Department of Plant Pathology, University of California, Davis, CA 95616, USA

Background and objectives
Methyl bromide is thought to be a significant stratospheric ozone-depleting chemical, and therefore will soon be phased out in the USA and internationally [1]. The research reported here is part of a continuing project supported by the California Strawberry Commission and ARS-USDA to find chemical and non-chemical alternatives to methyl bromide for pre-plant fumigation of soil in strawberry production, as practised in California [2].

Results and conclusions
Chemical alternatives to methyl bromide have been tested for several years in replicated field experiments. For example, in experiments with broadcast fumigation, total berry yields, relative to those (100%) obtained following standard fumigation with methyl bromide/chloropicrin (67/33% @ 364 kg/ha), were 94-96% with chloropicrin alone (336 kg/ha), 98-108% with Telone 11 (1,3 dichloropropene)/chloropicrin (70/30% @ 500 kg/ha), and 57-70% on non-treated soil. The incidence of plants with recognizable diseases (e.g. Phytophthora root and crown rots, Verticillium wilt) or collapse of unknown aetiology was low in these experiments, and yield differences were due largely to overall differences in plant growth and vigour. At a separate site where populations of Verticillium dahliae are high, fumigations confined to preformed beds required less chemical and were nearly as effective as those applied in a broadcast mode. While the chemical fumigants above, as well as methyl iodide, were generally effective in bed applications, chloropicrin did not reduce V. dahliae populations in soil as fully when used alone as when used in combination with methyl bromide or Telone. Experiments at this site also demonstrated that without fumigation, Verticillium wilt can cause total losses. The methodologies needed to practically employ most of the chemical alternatives to methyl bromide are not fully developed. Therefore research is continuing to optimize soil fumigation methods, including use of bed applications, high-barrier films, lower rates, emulsion delivery, and other chemical materials.

Fumigation with mixtures of methyl bromide (or some chemical alternative) and chloropicrin effectively controls V. dahliae, Phytophthora species, and most other major pathogens of known importance in soil. However, soil fumigation can nearly double strawberry yields even when known pathogens are not present, and we are researching mechanisms of this general yield response to fumigation. Relative to non-fumigated soils, total numbers of fungi are usually low for several months following fumigation, while total numbers of bacteria remain high with a heavy representation of Pseudomonas species. These differences in soil populations also occur, with some modifications, in the rhizosphere of growing strawberries. We are currently testing the effects of rhizosphere and root-associated microbes from fumigated and non-fumigated soils on the growth and root health of strawberries.

A variety of non-chemical alternatives to soil fumigation with methyl bromide have been tried. Crop rotation, in comparison to continuous strawberry, can be beneficial. For example, 1-year rotations with rye or broccoli increased subsequent strawberry yields 18-24% at one location, but had no beneficial effect at a second location. Furthermore, these 1-year rotations did not reduce Verticillium wilt significantly. Soil solarization has limited prospects because of a foggy coastal climate at the time treatment is feasible. Various organic amendments and approaches have been tried by others, but results are variable and none gives a consistent yield response approaching that obtained by soil fumigation. Although additional non-chemical approaches need to be tried, it is unlikely that non-chemical methods of soil treatment can fully duplicate the level of pathogen control and yield enhancement obtained with pre-plant fumigation. Furthermore, the use of effective soil fumigants for the production of quality runner plants in nurseries will remain critical to profitable strawberry production, regardless of the type of management approach finally used in berry production fields.

1. Ristaino JB, Thomas W, 1997. Plant Disease 81, 964-977.
2. Wilhelm S, Paulus AO, 1980. Plant Disease 64, 264-270.