Replacement fumigants troubling

In the search for alternatives to methyl bromide, some progress is being made with drip-irrigation applications of soil fumigants, but regulation questions still loom, according to USDA agricultural engineer Tom Trout.

The fumigant, named an ozone-depleting compound by international treaty, is due for complete phase-out, other than for quarantine and other special uses, by developed nations by Jan. 1, 2005.

As a host of a recent field day at the new but yet-to-be-occupied USDA Agricultural Sciences Center at Parlier, Calif., Trout said he and other researchers on the trail of a methyl bromide replacement are developing more confidence each year.

“The efficacy of chloropicrin and other drip-applied fumigants continues to be pretty good, but the real unknown is the regulatory issues.”

That uncertainty of how materials, once they are found to be effective, will be regulated by governmental authorities means, he added, that several options, including biological agents and fallowing of farmland, must be kept open.

Trout said he and other USDA scientists are prepared for the long haul in the search for an alternative. “When USDA started this effort in 1995, I thought that by 2005 we would have worked ourselves out of business. Now, that's looking less and less likely all the time. I expect to be in the business of looking for an alternative for the next five to 10 years, at least.”

He noted that other USDA and university specialists have been looking for means of controlling nematodes, diseases, and other pests for decades.

Replant problems

Trout leads a team observing trials with alternatives for control of replant problems in peaches. The causes of the disorder are obscure, thought to be perhaps a complex of soil-borne pests remaining from previous trees on the property.

Common practice has been to fumigate soil with methyl bromide before replanting a new orchard. New trees on fumigated ground have been more vigorous and uniform.

In studies with USDA colleagues Husein Ajwa and Sally Schneider, Trout said they found that, as with several crops, fallow periods or crop rotations reduce target pests. Studies to quantify the benefits of fallow in a replant situation began in 1996.

Among the various combinations of fallowing, a year of fallowing did increase tree growth, although differences were not significant. Trout said tree growth after three years was nearly as good as with methyl bromide fumigation in other studies with plums but not in peaches.

In trials with drip-applied Telone formulations, they also recorded tree growth similar to that of methyl bromide treatments. They showed better growth than the non-fallowed checks.

Chloropicrin fed by drip lines into the soil showed the best growth of the series of treatments, confirming earlier research and recent grower trials. However, the tear-gas-like compound, Trout said, “is not a good nematicide and has not been used for orchard replant. This may indicate that there are important aspects of the replant problem that are fungal based.”

Although some indications have been seen, Trout said all of the studies need at least another four years to determine the effects on tree productivity. Among questions are the results of spacing between trees and vigor and possible overestimation of pre-production trees in relation to the replant problem.

In summing up the replant research, he said increasing fallow periods did reduce replant disorder, but even three years may not be sufficient to control the problem as well as methyl bromide.

Fallowing costly

“Fallowing is an expensive option for orchard crop growers, especially for peaches that are replanted an average of every seven years in California.”

Noting that drip-applied Telone has been effective and uses with chloropicrin are worthy of more study, Trout also said his team wants to know more about the causes of replant disorder.

In another study at the USDA center, Schneider, a USDA plant pathologist, leads a group in the search for methyl bromide replacements for vineyard use, particularly nurseries whose stock must be free of nematode pests.

The site for the trials was formerly planted to a nematode-infested, 85-year-old Thompson Seedless vineyard which was removed in the fall of 2000.

Before replanting the site with Thompson Seedless and Cabernet Sauvignon vines and Freedom rootstock, Schneider made shank and drip fumigations with methyl bromide, Vapam, chloropicrin, and several other compounds and combinations of them.

“Use of such a site for a nursery field trial represents one of the most extreme situations that a methyl bromide alternative might face,” she reported.

“Even so, some shank- and drip-applied materials appear to provide the necessary level of nematode control to the 5-foot depth required to meet nursery certification.”

Six weeks after treatment, Schneider collected soil samples at the 1-foot depth and found citrus and rootknot nematodes predominant. Plants will be harvested in the winter of 2002 and roots analyzed for nematode control.

Further testing is needed, she said, “to determine the minimum planting interval following treatment to avoid phytotoxicity for each material and grapevine variety or rootstock.”

Plans are, she added, for nematode populations and plant growth to be monitored for another four years to evaluate the performance of the several fumigants.

In a non-chemical approach to nematode control on fruit tree rootstock, Becky Westerdahl, University of California, Davis Extension nematologist, has been monitoring trials using hot-water treatments. Similar treatments are used for daffodil and garlic planting stock.

At the USDA center, she planted in 1997 six rootstocks, including Lovell, Myrobalan 29C, and Paradox, that were treated for five lengths of time in water at temperatures ranging from 110 degrees to 130 degrees to establish baselines for thermal tolerance.

Thus far, said Westerdahl, “we have seen no evidence of reduced survival or vigor that could be attributed to fungal infestation that has typically been a complicating factor in hot water treatment of bulb crops.”

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