasture.

"And when much of the country is worried about heating bills and snowy, slippery driving conditions, we're planting and harvesting tomatoes, snap beans, cabbage, celery, sweet corn, and squash," says Richard T. Mayer. "Here in Florida we grow a few crops that are shipped every month of the year."

Mayer is head of the U.S. Horticultural Research Laboratory, the ARS research facility located in Orlando.

Mayer and his staff are looking for ways to keep food plentiful at economical costs. This can be done, he says, with research that will improve fruit and vegetable quality and protect crops from pests, diseases, and postharvest problems such as chilling damage during shipping or storage.

Turning Up the Heat To Keep Cool

Supplying fresh produce for distant parts of the country means that maximizing its storage life is essential.

At the ARS Orlando lab, this responsibility falls to horticulturist Roy E. McDonald and staff.

"One problem with shipping fresh fruit and vegetables is that each commodity has its own ideal shipping temperature," McDonald says. "Since fresh produce is alive, it breathes. And without proper atmospheric conditions and refrigeration, shelf life diminishes, leaving the produce more susceptible to decay."

This means produce must be shipped at reduced temperatures. But low temperatures can cause chilling injury and abnormal ripening, both of which decrease quality.

"This creates a tough situation--a real double bind. What we really need is a way to prevent chilling injuries," says McDonald.

And as contradictory as it may sound, he is using heat to do that very thing. For some fruit, such as mangos, heat treatments also reduce decay, slow down ripening, and can be used as a quarantine treatment to control insect pests.

T. Gregory McCollum, an ARS plant physiologist, has been working with McDonald to make fruit more tolerant to lower storage temperatures. McCollum has come up with a high-temperature conditioning technique that improves the shelf life of mangos and could expand the market for that crop.

"Being tropical fruit, mangos get chilled at temperatures below 50 [degrees]F. Rinds turn from pinkish-red to gray, sunken lesions appear on the skin, uneven ripening occurs, inner color and flavor development slow down, and the fruit becomes more susceptible to decay," he says.

McCollum transported mature mangos in an air-conditioned van from a commercial packinghouse to the ARS lab on the day the fruit was harvested. For 48 hours, he kept the mangos in a storage room at about 100 [degrees]F. After 2 days, he decreased the temperature to 41 [degrees]F--an appropriate shipping temperature--and left the fruit for 11 days.

"The mangos didn't look much different from the way they looked when they first came off the tree," says McCollum.

However, fruit in the control group that had not been subjected to high temperatures showed all the signs of chilling injury. "Heat preconditioning probably added a week to the test fruit's shelf life," McCollum says.

McCollum carried his research further. "Some growers may not be able to hold their produce at higher temperatures for 48 hours," he says. "So we took cucumbers from the field and instead of putting them in a high-temperature storage room, gave them a hot water bath."

For an hour the cucumbers sat in water heated to 108 [degrees]F. Then they were chilled at 36 [degrees]F for 2 weeks. McCollum says there was little rind pitting and tests showed few internal physiological changes.

Normally, cucumbers, which are very susceptible to chilling injury, are taken from the field, washed, precooled, and shipped at the temperature that is best for them. But if the heat conditioning protects them from chilling injury, they could be shipped in mixed loads with other commodities that require lower temperatures.

Chilling injury causes breakdown of the internal cell structure in fresh produce. But heat has been found to inhibit this cellular breakdown in cucumbers. McCollum's tests showed very few water-soaked areas in the cucumbers that had been soaked in hot water.

What goes on physiologically within the mangos and cucumbers to protect them from injury when they're hit with high, then low, temperatures?

According to McCollum, no one really knows. "We do know that when plants are exposed to nonlethal stress, they become more resistant to other types of stress."

Apparently, the stress of exposure to 100 [degrees]F temperatures conditions the fruit and helps it to adjust, or acclimate itself, to later drops in temperatures during shipping.

McCollum says novel compounds called heat shock proteins exist, but they aren't present until plants are exposed to high temperatures. "We haven't researched this idea yet, but it is one of our hypotheses," he comments.

Plant physiologist Elizabeth Mitcham says heat can keep tomatoes from ripening and softening too quickly.

"Softening is an important aspect of ripening, but it causes tomatoes to be more susceptible to bruising and decay," she says.

Mitcham stored mature green tomatoes at 104 [degrees]F for 4 days, then lowered the temperature to 70 [degrees]F.

"After 10 days, treated tomatoes were still firm, with just a slight red color," she says. "But the untreated fruit was blood red and completely ripe."

The main reason fruit softens during ripening is that changes occur to the structure of the cell wall. These include a loss of sugars--particularly, galactose and arabinose.

But heat treatment inhibited the loss of these sugars, delaying cell wall modification and fruit softening.

Tomatoes are usually picked mature green for later ripening in storage at temperatures between 57 [degrees] to 60 [degrees]F. To retard the ripening process, they must be chilled in the field, during transit, and in storage.

However, tomatoes are chilling-sensitive and cannot be stored below 55 [degrees]F without injury. Therefore, low temperatures can't be used to greatly delay ripening.

The new heat treatment could cut tomato losses during shipping and also reduce low-temperature storage costs. "We need more time to work out the optimum time and temperature combinations," Mitcham says.

The Florida fruit and vegetable industry anxiously awaits more information on using heat treatments to prolong shelf life of fruits and vegetables.

"We're very supportive of this research and are particularly interested in ARS' work with mangos," says Craig A. Campbell, director of research and development for J.R. Brooks & Son in Homestead, Florida. Campbell says perishability and disease are the main problems they face in shipping mangos.

One of the biggest shippers of tropical fruit and vegetables in the United States, J.R. Brooks handles about 100 million pounds of fruit annually, including about 15 million pounds of mangos. The company imports fruit for U.S. consumers from 10 countries. [This article was written before Hurricane Andrew severely damaged south Florida where Brooks is located.--Ed.]

"Central and South American growers are very interested in shipping mangos to the United States to fill the market gap that can't be met by our own growers. But again, the problem is short shelf life," Campbell explains. "Slowing down ripening with heat treatments would greatly expand market potential for mangos."

Since his company grows, packs, and/or ships 40 different tropical fruits and vegetables, Campbell hopes that the treatment will be tried on other commodities in addition to mangos, cucumbers, and tomatoes.

Before the Reaping--A New Pest Threatens Quality

ARS scientists are actually thinking about the quality of fruits and vegetables long before they are ready for shipping--in fact, long before harvest.

A new insect pest that has recently appeared in Florida is causing concern because it has the potential to devastate many vegetable, fruit, and ornamental crops.

First sighted in this country in southern Florida in 1990, Thrips palmi has spread into eight Florida counties.

"The pest is damaging in all stages of its life cycle," says ARS Orlando lab director Richard Mayer, "even the winged adult." Called a rasping insect, T. palmi drags its sharp mouthparts across leaves, growing tips, flowers, or fruit to cause them to ooze fluid, which it feeds on. Leaves and fruit of damaged plants are bronzed, stunted, or severely scarred.

To find a solution, ARS joined forces with scientists from the University of Florida, Florida State Department of Agriculture, and USDA's Animal and Plant Health Inspection Service, and with growers, processors, and commodity groups.

There is no wholly satisfactory chemical control for the new pest. Field trials using several different pesticides and combinations of pesticides have been tried.

"Although now confined to Florida, T. palmi could spread through the Gulf Coast states and parts of the Southwest and could easily infest greenhouses in the North," Mayer says.

Growers have found it on eggplant, green and wax beans, jalapeno and bell peppers, potatoes, okra, zucchini, cucumbers, passionfruit, mangos, and several ornamentals.

Working with University of Florida scientists, ARS entomologists William J. Schroeder and Kim Hoelmer are rearing an experimental colony of T. palmi on a variety of vegetable plants at Apopka, Florida.

"We're using this colony to evaluate natural controls such as fungi and other disease-inducing organisms, nematodes that might be introduced into the soil, and possible predators and parasites," Schroeder says.

ARS is also exploring potential biocontrol agents at overseas laboratories. Candidate biocontrols could be tested at ARS quarantine facilities in the United States.

Also called melon thrips, T. palmi is thought to be native to the Malaysian-Indonesian region. In addition to Japan, it is found in Hawaii and the Caribbean.

According to Hoelmer, the female lays hundreds of eggs in leaf tissues. In 11 to 24 days, these microscopic white eggs become mature thrips.

"One of the major problems with this pest is that populations grow so rapidly they displace other, less damaging insects," Hoelmer says.

Jack Lee, president of Spectrum Farms, Inc., grows vegetables on about 700 acres in Homestead. "This pest is as bad as--if not worse than--any freeze that has ever hit Florida," he says.

Crop loss and extra chemical spraying have already cost him well over $150,000.

Grower D. Webster Williams, who raises potatoes for the fresh market, says, "I've never seen anything like this in all my 41 years of farming." Webster lost about a fourth of a 50-acre potato field to T. palmi last year.

Since crop production can be year-round in Florida, growers are anxious for researchers to identify possible ways to fight this devastating pest.

Working with cooperators, ARS has identified several areas of needed research, which include controlling the pest, treating affected produce, and searching for host plant varieties that can resist T. palmi infestation.

Heading Off Citrus Diseases and Nematodes

A major problem in keeping citrus and other crops healthy is identifying diseases and other problems before they destroy crops.

Especially potent or severe strains of citrus tristeza virus (CTV) pose a serious problem to fruit growers. These strains of the virus can kill trees within a matter of weeks after the first symptoms appear.

Surprisingly, mild strains of the virus may actually protect trees from infection by severe strains. But it has been impossible until now to differentiate between types of CTV.

"We've developed a quick, practical, and reliable diagnostic test using a monoclonal antibody," says David T. Kaplan, who leads ARS subtropical plant pathology research at Orlando.

The antibody was developed and patented by plant pathologist Stephen M. Garnsey and former ARS research associate Tom Permar.

"The State of Florida already uses our testing method to certify that budwood used for propagating new citrus groves is free of severe, decline-inducing strains of CTV," Kaplan says. It is also used to identify severe strains in Central America and the Caribbean Basin that, if accidentally brought into the United States, could threaten our citrus. And the test can identify beneficial protective strains of CTV.

In another research approach, Kaplan says, "We've identified a gene in a citrus relative that would make citrus resistant to CTV." ARS pathologists and horticulturists are working to breed this gene into new scion and rootstock varieties.

Citrus production is also hampered by parasitic nematodes--microscopic worms that feed on and destroy roots, lessening plant vigor and productivity.

Kaplan says that nematodes cause $4 billion in lost revenue annually worldwide. These losses will likely increase in the United States, since most nematicides previously used in citrus groves have been banned for environmental concerns.

Some citrus rootstocks resist nematodes, but some nematodes can overcome this resistance. It is almost impossible to identify these extra-aggressive nematodes. But Kaplan is working with ARS plant breeders at the Orlando lab to develop rootstocks with natural resistance that even the toughest nematode can't overcome.

Exploiting Natural Plant Defenses

Protecting crops from pests and diseases and improving the quality of fruits and vegetables are important parts of the diverse Orlando research plan.

So is the research of entomologist Jeffrey P. Shapiro, who studies the interactions between insect pests and the natural chemicals present in their host plants.

"This interaction is vital to both the plant's self-defense and the insect's survival," he says.

According to Shapiro, the successful exploitation of natural products against insects is still in its infancy. "Future success hinges on discovering, breeding, and genetically engineering effective deterrent compounds into host plants."

He has isolated and is now identifying several potential defensive compounds from citrus. And he advocates manipulation of insect defenses as well.

For example, "We've found certain blood proteins in root weevil larvae that are responsible for absorbing and transporting plant compounds. Therefore, disrupting or synergizing these proteins could lead to new strategies to control the weevil," he explains.

Along with ARS entomologist Desmond Jimenez, Shapiro is also studying plant proteins. They've found proteins in squash and pumpkin that appear or disappear when attacked by some strains of whitefly.

"Whether these proteins help the plant defend itself against the pest or not, their genetic control may yield clues to natural defensive responses," says Shapiro.

He predicts that the genetic engineering of plant defenses, combined with biological and cultural controls, will eventually replenish our declining arsenal of synthetic insecticides.--By Doris Stanley, ARS.

Scientists mentioned in this article are with the USDA-ARS Horticultural Research Laboratory, 2120 Camden Road, Orlando, FL 32803. Phone (407) 897-7300, fax number (407) 897-7309.

Major Milestones for the Horticultural Research Laboratory

1930 - Released the Orlando tangelo to Florida nurseries. In 1990, growers harvested a crop valued at $10 million from 1,215,000 Orlando tangelo trees.

1945 - Established fresh fruit maturity standards for marketing oranges and, in 1952, for grapefruit.

1972 - Introduced a family of microhymenoptera parasites to control scale insects and, in 1979, helped Florida state scientists (who had originally used a chemical control) identify a different family of the parasites that control citrus blackfly. Because of this work, neither scale nor blackfly is considered a threat to Florida citrus.

1973 - Designed the fiberboard box now used for exporting all citrus fruit.

1974 - Released swingle citrumelo citrus rootstock to nurseries. Determined its resistance to citrus nematodes in 1981 and its tolerance to citrus blight in 1988. Swingle now makes up 51 percent of Florida's nursery stock.

1979 - Helped citrus industry design a semirigid insulation wrap that protects young citrus trees from freezing. Now used extensively by citrus growers.

1982 - Developed cold treatment for grapefruit that allowed export to Japan and other quarantine countries and states. In the same year, developed heat treatments to disinfest citrus nursery trees of nematodes and for specific nematode problems in 1985.

1989 - After 26 years of breeding, introduced Ambersweet, the first cold-hardly orange. Growers have already planted at least 10 million trees.

1990 - Found three nematode species that biologically control citrus root weevil. Growers throughout Florida are now using the nematodes, which are being produced commercially.

1990 - Developed a test to rapidly diagnose citrus blight, a disease that kills about half a million citrus trees annually.

1992 - Patented a practical, reliable test to differentiate strains of citrus tristeza, a virus that can kill trees.
COPYRIGHT 1992 U.S. Government Printing Office
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Author:Stanley, Doris
Publication:Agricultural Research
Date:Oct 1, 1992
Previous Article:Natural antifreeze for underage trees.
Next Article:Food poisoning cases linked by DNA fingerprints.

Terms of use | Copyright © 2017 Farlex, Inc. | Feedback | For webmasters