Methyl bromide alternative projects bear diverse fruit

by Sam Prentice and Jenny Broome, SAREP

In 1998 SAREP was awarded a one-time legislative augmentation to support research into alternatives for the agricultural fumigant methyl bromide. Six projects were funded with the special allocation (AB 1998)
sponsored by Assemblywoman Helen Thomson (D-Yolo County) with a friendly amendment by then-State Senator Mike Thompson (now U.S. Congressman, Napa) and funded through the Department of Pesticide Regulation. Methyl bromide is a broad-spectrum fumigant widely used to control insect, pathogen, nematode, weed and rodent pests. It has also been identified as a Class I ozone-depleting substance. Under the Clean Air Act, the U.S. Environmental Protection Agency (EPA) has prohibited the production and importation of methyl bromide starting January 1, 2005. In addition, the United States has joined 140 other nations in signing the Montreal Protocol, which in 1994 froze production and importation of methyl bromide at 1991 levels, and which requires use to be reduced in developed countries by 25 percent in 1999, 50 percent in 2001, 70 percent in 2003 and 100 percent in 2005. According to EPA, continued use of methyl bromide as an agricultural pesticide may contribute five to 15 percent to future depletion of the ozone layer if it is not phased out.

This phase-out has significant implications for California agriculture, as methyl bromide is widely used as a pesticide for the production and export of high-value crops and commodities produced statewide. Approximately 90 percent of the methyl bromide used in California is for pre-plant soil fumigation to control soil-borne pathogens and pests principally in strawberries, nursery crops, grapes, and tree fruits and nuts. When used in this manner, about 50 to 95 percent of the methyl bromide injected can enter the atmosphere. Postharvest commodity treatment accounts for another five to 10 percent of the methyl bromide use statewide and is directed largely at insects that damage nuts, cherries, grapes, raisins, and at imported materials. About 80 to 95 percent of the methyl bromide used in a commodity treatment eventually enters the atmosphere. Structural fumigation accounts for most of the remainder of the methyl bromide use in California.

Several potential chemical and non-chemical alternatives to methyl bromide have been identified nationally and internationally and some of these alternatives have been and are being evaluated in California. Below are final summaries from three of six projects funded by SAREP that investigate potential alternatives in several different cropping systems; the remaining three will be published in the next edition (Vol. 15, No. 3) of Sustainable Agriculture. Overall, it appears that there is no single alternative for the use of methyl bromide that is both effective and economical. Rather, SAREP’s research indicates that a matrix of alternatives is necessary to manage pests currently controlled by methyl bromide within California farming systems. Research is ongoing, as there is an urgent need to develop and evaluate effective, economical alternatives to the agricultural use of methyl bromide as a pre-plant soil fumigant and postharvest commodity and quarantine treatment.

Cultural Control and Etiology of Replant Disorder of Prunus spp.
Principal Investigator: Greg Browne, USDA- Agricultural Research Service (ARS), UC Davis plant pathology department. Cooperators: Russ Bulluck, UC Davis plant pathology department; Tom Trout, USDA-ARS Water Management Lab, Parlier; Andreas Westphal, UC Davis plant pathology department. Submitted February 2003. Updated May 2003.

Objectives
The overall goal of this research was to reduce dependence on pre-plant fumigation with methyl bromide for control of replant disease. The specific objectives were to:

1. Determine effects of pre-plant bare-fallow periods and pre-plant cover cropping on development of replant disease on peach in California.

2. Determine if cross-specificity exists between replant disease of peach and grape.

3. Determine organisms and factors that cause replant disease on selected Prunus spp. in California.

Summary
Replant disease (RD, also known as replant disorder) of Prunus species can complicate establishment of stone fruit and nut orchards planted after removal of a closely related crop. It results in poor growth, delayed crop production, and, in severe cases, tree death. RD is most clearly evident when it occurs in the absence of known causes of other replant problems, which can include plant-parasitic nematodes, Armillaria mellea, Phytophthora species, Verticillium dahliae, or chemical or physical soil inadequacies. It can be prevented by pre-plant fumigation with methyl bromide (MB). Past research indicates that RD can severely limit tree performance even in the absence of the known causes for replant problems. Current data indicate that the causes of RD are primarily biological; RD symptoms can be prevented by soil heating (50 to 60 C) or by treating soil with diverse fumigants. The research on Prunus RD reported here was conducted at sites or with soils that lacked significant populations of plant parasitic nematodes.

Effects of zero- to three-year pre-plant fallow periods on performance of peach and plum trees on old orchard sites were determined in four experiments conducted by the USDA-ARS Water Management Research Lab near Parlier. In each test, a pre-plant MB fumigation treatment (350 lb/A, 0-year fallow) was included as a standard, and trunk growth and marketable fruit yields were used to assess treatment benefits. Without fumigation, each additional year of pre-plant fallow from zero to three years incrementally increased the amount of tree growth produced during the first several years after planting. However, not all fallow-induced growth increases were statistically significant, and they were not all accompanied by significant increases in first year marketable fruit yields. The results indicated that at least three years of pre-plant fallow are needed to match the growth and first-harvest yields produced following pre-plant fumigation with MB. It was not clear that one year of fallow provided a significant yield benefit, compared to no fallow, but growers should consider that optimal cultural preparation for replanting often requires a year of fallow.

Effects of pre-plant cover cropping on RD were investigated in two greenhouse experiments and are now being tested in field microplots. For the greenhouse tests, soil samples from old peach and plum sites were planted with 10 different cover crops in pots in a greenhouse. Non-cropped and MB treatments were included for comparison. After four months of growth, the cover crops were shredded, incorporated into the soil, and allowed to decompose for one month. Nemaguard peach seedlings were transplanted in all of the soils and used to assay them for RD potential. In these tests there was no consistent effect of pre-plant cover cropping or fumigation on peach plant performance or incidence of root-associated fungi on peach. It is not certain that the greenhouse tests adequately represented field settings. To address the potential shortcomings of greenhouse tests, field microplots were established in 2002. The microplots, filled with soil from a peach RD site, received different pre-plant treatments starting in summer 2002, including short-term fallowing (one year when complete), short-term crop rotations (a summer crop of corn or Sudan grass, or a fall/winter crop of wheat) and standard fumigation with MB/chloropicrin (50:50, 400 lb/A, imposed in November). Nemaguard peach seedlings were planted in the microplots in early 2003 and will be used to test for potential benefits of the pre-plant treatments under field conditions.

Cross specificity between peach and grape RD was studied in the greenhouse and is now being investigated in field microplots. The greenhouse experiments did not provide conclusive evidence of such cross specificity, although in two of three tests, peach plants produced healthier roots in non-fumigated grape RD soil than in non-fumigated peach soil. Conversely, in the same two tests, grape plants produced healthier roots in non-fumigated peach RD soil than in non-fumigated grape soil, but the specificity was less pronounced for grape than for peach. For both crops, pre-plant fumigation with MB:chloropicrin (67:33) or autoclaving of the soil consistently increased growth (i.e., plant mass). A microplot experiment was initiated near Parlier in 2002 to test for the cross-specificity effects under field conditions.

Factors and organisms that cause or contribute to RD on Prunus species were investigated near Chico and Parlier using coordinated field, greenhouse, and lab experiments. At both locations, symptoms of RD in the experimental trees’ shoots (i.e., growth cessation, wilting, or defoliation) appeared to result from poor root system development. Fewer healthy feeder roots were present on trees with RD symptoms in non-fumigated plots than on healthy trees in chloropicrin- or methyl bromide-fumigated plots. Isolations from the feeder roots on healthy and RD- affected trees revealed occasional association between root infection with Cylindrocarpon or Fusarium species and incidence of the disease. Greenhouse tests confirmed pathogenicity of these fungi, indicating that they can play at least a partial role in causing RD. Hundreds of bacteria were systematically isolated and preserved from the rhizospheres of healthy and RD- affected trees in the Chico and Parlier trials. The collection will facilitate future research to determine whether certain culturable bacteria play a significant role in RD. When semi-selective fungicidal and nematicidal treatments were imposed on RD soil in the greenhouse, one of the fungicides, but not the other chemicals, resulted in less severe root symptoms of RD on test plants (Nemaguard peach and Marianna 2624 plum), providing additional evidence for fungal involvement in the disease. In field experiments, chloropicrin, which is known for effective control of several soilborne diseases caused by fungi, was more effective in preventing RD than either 1,3-D or MB.

Continued work is needed on most aspects of this research. As the work progressed, it became apparent that the field environment is needed for full expression of RD. Therefore, microplot studies were established at Parlier to augment the greenhouse experiments. Although the results indicate that some fungi not previously known as important pathogens of Prunus spp. contribute to RD, more work is needed to characterize them and their pathogenicity and to determine involvement of other microbes. Although this research accumulated an extensive collection of bacteria from the healthy and diseased trees, continued work is needed to characterize the sample populations and determine their effects on crop health. It is apparent that molecular approaches are needed to augment culture-based approaches to determining RD etiology, because most soil microbes are not culturable.

Field Update: With support from the Almond Board of California, Greg Browne has continued his research on replant disease. Microplots established in Parlier are being used for continuing determinations of RD etiology and management using fallow periods and cover crop rotations. Similar work on reducing incidence of RD is being carried out in the Sacramento Valley with the help of UC farm advisor Joe Connell. In addition, Bruce Lampinen and Browne have been recommended for funding by USD/CSREES for research on development and assessment of alternative pre-plant fumigation strategies for nut crops. The CSREES-funded research is a multidisciplinary project, involving a pomologist, two plant pathologists, a weed scientist, an economist, and several UC farm advisors. The research will include nursery as well as orchard experiments and will occur over a 3-year period.

Development of Grape Rootstocks with Multiple Nematode Resistance
Principal Investigator: Howard Ferris, UC Davis nematology department. Coop-erator: Andrew Walker, UC Davis viticulture and enology department. Submitted December 2002, updated May 2003.

Objectives
1. To continue the development of grape rootstocks with resistance to a broad range of nematodes species and aggressive strains.

2. To evaluate the durability of resistance in advanced selections with multiple nematode resistance.

3. Field-testing of selected rootstocks for horticultural characteristics and durability of nematode resistance.

4. To develop and employ new rootstocks with resistance to a broad range of key nematode species as a sustainable alternative to the use of preplant fumigation.

Summary
Several species of plant-feeding nematodes are present in most vineyards, however few rootstocks have resistance to more than one species. The project screened rootstock candidates against the root-knot nematode (Meloidogyne incognita race 3), two strains of root-knot nematode that overcome the resistance of Harmony rootstock (Meloidogyne arenaria strain A and Meloidogyne incognita strain C), and the dagger nematode (Xiphinema index). Crosses made among a series of Vitis and Muscadinia species resulted in selection of candidate rootstocks with multiple nematode resistance. Of the 6,000 seedlings produced from these crosses, only 12 graduated from rooting trial and individual nematode screening trials with broad resistance to all four nematodes. These rootstock selections were tested for their susceptibility to the ring nematode, Mesocriconema xenoplax. Several appeared to have some resistance to this nematode as well.

To test the durability of the resistance, rootstock selections resistant to all four nematodes when inoculated individually were exposed to all of the species at the same time. When inoculated together there was a very small amount of galling on some of the broadly resistant lines. Two rootstock candidates (9407-14 and 9449-27) appeared to have broad resistance to dagger nematode and three root-knot nematode strains when exposed concurrently. These are extremely valuable plants. There are no other known examples of rootstocks for perennial crops selected for broad (multi-species) nematode resistance.

Some rootstock candidates have now progressed to field trials for tests of horticultural characteristics and to assess the durability of the resistance against field populations of nematodes in a range of environments. Ongoing and future studies will test the durability of resistance to root-knot and dagger nematodes when the plants are inoculated with other nematode species not yet tested, including citrus, pin and lesion nematodes. The research will also determine under what conditions, if any, the resistance breaks down. Preliminary experiments suggest that the resistance to root-knot and dagger nematodes may break down at high soil temperatures in some of the selections.

Alternatives to Methyl Bromide for Control of Soil-borne Fungi, Bacteria and Weeds in Coastal Ornamental Crops
Principal Investigator: James MacDonald, UC Davis plant pathology department. Submitted July 2001. Updated May 2003.

Objectives
To determine the efficacy of soil solarization (with organic amendments) for the control of selected root pathogens and weed pests of field-grown ornamentals in coastal climates

Summary
The coastal regions of California represent a highly valued and productive component of California’s ornamental industry, but the productivity of these regions is seriously threatened by the pending loss of methyl bromide. Some of the alternatives that have been proposed for strawberries [e.g., chloropicrin plus 1,3-D (Telone)] probably will not be suitable for ornamental production. This is because production of these specialty crops tends to be dispersed on many small parcels of land near homes and business, and cannot easily accommodate the ever-increasing buffer zone requirements. The goal of this project was to research the efficacy of biofumigation, an effect created by the decomposition of Brassicaceae (e.g., broccoli, cauliflower, mustards) in soil to release isothiocyanates (ITCs). In laboratory experiments, ITCs volatized from macerated plant tissues have been shown to kill fungi (Fusarium oxysproum f.sp. dianthi) and nematodes (Tylenchulus semipenetrans and Meloidogyne javanica). Members of the Brassicaceae differ in the amounts and types of ITC precursors produced, so aspects of the research focused on identifying plant species that produce the most biologically-active decomposition products, and whether there are periods in a plant’s development when the products peak.

Field experiments have been carried out simultaneously at Davis and Watsonville to determine the efficacy of biofumigation in natural soils. These experiments have generally involved the burying of fungal propagules (Sclerotium rolfsii, Fusarium oxysporum f.sp. dianthi, Rhizoctonia solani, and Verticillium dahliae), nematodes (Tylenchulus semipenetrans [citrus nematode] and Heterodera schactii [cyst nematode]), and weed seeds (Amaranthus retroflexus [rough pigweed], Portulaca oleraceae [common purslane], Malva parviflora [cheeseweed], Convolvulus arvensis [field bindweed] and Poa annua [annual bluegrass]) at different soil depths to expose them to biofumigation or chemical treatments. At intervals of 2-6 weeks following treatment, the buried organisms were recovered to quantify survival. While results have shown a beneficial effect of biofumigation, the effect is inconsistent and efficacy does not approach that of metam sodium, the chemical treatment used as a control standard. In experiments done at Davis, tarping caused a solarization effect that dominated the treatments, although in some experiments a synergistic effect between solarization and Brassicaceae incorporation was detected. In the cooler coastal regions, a solarization effect is difficult to demonstrate, but in combination with Brassicaceae incorporation, a suppressive effect can sometimes be demonstrated. The inconsistency of biofumigation treatments is likely related to a general lack of knowledge of the factors influencing ITC volatization from tissues in soil.

Work has continued on methyl bromide alternatives. The project continues to study Brassicaceae for their ability to reliably produce ITCs in soil. Project team members have also done experiments with a USDA grant to study chemical alternatives. They have found iodomethane plus choloropicrin and metam plus telone C35 to be among the most consistently effective treatments in a variety of field locations.

A manuscript covering these biofumigation experiments will be submitted to Plant Health Progress.

[Editor’s Note: Three more SAREP-funded projects investigating sustainable alternatives to methyl bromide will be addressed in the next issue of Sustainable Agriculture, Vol. 15, No. 3.]