...From the pages of Agricultural Research magazine
Strawberry Growers Test
||Sweet, juicy strawberries are one of America's favorite fruits. We each eat about 4 pounds of the plump, bright-red berries every year. Fat-free and low in calories, strawberries are loaded with vitamin C. They also furnish folatea B vitaminplus potassium and fiber. What's more, strawberries provide ellagic acid, a compound that fights cancer. To raise perfect berries, most growers rely on a fumigant, methyl bromide. Berry farmers inject methyl bromide gasalong with a companion chemical, chloropicrininto the soil a few weeks before planting. The chemicals zap soil-dwelling fungi and bacteria that can cause plant diseases. And they quell weeds that would otherwise compete with young berry plants for water, sunlight, space, and nutrients. Wide Range of Tactics Probed America's strawberry farmers will be among the hardest hit as the nation phases out production and use of methyl bromide. The phaseout stems from evidence that methyl bromide is an ozone depleter, that is, a substance that destroys the protective ozone layer of Earth's atmosphere. The impending loss of methyl bromide has sent growers and researchers scrambling to find safe, effective, economical, and easy-to-use alternatives.|
||Agricultural Research Service scientists in Californiawhere 80 percent of the nation's fresh-market strawberries are grownare investigating a range of new tactics for producing high-quality harvests. The researchers anticipate that growers will likely end up using an assortment of biological control and chemical optionsinstead of relying on a few magic-bullet chemicals like methyl bromide paired with chloropicrin. Scientists from ARS labs in Fresno, Salinas, and Davis, California, are measuring how various compounds affect berry harvests. Their tests include not only today's leading commercial strawberry varieties, but also more traditional cultivarsones that dominated berry fields decades ago. Some experiments scrutinize the impact of the promising chemicals on beneficial and harmful soil microbes. Others explore the effects of adding helpful microbes to fields to boost growth and provide protection from disease organisms. In this array of projects, researchers have teamed up with university scientists and with specialists from industries that produce biological control or chemical products. Thanks to the grower-funded California Strawberry Commission, farmers from major berry-producing regions of the state have willingly set aside portions of their fields for the researchers to use and have hosted tours of those sites so neighboring farmers can also benefit from what's being learned there.|
||Applying Chemicals Through Drip Irrigation Systems Most growers send irrigation water to their strawberry plants though slender polyethylene tubes known as drip tapes. Each of the raised beds in which strawberries grow has one or two of these tapes, buried an inch or more beneath the soil surface. These same drip tapes could be used to carry fumigants. Soil scientist Husein A. Ajwa and agricultural engineer Thomas J. Trout at Fresno have probably explored more variations of that idea than any other recent scientific team. They have looked at more than half a dozen chemicalsused alone or in combination with other compounds. In some tests, they tried different concentrations of the chemicals and applied them at different rates, as well. In other experiments, they scrutinized emulsifiers that make the compounds soluble in irrigation water.|
|Aided by a sophisticated, field-ready version of an instrument known as a gas chromatograph, they have probed different kinds of soils to track the compounds, learning how far downand how far across the raised bedsthese materials move and whether the spread is uniform. By tracking how long it takes the chemicals to deteriorate, they've determined when it is safe to re-enter fields to plant the beds. Ajwa says that applying fumigants through drip irrigation systems may reduce worker exposure and decrease the amount of chemicals needed to treat fields. Among the best performing of the compounds that he and Trout have examined is InLine, a combination of about 60 percent 1,3-dichloropropene and up to 35 percent chloropicrin. The manufacturer, Dow AgroSciences LLC, is pursuing registration of this material for use in strawberry fields. InLine is the water-soluble formulation of Telone C35, a parent compound that is already approved for use on strawberry fields. But it requires a buffer zone and has a maximum-use-per-township limit, meaning that not every grower on every farm could use as much as would be needed.|
|At some sites, marketable yields of InLine-treated plots were from 95 to 110 percent of those from plots treated with methyl bromide, Ajwa reports. Plots that he treated with another optionchloropicrin alone, at a lower rateprovided a yield about 94 to 100 percent that of the methyl bromide sites. Ajwa also found that reduced rates of InLine, or of chloropicrin, can also produce yields equivalent to methyl bromide if either alternative chemical is used in tandem with metam sodiumanother fumigant. Metam sodium would be applied 1 week later through the same irrigation system. "Or a compound known as methyl iodide can be applied with chloropicrin, using drip irrigation," Ajwa adds. "We got about the same yields from this combination as from methyl bromide." That test was likely the first to apply methyl iodide in a drip irrigation system.|
|"We've also seen very promising results with a chemical called propargyl bromide," says Ajwa. "It gave yields that were up to 110 percent of those from the methyl bromide plots." This year Ajwa plans to find out how much of the drip-applied fumigants escape into the air. Those figures may be particularly valuable if the manufacturers of candidate compounds, such as methyl iodide or propargyl bromide, seek U.S. Environmental Protection Agency and State of California approvals for this use of their products. Harnessing Helpful Microbes When fields are fumigated, good and bad microbes alike may be wiped out. But what if beneficial microbes were routinely added back to the soil? Plant pathologist Cynthia Eayre at Fresno is investigating rhizobacteria, which live on or around plant roots in the soil zone called the rhizosphere. In particular, she's looking at plant-growth-promoting rhizobacteria.|
|Eayre has screened more than 130 strains of these friendly bacteria to pinpoint those which might boost the vigor and yield of young berry plants. She's working with plants growing in nonfumigated soil and those growing in beds fumigated with methyl iodide or chloropicrin, for instance. How do these microbes help plants? "We think they use nutrients that might otherwise be taken up by harmful bacteria," says Eayre, "Or the rhizobacteria might colonize sites on strawberry roots, edging out pathogenic bacteria. "The rhizobacteria can also release hormones that encourage root growth and can produce compounds that inhibit the harmful bacteria. In some other crops, they can enhance disease resistance, but we don't know exactly how they do that."|
|From her screening of these microbes, Eayre has found about a half-dozen worth further scrutiny. Among them is a strain from EcoScience Corp., in Longwood, Florida. In one test, yields from plants that she inoculated with the EcoScience formulation and then planted in chloropicrin-fumigated soil were equal to those from methyl bromide plots. Ideally, instead of hand-dipping the plantlets into the inoculumas Eayre had to dotomorrow's planting stock could be quickly and easily treated by adding a freeze-dried version of the microbial product into drip irrigation systems. Foiling Phytophthoraa Formidable Fungal Foe One of the worst soilborne enemies of strawberry plants is a fungus called Phytophthora. "Phytophthora can cause severe root rot and crown rot, not only at the nurseries where strawberry plants are started, but also later in the fruiting fields where the berries are produced," says Greg T. Browne. His approaches to fending off Phytophthora include evaluating strawberry varieties for strong natural resistance that could be bred into new commercial cultivars.|
|An ARS plant pathologist at Davis, Browne is also testing fungicides to provide new and more detailed information about their effectiveness.The screening procedure that Browne developed for ferreting out gene-based resistance to Phytophthora has been adapted for use in the University of California's strawberry breeding programthe state's oldest and largest. Browne's procedure involves growing Phytophthora colonies in a nutrient-rich mixture that's later stirred, or rototilled, into soil at experimental sites in either the nursery or berry-producing fields. Says Browne, "We think this research procedure mimics natural conditions in which the fungus grows and spreads. Some plants collapse from fungal infection early in the field tests. Those which don't are monitored for their marketable yield." Browne's test of fungicides included two compounds already approved for use on strawberriesRidomil Gold, which is applied to soil, and Aliette, which is applied first as a dip before planting, then as a spray after planting. "Our preliminary tests," he says, "indicate these chemicals may be an important component of an overall strategy that uses many different weapons and tactics."|
| Other Pathogen Problems Probed
While powerful pathogens such as Phytophthora can clobber strawberry
plants outright, slower-acting, stealthier microbes can't be ignored.
These pathogens include "root nibblers," as Frank N. Martin
refers to them.
"Collectively, they may not kill plants," he says. "But
they can have dramatic effects on yields by damaging the fine roots that
bring water and nutrients to plants."
In his Salinas studies, Martina plant pathologistis tracking
the fate of such soil-dwelling foes as Pythium, Rhizoctonia, and
Cylindrocarpon species, among others. In nursery, greenhouse, and
field tests, he has scrutinized more than a dozen different commercial
strawberry cultivars for any signs of resistance to these microbes.
His field trials with 17 cultivars planted in soil inhabited by the root-nibblers
showed that losses in marketable yield ranged from minimal up to about
80 percent, compared to plants grown on methyl-bromide-fumigated plots.
"Even though some of the more tolerant cultivars are no longer grown
commercially," Martin says, "it may be possible to breed that
tolerance into new cultivars."
In other work, Martin and University of California colleagues Krishna
Subbarao and Steven T. Koike are finding out how strawberry pathogens
fare if the crop is rotated with broccoli or Brussels sprouts. Earlier
work, done elsewhere, showed that these plants contain substances called
glucosinolates, which decompose naturally into compounds that are thought
to kill microbes.
Chemical-Free Strategies Eyed
Conventional growers may learn from the experience and expertise of organic
farmers, "who never relied on methyl bromide in the first place,"
says Carolee T. Bull, a plant pathologist with ARS at Salinas. Her work
with organic producers has included testing commercial cultivars on fumigant-free
farms, to get a precise picture of comparative yields. This work is likely
some of the first of its kind in California.
"These varieties were developed for conventional farmlands treated
with methyl bromide," explains Bull. "So of course they were
tested on methyl-bromide fumigated fields. That means there wasn't a pressing
need to select them for natural resistance to the pathogens that methyl
She's also scrutinizing beneficial microbes known as Myxobacteria.
"We're examining organisms like Myxobacteria flavescens, M. fulvis,
and M. zanthus," she says, "because they may produce
compounds that can weaken or kill strawberry pathogens. We may be able
to use them as biological control agents by adding them to soil to increase
Organic growers often produce lower yields, but they can offset those
losses when their crop commands top prices. The number of organic strawberry
growers in California has increased in the past several decades, with
some of the state's largest and best-known growers now converting some
of their acreage to organic production.By Marcia
Wood, Agricultural Research Service Information Staff.
This research is part of Methyl Bromide Alternatives, an ARS National
Program (#308) described on the World Wide Web at http://www.nps.ars.usda.gov.
To reach scientists mentioned in this article, contact Marcia Wood, USDA-ARS Information Staff, 800 Buchanan St., Albany, CA 94710; phone (510) 559-6070, fax (510) 559-5882.
"Strawberry Growers Test Methyl Bromide Alternatives" was published in the January 2001 issue of Agricultural Research magazine.