Ornamentals, Grapes & Orchards:
Working on Alternatives to Methyl Bromide
By Lyra Halprin, SAREP

Replant disorder caused poor vigor in this peach orchard in Southern San Joaquin Valley. (photo by Greg Browne)

Cut flowers and bulbs, grape rootstocks and orchard replants are the focuses of three of the seven SAREP-funded projects seeking alternatives to the agricultural fumigant methyl bromide. The three projects are funded with the 1998 special allocation from the state legislature promoting the development of alternatives to methyl bromide (AB 1998) sponsored by Assemblywoman Helen Thomson (D-Yolo County) and funded through the Department of Pesticide Regulation.

Researchers are growing peach seedlings in soil from replant sites to test for pathogens associated with replant disorder. (photo by Greg Browne)

“The planned phase out of methyl bromide, which is often used as a preplant fumigant to eliminate nematodes, weeds and pathogens in many ag production systems, will hit California growers hard over the next few years,” says Sean L. Swezey, SAREP director. “The chemical is a Class I ozone depleter, which is scheduled for 100 percent use reduction in this country by 2005.”

More than 500,000 pounds of methyl bromide were applied to nursery flowers and floral greens in California in 1998 according to the Department of Pesticide Regulation’s Pesticide Use Reports. Growers used 1,116,000 pounds of the product on grapes, and applied 1,380,000 pounds on almonds, nectarines, peaches, plums and prunes in the same year, according to the Pesticide Use Reports and further analysis of the soil fumigation data which applies to perennials in Fresno and Tulare counties.

Orchard Replant Disorder Project

Researchers in the plant pathology department at UC Davis, the USDA-Agricultural Research Service (ARS) in Davis and Fresno and farm advisors in Fresno, Tulare and Butte counties are trying to reduce dependence on methyl bromide fumigation in almonds, nectarines, peaches, plums and prunes (Prunus spp.). The goal of their collaborative project is to help reduce dependence on methyl bromide fumigation for control of replant disorder (RD), which causes poor and irregular tree growth, delays economic production, and in severe cases leads to tree loss in young orchards.

“RD can occur when trees or vines are planted without special precautions at sites previously planted to a closely related crop,” says principal investigator Greg Browne, research plant pathologist for the USDA-ARS in the plant pathology department at UC Davis.

There are thought to be many causes of RD including attack by parasitic nematodes, oak root fungus, or Phytophthora, but some key causes remain unknown, as it can occur in the absence of these known pests and pathogens. Browne and co-principal investigators Tom Trout, supervisory agricultural engineer at the USDA-ARS water management research lab in Fresno, and Russ Bulluck, visiting postdoctoral scholar at UC Davis, are trying to determine other unknown organisms or factors that contribute to RD and devise improved approaches to its management.

“We believe an improved understanding of the origins of RD may be one of the most important keys to developing methyl bromide alternatives,” Browne says.

The investigators and their ARS colleagues based in Parlier, Fresno County, are also testing the potential impacts of the amount of time a field lies fallow, the use of cover crops, and previous crop history on RD occurrence and severity. Browne notes that improved basic knowledge of the causes of RD will help with the development of new management strategies, and data on the impacts of fallowing and cropping patterns will help growers assess risk and develop the most favorable replant schedules.

Effects of Preplant Fallow Periods

Peach replant field trials are underway at the USDA-ARS research farm in Parlier to determine effects of preplant fallow periods and chemical alternatives to methyl bromide on RD. The previous orchards at the Parlier experimental replant sites were farmed using conventional commercial practices until removal. Replanting for the first trial began in March 1998.

Fallowing (leaving fields barren) and crop rotation can provide suppression of plant diseases. Without suitable hosts, soilborne pathogens gradually decline.

“Four years of dry fallowing prior to replanting may be sufficient to avoid most of the replant problem in California,” says Trout, who is managing the Parlier experiment sites. “Unfortunately, that length of fallow is usually not economically feasible. Almond, peach and plum growers in the San Joaquin Valley usually choose between four-month and 16-month fallow periods before replanting the same crops, and more information on how much replant disorder control that provides is needed.”

The project treatments included a 17-month fallow period, begun in September 1996, and a five-month fallow period begun in September 1997. Additional treatments included applications of methyl bromide, and an alternative chemical standard, Telone plus Vapam. Trial 2 began in February 1999. Preplant procedures were similar.

According to Trout, growth measurements to date suggest that a 17-month fallow period rather than a five-month fallow period between tree removal and replanting will improve early tree growth in some, but not in all cases.

“It is noteworthy that first season trunk diameters in non-fumigated plots of the first trial were similar to those in fumigated plots in the second trial,” says Trout. “This suggests that environmental factors other than the treatments played an important role in the rate of replant tree growth.”

Additional experiments regarding the effects of preplant fallowing are underway, he says.

Effects of Preplant Cover Crops

Although results are not yet available, the project has begun three replicated cover crop experiments, each with soil from a different replant site at the Parlier fields.

“The hypothesis is that growth of some cover crops during the period between old orchard or vineyard removal and new tree or vine planting may shift the soil microbial community and reduce the severity of RD,” Browne says.

Peach seedlings planted in cones containing replant soil that is being tested for the presence of pathogenic organisms. (photo by Greg Browne)

In the summer of 1999, the investigators collected soil from replant sites in one peach orchard and one plum orchard where the trees had been removed the previous fall. In the former peach orchard, the soil collection sites included methyl bromide-fumigated and non-fumigated plots, but in the plum area, none of the collected soil had been fumigated.

Researchers are trying to determine if cover crops grown during the period between old orchard removal and new tree planting change soil microbial communities and reduce severity of replant disorder. Cover crops are being tested in greenhouse experiments with soil from replant sites. (photo by Greg Browne)

Ten cover crops were germinated and transplanted into the potted samples of soil from the replant sites. Additional potted portions of the soil were subjected to dry or moist fallowing without fumigation or cover crops, or moist fallowing followed by methyl bromide/ chloropicrin fumigation. After four months of growth, the cover crops were uprooted, chopped, incorporated into the soil and allowed to decompose for two months. The soil samples were replanted with Nemaguard peach seedlings; effects of the preplant treatments are being assessed regarding shifts in microbiological soil populations and the growth and root health of the replants.

Cross-specificity between Peach and Grape?

Growers intending to plant peaches or related crops on land previously devoted to grapes, or growers shifting land from peach to grape production are concerned about the degree of specificity between peach and grape replant disorder.

“So far, results from soil samples from fields previously planted to peach and grape have provided no clear evidence for cross-specificity between peach and grape replant disorder,” Browne says. Regardless of whether the soil was collected from an old vineyard site or an old peach orchard site, preplant heating and/or fumigation usually resulted in greater peach and grape seedling root or shoots weights and reduced the severity of root discoloration.

Favorable responses to heating were more pronounced for grape than for peach, Browne says. For grape, fumigation alone usually was not as effective as preplant treatments involving heat. He also notes that there were no significant interactions between the soil source and the seedling type.

“Our results should be repeated and confirmed with field observations before application to commercial situations,” Browne says.

Organisms that Cause RD

Browne notes that although it is not certain that all RD problems are caused by soil microorganisms, positive results from using preplant soil fumigants like methyl bromide suggest that a biological origin should be explored. One approach to identifying microbes that cause the disease is to impose heat or chemical treatments that will “fractionate” soil microbe populations by killing or suppressing some of them, leaving a less complex mix of pest or pathogen candidates.

Several heat fractionation experiments were completed in a greenhouse with soil from peach and almond replant sites near Parlier, and in Chico, Butte County. For the experiments, soil samples were heat-treated in water or autoclaved. Heat treatments consistently reduced the severity of root problems.

Researchers also conducted a chemical fractionation experiment using preplant soil treatments of antibiotics, fungicides, a nematicide and a non-treated control on either autoclaved or non-autoclaved samples of the replant soil. Nemaguard peach seedlings grown in the non-autoclaved, non-chemical-treated soils developed root discoloration symptoms, while those grown in autoclaved soil had healthy roots. Among the seedlings grown in non-autoclaved soil, one of the fungicides significantly reduced the severity of root discoloration suggesting fungal involvement in the symptoms, Browne says.

Organisms associated with symptoms of root discoloration in the fractionation tests on RD-affected trees in the field have been isolated and are being tested for their ability to cause disease on peach roots. Browne says that although several fungi are RD suspects, no new conclusions on causes can be made yet.

Almonds, too

Joe Connell, Butte County farm advisor, is working with Browne on the segment of the project related to almond replant disease.

“My part of the project is just getting underway, so we don’t have any results yet,” Connell says, “But we do have a big problem with replant disease.”

Connell and Browne are conducting replicated trials in a Chico area orchard, comparing the success of untreated plots with different rootstocks, and with plots fumigated with either methyl bromide, Telone/chloropicrin, or chloropicrin alone. They are hoping to identify pathogens that cause the almond replant problem, and are looking for effective preplant treatments.

Growers Count on Science

“When it seems like we’re faced with a disaster, like the elimination of methyl bromide, it motivates change,” says Rod Riffel, farm manager for Enns Packing in Kingsburg, which grows peaches, plums, apricots, and nectarines. “There’s a lot of hope out here that biological alternatives, chemical alternatives, fallowing or a combination of these may provide a better situation for replanting than we’ve been using.”

Riffel is glad nursery operators, plant breeders and other researchers are developing stronger, more resistant rootstocks, and is excited about research being done with microbial inoculants.

“Microbial inoculants and their effect on root health, particularly in their relationship to pathogen and nematode antagonism, is very interesting,” Riffel say. “I’m happy to be working with Tom Trout and Greg Browne on the different trials. Their open-minded, practical and thorough approach to exploring the possibilities will help us find the best options.”

Riffel says that biological controls will be much welcomed. “We look at biological controls first,” he says. “That’s just our approach.”

Ken Enns, a family partner in Enns Packing, is hoping for scientific breakthroughs.

“The bottom line for me right now is that we must rely on science and/or politicians to come up with a viable means to fumigate,” he says. “For our family, we need to fumigate land when we replace our permanent crops, and we need fumigation for post-harvest treatment on stone fruit. We currently use methyl bromide on about fifty or less acres per year and we do well over 100 chamber fumigations per summer for export purposes.”

“Methyl bromide gives us the best protection against some of the organisms in the soil right now,” says Gary Van Sickle, research director at the California Tree Fruit Agreement. “As we lose methyl bromide, we could lose 15 to 30 percent of our potential yields, in stunted growth and reduced crop loads on trees. That’s tough for farmers. The market for fruit hasn’t gone up that much in the last 20 to 30 years. The only way we can overcome low farm prices is to increase efficiency and yields. That’s the only way we can squeeze out profits. As we lose this chemical tool, we’ll go backwards.”

Van Sickle notes that tree fruit varieties are becoming obsolete very fast.

“Plant breeders are putting out new varieties with improved yield so quickly now that farmers are replanting 10 percent of their field each year and have about 20 to 30 percent of their fields out of production during any given year,” he says. “If they had to leave their land fallow for three years to avoid RD, some would have 50 percent out of production annually.”

Researcher Browne is aware that the loss of methyl bromide as a tool is a serious one for the agricultural community.

Range of responses to dagger nematode damage on grape seedlings ranging from resistant(left) to very susceptible(right). (photo by Yimin Jin)

“We all appreciate the valuable cooperation and input we’ve received from growers, other ag representatives and businesses, and UC Cooperative Extension,” he says. “In light of the phase-out of methyl bromide, we all hope the project’s efforts will lead to improved cultural management practices for replant disorder.”

Grape Rootstock Project

Researchers sample vegetation at dagger nematode and Phylloxera resistance rootstock trial in Napa Valley. (photo by Andrew Walker)

Methyl bromide is commonly used as a preplant fumigant for vineyard establishment in the San Joaquin Valley, and Central Coast valleys. Its phase-out will present California grape growers with a critical problem: lack of suitable rootstocks with nematode resistance. This problem will be particularly severe where new vineyards are replanted over a previous vineyard. Available rootstocks may not have the right horticultural characteristics for all situations or they have insufficient resistance against aggressive nematode strains and species.

UC Davis nematologist and professor Howard Ferris and Andrew Walker, associate professor in the UC Davis viticulture and enology department, are evaluating grape rootstocks for resistance to two major nematode pests: the dagger nematode and standard and aggressive strains of the root-knot nematode.

“Fumigation has a place in vineyards with high nematode populations and where growers are using less than fully resistant rootstock, but we have seen very little impact when fumigation is used to control Phylloxera,” says Walker, whose specialties include grape breeding and genetics.

“We see the same result in the case of fanleaf degeneration vectored by the dagger nematode,” he says. “Fumigation has little utility in controlling the problem because it cannot penetrate the soil deeply enough to kill all the nematodes, and the remaining ones continue to infect roots and vector virus.”

Ferris says approximately 45 selections from crosses made with wild grape relatives have been screened against the root-knot nematode. These selections have previously shown resistance to the dagger nematode. Of those tested, 23 were also resistant to the root-knot nematode. Seventeen selections from wild grape crosses have also been tested for root-knot nematode resistance; six exhibited resistance.

A set of 33 selections, which previously tested resistant to root-knot nematode, is in the final stage of screening for resistance to the dagger nematode.

“We’re building up population levels of three root-knot nematodes populations that are aggressive on the Harmony rootstock, a major nematode-resistant rootstock used by wine grape growers,” Ferris says.

Then, he says, promising rootstocks will also be challenged by both dagger and root-knot nematodes.

“So far our project results have been productive,” Ferris says. “We’ve overcome some challenges related to expanding our program and using borrowed greenhouse space, and developing appropriate cultivation practices for the rootstock selections we are testing.”

Ferris notes that the ultimate benefits of this project will be gauged by the use of the rootstocks that are developed.

“Currently available rootstocks have viticultural flaws like excessive vigor, or they lack effective resistance,” Ferris says. “There is an excellent chance that our ongoing research will produce better rootstocks, and when it does, that they’ll be accepted and used.”

Most of the extension and outreach activities of this project will occur during and after the third year, according to Ferris. At that point, rootstock trials will be set up with farm advisors and growers in the North Coast focusing on grapevine fanleaf virus areas, the northern San Joaquin Valley with an emphasis on pressure from root-knot nematodes and grapevine fanleaf virus, the Central Coast for root-knot nematodes, and the central and southern San Joaquin Valley for root-knot nematode complexes, lesion nematode, and citrus nematode.

“We’re hoping that data from field trials could result in recommendations on the use of new rootstocks in as few as five years,” Ferris says.

Grower Hopes

Xiphinema index, the largest plant parasitic nematode, a major problem in grape rootstocks. (UC Davis Plant Pathology photo)

Egg masses of root-knot nematode on grape rootstock. Researchers are evaluating rootstocks for resistance to this major grape pest. (photo by Peter Cousins)

Martin Mochizuki, of Walsh Vineyard Management in Napa, is cooperating with Walker in a Beaulieu Vineyards test plot near Rutherford.

“I really like the idea of resistant rootstock,” Mochizuki says. “That’s what we’re hoping for. Otherwise, when you plant new vines, they struggle. Letting the winegrape land go fallow is not really economically feasible here, so we’ve been fumigating. We’re hoping to fumigate and replant any older vineyards in heavy fanleaf-infested areas before methyl bromide is phased out. We hope before we have to replant again in 15-25 years, there will be more fully resistant rootstocks. Right now we’re using the 03916 rootstock, which is somewhat resistant. It can pick up fanleaf, but the disease isn’t expressed in the fruit.”

Coastal Ornamental Crops Project

Researchers are testing solarization and broccoli residue as alternatives to methyl bromide in fields planted to callas like these near Monterey. (photo by Golden State Bulb Growers)

Cut flowers, bulbs and greens are highly productive and valued components of Califor- nia’s ornamental crops industry, which is seriously threatened by the pending loss of methyl bromide. Since solarization alone does not create enough soil heating to be effective at killing fungi, bacteria and weeds in the coastal regions, researchers at UC Davis are combining solarization with the addition of organic amendments to stimulate the phenomenon of bio-fumigation. Their project is focusing on controlling the soil-borne fungus Fusarium oxysporum, the soil-borne bacterium Erwinia carotovora and several weed species. Microplot experiments are being conducted in several locations; in-field experiments are taking place with two different bulb crops, Dutch iris and calla.

James MacDonald of the UC Davis plant pathology department and Clyde Elmore, of the UC Davis vegetable crops/weed science department are working with Steve Tjosvold, a UC Cooperative Extension farm advisor in Watsonville specializing in horticulture.

In the first year of the project, researchers started laboratory and greenhouse studies to determine the effects of broccoli residues on the survival and growth of selected fungal pathogens.

“Broccoli residues release isothiocyanate upon decomposition, which has been an effective fungicide,” MacDonald says. “We wanted to quantify the fungicidal effects.”

Researchers collected leaf and stem tissues of mature broccoli, which were macerated (ground up) in food processors and placed in culture dishes with fungi. A similar set of experiments was done in which the macerated material was mixed with soil.

“Our experiments showed that volatile materials released from macerated broccoli significantly reduced radial growth of the fungi we tested,” MacDonald says. “We also noticed that fungal inhibition was generally enhanced at higher soil temperatures.”

Additionally, field experiments were done at four locations: UC Davis (as a Central Valley location where solarization would be maximized), 3-Way Farms in Royal Oaks/ Watsonville, Watsonville Nursery, and the Monterey Bay Academy. The three Monterey sites were chosen for variation in climate; the Monterey Bay Academy site is several hundred yards from the ocean, while the other sites are several miles inland.

The general methods employed in all experiments involved burying small packets (polyester sachets) of soil containing known populations of the fungus in test plots prior to treatment. At one site, packets containing pieces of potato colonized by soft rot bacteria also were buried. Additionally, bulbs of gladiolus or calla were buried, along with soil temperature sensors. Bulbs of previous crops are significant weed problems for cut flower growers.

The following treatments were used in field experiments: no treatment (control), clear polyethylene for various lengths of time, clear polyethylene over water bags for six weeks (heat retention), various concentrations of Metham sodium, blood meal and solarization, composted chicken manure with and without solarization, corn gluten meal with and without solarization, acetic acid and solarization, ammonia and solarization, and broccoli chop with and without solarization.

At the conclusion of each field experiment the sachets containing fungi, those containing bacteria and the bags of flower bulbs were recovered to assess post-treatment survival. The stands of native weeds in each plot were also quantified to determine treatment efficacy.

Evaluations of harvested plots have been essentially completed, MacDonald reports. Researchers compared soil temperatures at different depths and with different treatments, the survival of several fungi at different depths and with different treatments, weed germination/survival with different treatments, bulb survival with different treatments, and bulb infections following different treatments.

In comparing the soil temperatures achieved through solarization, MacDonald says the degree hours accumulated in the three Watsonville-area plots were substantially reduced compared to the Davis plots.

The Monterey Bay Academy site never achieved soil temperatures of 40°C, he says. The soil temperatures achieved through solarization were insufficient to reduce germination of calla bulbs relative to controls at that site.

“This was important to determine, as growers consider residual bulbs from previous crops to be one of their greatest weed problems,” says Tjosvold.

Tjosvold reports that excellent control was achieved at all experimental sites with Metham sodium (100 gal/acre) overlaid with plastic tarps for six weeks. Good control also was achieved when broccoli residues were incorporated at the rate of five tons/acre (dry weight basis) in the top three inches of soil, and overlaid with plastic tarps for six weeks. Broccoli residues without tarping had no effect, he says.

“We think the beneficial effect of tarping was related to trapping of volatile materials within the soil rather than a solarization effect,” he says.

Calla bulbs were recovered from the plots at the end of the treatment periods and were assayed to detect the presence of two fungi species; both were present.

“While these results were entirely qualitative, they did indicate that these microbes could survive, at least to some extent, in the presence of these soil treatments,” Tjosvold says.

In experiments in which sachets of infested soil were buried in plots prior to treatment, and recovered after treatment to compare survival, researchers found that the results at Davis differed greatly from the Watsonville sites. At Davis, all treatments (except composted chicken manure alone) caused significant reductions in fungi populations. At the Watsonville Nursery site, there was no statistically significant difference between any of the treatments and the controls with respect to fungi survival. However, at the Monterey Bay Academy site, Metham and tarping resulted in a significant reduction in one fungus population relative to the control at the 5-cm depth. At this same depth, the survival of a second fungus species was lowest in the broccoli residue treatments, but Tjosvold says the results were highly variable and no statistically significant results were obtained.

During the summer of 2000 researchers repeated experiments with Metham and broccoli residues.

“These were the treatments that appeared to warrant further research based on our first-year results,” MacDonald says.

“Growers of cut-flower crops will be especially hard-hit by the loss of methyl bromide, and larger, field-scale experiments will give a better indication of the cost effectiveness of these alternatives,” he says.

MacDonald reports that the results of this research will be disseminated through presentations at the annual Methyl Bromide Alternatives conference, the California Ornamental Research Foundation tours and newsletters, and through the California Cut Flower Commission.

Grower Concerns

Snapdragon and godetsia seedlings were planted in UC Davis test plots after six weeks of solarization with tarps(left) or the incorporation of 35 tons of fresh broccoli (right) for weed control.(photo by Clyde Elmore)

Eric Overeem, farm manager, pest control advisor and certified crop advisor at Golden State Bulb Growers in Santa Cruz and Monterey counties, says that the loss of methyl bromide as a preplant fumigant will hit their industry hard.

Golden State produces calla bulbs (Zantedeschia spp.), predominantly for the smaller colored flowers that range from yellow to purple to red and multicolored, and the traditional larger-sized white flowers. The vertically integrated operation grows and sells the bulbs.

Overeem says one of the problems for his operation is that they can’t cultivate or use dust mulch (“dirt smothering”) to reduce weeds. The plants are 11/2 to 3 inches apart; tools for cultivation or mulching would damage them.

“We’ve also used methyl bromide to kill volunteer callas, because they are sources of disease inoculum and are reservoirs for viruses,” Overeem says.

Overeem says Elmore’s broccoli residue combined with solarization did suppress volunteers on one of Golden State’s fields, but he doesn’t think it’s economical yet for their operation.

“We have a very short window of opportunity to plant in the spring,” he says. “If I had to solarize for six weeks, I’d be out of business. We need to maximize the amount of growth we develop the first season or we don’t get salable bulbs the following season. It’s an 18-month crop.”

“Until there are more economically feasible alternatives, we see using more chemicals to achieve the same result we got with methyl bromide,” he says. He anticipates using Telone and chloropicrin for base fumigation. “And then we’ll have the weeds to deal with,” he adds.

Overeem says the post-methyl bromide regime will include a more extensive herbicide program and/or more expensive hand weeding.

“As far as disease control, I anticipate having to spray more frequently,” he says. “I might end up putting more pounds and more types of toxins in the environment than I would have with methyl bromide.”

“We’ve grown broccoli as a cover crop before, but didn’t see much pest-suppressing effect,” he says. “In theory, it sounds great, but the logistics make it complicated.”

Pat Treffry, general manager at 3-Way Farms based near Watsonville in Royal Oaks, is very concerned about the phase-out of methyl bromide, particularly because undeveloped countries have a longer time frame in which to accomplish it. U.S. farmers must stop using methyl bromide by the year 2005, while undeveloped countries have until 2015. The gradual phase-out has also meant a dramatic increase in the price of methyl bromide.

“The cost went up 40 percent this year,” Treffry says, noting that the price will increase again each year the phase-out of the chemical requires a reduction in use.

In the meantime, floral greens producers like Treffry are looking for alternatives for postharvest pest control and soil fumigation.

“The solarization and the tarps would be so terrific in the Central Valley with that heat,” Treffry says, commenting on one of Elmore’s polyethylene tarping and broccoli trials, which took place on a 3-Way field. “It’s harder to see them being effective here when the sun might not break through until after 11 a.m.”

He says, however, that the solarization methods might be very useful on 3-Way’s farm in the Central Valley community of Los Banos, where they raise German statice, caspia and larkspur seeds.