As methyl bromide alternatives tested

Drip and micro-sprinkler system delivers fumigants USDA scientists have developed a method to use drip irrigation and micro-sprinkler systems to deliver fumigants other than methyl bromide to replanted orchards and vineyards.

Tom Trout, agricultural engineer at the Water Management Research Laboratory in Fresno, reported on his work during a field day at Parlier showcasing projects aimed at finding alternatives for methyl bromide due, to phased out of use by 2005.

Methyl bromide fumigation, with the conventional shanked and tarped method, has long been applied prior to replanting of tree and vine crops to control nematodes and diseases.

It also minimizes the so-called "replant problem," blamed on the nematodes and fungi that remain with roots of the previous crop mostly in the top two feet of soil, and poor soil qualities.

In some crops, fumigation causes growth-stimulating shifts in soil microbial populations. Scientists say other, unknown biological factors also come into play in the replant problem.

Trout and Husein Ajwa, soil scientist at the lab, used emulsified formulations of Telone C-35 as a nematicide and chloropicrin as a fungicide injected through drip and microsprinkler lines.

Their work with trees and vines is an expansion of earlier research using drip lines to apply fumigants to strawberries, another crop that depends heavily on fumigation to control soilborne pests.

The application technique was used on peaches, grapes, and plums from 1996 to 1999 in test plots and was used in 1999 and this year in a grower's field.

"This application method," Trout said, "is economically viable for fields set up for micro-irrigation systems. Many California growers convert to micro-sprinkler irrigation when they replant an orchard, or to drip irrigation for new vineyards."

Telone and/or chloropicrin are applied through the drip tape buried about a foot in the tree or vine row. Ideally, the soil is dry and will absorb 3 to 6 inches of water to carry the fumigants down 4 to 5 feet.

Before the drip system is activated, micro-sprinklers apply about 3/16-inch of water to create a 3- to 4-inch seal and prevent the fumigants from escaping.

"Application to this depth," Trout said, requires a soil with reasonably high infiltration, one that can absorb the required irrigation water in less than 24 hours."

Following the application, the micro-sprinklers, with Vapam injected, are turned on again to apply another 3/16-inch to treat the soil surface for pathogens and weeds. Following the treatment, the same sprinklers can be used to irrigate the orchard, although they may be removed during the planting.

Although pumping, filtration, and water delivery systems are used, addition of proper backflow prevention devices and chemical injection points may be needed.

The method requires uniform water application and a water table deep enough to insure the chemicals break down completely before reaching groundwater.

Trout estimates the costs of the application system, mainly for the irrigation drip tubing and installation, in the range of $300 to $400 per acre, or near the costs of plastic tarps used for conventional shank fumigation. After the fumigation, the tubing is unused and left in place.

Although Telone performed well in this and other trials, it is subject to caps, or limits, on use by townships in California.

Replant difficulties In another project under way at USDA's test plots at Parlier, Greg Browne, USDA plant pathologist at Davis, is leading a team project to learn more about the unknown elements of the replant problem in stone fruit. These have become more critical with the phase-out of methyl bromide and the search for alternatives.

"We have a real haystack here, but we have found a few needles," Browne told visitors at the field day. He went on to say samples of soils and fine feeder roots are being analyzed and reveal useful information.

Partial root excavations showed that fumigation with methyl bromide or alternatives caused large increases in root growth.

In one trial, 7,245 pin nematodes were found in a sample of non-fumigated soil, but 4,005 remained in another sample after methyl bromide fumigation.

Some of Browne's initial tests identified fungi pathogenic to peach, and additional tests are being done. Greenhouse tests are screening cover crops that suppress the diseases in hopes cover crops might benefit management of the problem.

Trout, a collaborator with Browne, noted that fumigation doesn't kill all soilborne organisms and can shift the abundance of one species or another.

Plant pathologist Cynthia Eayre with USDA in Fresno is researching methyl iodide and said it offers control of replant disorder on peach comparable to that of methyl bromide.

Tomen Agro, she said, is pursing registration of methyl iodide and anticipates four years of trials, followed by one year on EPA's "fast-track" schedule.

Eayre is also working with plant growth promoting rhizobacteria on peaches and strawberries. Although the exact mode of action is not known, a hormonal effect is suggested, and the material may also produce antibiotics that work against harmful organisms.

Grape nematodes Sally Schneider, USDA nematologist in Fresno, is investigating replant disorder in grapes. Her initial trials are on a two-year-old vineyard of own-rooted Thompson Seedless and Merlot on two different rootstocks. They were replanted on land at the USDA farm near Parlier where an old Thompson vineyard was pulled.

Telone-Vapam combinations and methyl iodide have controlled nematode populations as well as methyl bromide to date, she said. Fallow treatments of 18 months also reduced the number of rootknot nematodes.

She found that rootknot resistant rootstock reduced rootknot populations but not citrus nematodes.

"In the short term," she said, "novel applications of currently available chemicals seem to be the most likely alternatives to methyl bromide.

"These will serve as stepping stones during the transition to an integrated systems management approach. That will be based on an understanding of the interactions and spatial variability of biological, chemical, and physical factors of the agro-ecosystem."

That system, she explained, will use various practices to curb pests, enhance beneficial organisms, promote good plant growth, kill old roots that harbor pests, and protect the environment.

Summarizing the several projects, Trout said, "We see promise in some of the alternative fumigants we are looking at. Telone and chloropicrin can do a just as good a job as methyl bromide in some cases.

"The long-term objective is to find some non-fumigants or less chemical alternatives, but we have a ways to go with that.

Although a couple of new chemicals have potential, he said, they are at least five years away from commercial release, so the growers are going to use a mixture of practices that work for them with available chemicals.

"Even though 2005 is the deadline for methyl bromide, that doesn't mean we will be finished looking for alternatives," Trout said.

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