Strawberry Growers Test
Methyl Bromide Alternatives
America's strawberry growers are
testing various new methods of
growing beautiful berries like
this one without using methyl
bromide, an effective but
environmentally unfriendly soil
fumigant that's scheduled for
phaseout by 2005.
|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.
Plant pathologist Carolee Bull (left) and technicians Joel Stryker (center)
and Adria Bordas discuss the effectiveness of biological weed and disease
control at a research site on grower Rod Koda's ranch.
|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
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
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.
To gauge the effectiveness of crop rotation strategies and methyl bromide
alternatives, technician Cameron Blackford counts colonies of the root
pathogen Pythium ultimum that survived the soil treatments.
|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.
The healthy looking strawberry plant was grown in soil
treated with methyl bromide alternatives. The other is from untreated
and unfumigated soil and is infested with Verticillium wilt.
|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.
Farm manager Arthur Ayala and Carolee
Bull discuss commercial variety trials
being conducted at the farm of Dale
and Christine Coke in San Juan
|"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
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.
Ranchers in California set aside
portions of their farms for collaborative studies on methyl bromide
alternatives for strawberries. Carolee Bull (right) and Adria
Bordas evaluate biologically based methods for weed and disease
control at Rod Koda's ranch.
|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
"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."
Soil scientist Husein Ajwa (right) and postdoctoral
research associate Shad Nelson inspect strawberries from a test
plot near Watsonville, California.
|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 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.
Plant pathologist Frank Martin examines cultures
of different root pathogens that can reduce yields of strawberries
grown in poorly or nonfumigated soil.
|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
"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.