View From The West: Fumigant Restrictions Challenge Growers

Charcoal Rot On Strawberries

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California agriculture can be viewed as a model for developing agricultural practices, fashioning innovative farm systems, and solving problems for farmers. Currently, California can also serve as a model and example of the struggle for agricultural survival in the face of change.

Changing availability of pre-plant soil fumigants and the regulations governing their use will challenge many California industries to make adjustments and to remain productive and profitable. California commodities such as strawberry and field-grown flowers have a history of success due in large measure to pre-plant fumigation with methyl bromide and chloropicrin. The impending total loss of methyl bromide by 2015, coupled with increasing use restrictions on the remaining fumigants, will make control of soilborne pests more difficult for many high-value crops.

The list of threatening soilborne issues is daunting. Perhaps most damaging are the soilborne pathogens that cause a wide range of root and crown rots, vascular wilts, and other diseases. Soilborne nematodes not only cause growth and yield reductions but can also harbor viruses that further damage some crops. Changes in the availability of soil applied fumigants can also affect a farmer’s ability to manage soilborne insects and weeds.

Less Effective Fumigants

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Increased production problems associated with reduced use of methyl bromide and more stringent regulations on soil fumigants are already evident in the field. Some California strawberry growers have made the switch away from methyl bromide combinations to less effective fumigant products. In some of these regions, soilborne diseases new to the state are appearing and reducing strawberry yields. Thus far, a strong correlation exists between bed fumigation with alternative chemicals and the occurrence of new soilborne issues such as charcoal rot (caused by Macrophomina phaseolina) and Fusarium wilt (caused by F. oxysporum f. sp. fragariae).

Changes in fumigation practices for one crop may also have implications for others. For example, in coastal California lettuce growers plant lettuce in ground that was planted to conventional strawberry the previous season; in such a rotation, the strawberry planting was usually placed into broadcast fumigated soil. The subsequent lettuce crop is often vigorous and disease-free due to the carry-over benefits from that fumigation. However, researchers recently discovered that the Verticillium wilt pathogen that damages strawberry also infects lettuce. If future soil treatments are less effective at controlling strawberry Verticillium wilt, then higher populations of this soil fungus will remain to infect and damage the lettuce crop that follows.

Future Changes

What will be the strategy for the future and how will these agricultural industries deal with such changes? First, resistant or tolerant cultivars are essential. There is an increased urgency to develop plants with genetic traits that can withstand these soilborne pathogens. Strawberry breeding programs in California have made substantial progress in developing cultivars that are tolerant to Verticillium wilt; additional resistance factors are now needed for Macrophomina and Fusarium. Second, growers will need to rely more heavily on established integrated disease management principles such as rotating to non-host crops, avoiding back-to-back planting of susceptible crops, and placing crops in sites not known to harbor the pathogens.

Clean Equipment

Another important management principle is the cleaning of equipment. Soilborne pathogens and parasites such as Fusarium, Verticillium, Macrophomina, and plant parasitic nematodes are spread to uninfested fields by equipment and vehicles contaminated with infested soil. Although washing equipment and vehicles is bothersome and time-consuming, such measures help slow down the spread of these infectious pests.

The use of highly effective pre-plant soil fumigants had previously allowed growers to skip or de-emphasize some of these measures. However, the potential loss of fumigants and emergence of new soilborne pathogens will force the industry to revert to these basic practices. Third, alternative fumigants will have to be applied at high rates under retentive plastic tarps to enhance the effective dose required to control soil pests. The challenge here is that some counties limit application rates to levels that are not efficacious to control these targets.

Agricultural commodity groups, state agencies, and other agricultural industries need to invest in research to cope with emerging agricultural problems and to develop additional pest control options for growers. In this regard, California again serves as a model. Strawberry, leafy greens, peach, pistachio, tomato, and other California industries have long supported research and thereby invested in their own futures. Innovative research efforts such as the California strawberry industry’s “farming without fumigants” program are needed to equip California growers for these future challenges.

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