Greg Browne and Ed Civerolo, Research Plant Pathologists, Crops Pathology and Genetics Laboratory, USDA, ARS, Davis, CA 95616; and Becky Westerdahl, Extension Nematologist, Dept. of Nematology, and Dave Rizzo, Assistant Professor, Dept. of Plant Pathology, University of California, Davis, CA 95616.

Methyl bromide is widely used in California as an effective preplant fumigant to control soilborne pests that affect deciduous tree fruits and nuts and grapevines. Depending on the particular commodity, orchard and vineyard lifetimes typically exceed 1-5 or more decades, and the establishment costs are relatively high. Therefore, sustained and profitable production of these crops requires prudent preplant preparations and postplant pest management strategies.

The pests of concern include a broad range of weeds, nematodes, insects, and fungi. Many of the pests and their effects and biology are well documented. For example, several species of plant parasitic nematodes and pathogenic soilborne fungi are commonly encountered in California agriculture, especially when orchards or vineyards are replanted at old sites previously devoted to the crops. These pests can severely debilitate and, in many cases, kill trees and vines. Most investigators believe that additional factors or pests (sometimes referred to as the "replant problem"), although not well documented or understood, operate at "replant" sites and reduce vigor and productivity of the perennial crops unless preplant precautions, such as methyl bromide fumigation, are taken.

At USDA-ARS and University of California (UC), Davis, scientists are working cooperatively and independently on development of alternatives to preplant fumigation with methyl bromide. Both chemical and nonchemical alternatives are under investigation, and the target problems include well-documented "acute" pests, as well as the more "chronic" and poorly understood general replant phenomenon.

Work led by Dr. Greg Browne, research plant pathologist with USDA-ARS, Davis, emphasizes genetic and cultural alternatives to methyl bromide for control of Phytophthora root and crown rots of deciduous trees and vines. With walnuts, Browne is evaluating promising clones of paradox (J. hindsii × J. regia) rootstock as well as selections and hybrids of Chinese wingnut that offer hope for superior resistance to Phytophthora spp. The evaluations are the next step in testing "elite" paradox rootstock clones that were preserved from previous screens for resistance to Phytophthora spp. led by ARS research plant pathologist Dr. John Mircetich (retired). UC pomologist and walnut breeder, Dr. Gale McGranahan, has backcrossed selections of paradox with English walnut to obtain tolerance to cherry leafroll virus, and Browne's group will determine relative resistance of these paradox backcross clones to Phytophthora spp.

While a student with Mircetich, Dr. Michael Matheron determined that Chinese wingnut is highly resistant to the most damaging Phytophthora spp. that affect walnuts in California. Because the wingnut selections evaluated are graft compatible with only some commercial English walnut varieties, additional research is needed to determine if broad graft compatibility can be combined with the resistance to Phytophthora spp. in an improved wingnut rootstock. To this end, Browne's team is screening additional wingnut selections and attempting hybridization of wingnut with English walnut. Improved genetic resistance to Phytophthora spp. is also needed in rootstocks for commercial Prunus spp. and grapevines. Phytophthora root and crown rots sporadically cause devastating losses in almond and peach plantings. Due to Phylloxera, the grape industry is shifting toward use of rootstocks, some of which may be relatively susceptible to Phytophthora diseases despite their resistance to the root aphid. In collaboration with plant geneticists, Dr. Craig Ledbetter, research horticulturist, USDA-ARS, Fresno, and Dr. Andrew Walker, professor of viticulture, UC Davis, Browneís group is seeking improved resistance to Phytophthora spp. in germplasm of Prunus and Vitis.

To evaluate a cultural alternative to methyl bromide for control of Phytophthora root rot of walnuts, Browne is cooperating with Dr. Robert Hutmacher, research plant physiologist, USDA-ARS, Fresno, to test microsprinkler water management strategies in a commercial orchard setting. For control of southern blight in apples, UC Farm Advisor Joseph Grant and Browne are evaluating chemicals and the biocontrol agent Gliocladium virens GL-21 (SoilGard^TM) as alternatives to methyl bromide + chloropicrin. In year one of the experiment, metam sodium performed as well as the methyl bromide/chloropicrin mixture for control of the disease at tree replant sites.

Browne is also seeking a better understanding of general replant problems that do not necessarily kill plants but do suppress general vigor and productivity of young trees and vines. In cooperation with J.M. Duniway, professor of plant pathology, UC Davis, Browne will determine the role of fungal and bacterial components of the rhizosphere soil microflora in replant problems with grapevines and deciduous fruit and nut trees. Browne and Duniway will determine if important effects of preplant fallowing, soil amendments, and chemical treatments may be mediated through rhizosphere microflora.

Dr. Becky Westerdahl, extension nematologist, UC Davis, is continuing evaluation of sodium tetrathiocarbonate (STTC, trade name Enzone), which is a candidate alternative to methyl bromide for control of plant parasitic nematodes in deciduous tree crops. STTC, which releases carbon disulfide in soil, offers potential as a postplant as well as a preplant treatment. To date, pre- and postplant STTC treatments have reduced populations of ring, root knot, and lesion nematodes on selected crops. In cooperative research supported in part by USDA-ARS, Westerdahl will determine efficacy of alternative hot water treatment strategies for eradication of nematodes on deciduous tree nursery stock. The cooperative support will also foster Westerdahl's continuing evaluations of chemical alternatives to methyl bromide for preplant nematode control, including STTC, formulations of a nematicidal fungal toxin from Abbott Laboratories, and ozone.

In another project on alternatives to methyl bromide for tree crops, USDA-ARS is working cooperatively with Dr. Dave Rizzo, assistant professor of plant pathology, UC Davis. Rizzo is studying the biology and control of Armillaria root rot (oak root fungus disease) in California. In the cooperative research, Rizzo is evaluating pre- and postplant Enzone treatments as an alternative to methyl bromide for control of Armillaria in pears. Other lines of investigation include the effects of soil-water management on disease expression; this includes a comparison of flood- and sprinkler-irrigated orchards.