Metam Looks Good

Metam Looks Good

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Fumigation, which has long been a common pest management practice for sweetpotato growers, has also represented a thorny problem. However, research conducted by Scott Stoddard, a University of California Cooperative Extension vegetable crops farm advisor, has revealed that growers do have cost-effective options. Unlike previously thought, recent field trials conducted in the San Joaquin Valley have shown that the method of shanking in metam sodium or metam potassium is effective in reducing nematode populations in sweetpotato fields.

Growers’ fumigation options have changed many times over the years. Methyl bromide is effective, but it’s become far too expensive to use in the field. Growers still use it in their hotbeds for intensive transplant production (See “Hotbeds Can Be Cool”) where they have critical use exemptions under the Montreal Protocol, but it doesn’t pencil out to use in the field.

Telone (1,3-dichlorpropene, Dow AgroSciences) has long represented a great option for production fields, but production and regulatory restrictions have made its use more complicated in the past few years. First, the California Department of Pesticide Regulation restricted use by implementing “use caps” for the areas in the state with the greatest need.

Second, more recently the nationwide recession has reduced production of the active ingredient, 1,3-dichlorpropene. Telone is a by-product of the resin manufacturing process. Those resins weren’t produced so much in recent years because Americans’ demand for products that utilize them, including boats and campers, declined. “But even with full production, the demand for Telone far exceeds what the use cap will allow,” says Stoddard.

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Two-Pronged Approach

Metam sodium (Vapam, Amvac Chemical Corporation) and metam potassium (K-Pam, Amvac Chemical Corporation) are liquid soil fumigants that have proved to be good herbicides, but were not as effective on nematodes, especially root-knot nematodes, the species that causes the most problems for growers. In the past, metam products were either flooded or sprinkled onto fields, and Stoddard wondered if by changing application methods, he could improve efficacy.
He set up a trial in a commercial field with numerous different treatments involving metam products as well as Telone both alone and in combination with different rates. All the treatments were applied with a specialized fumigation rig built by Crop Production Services. As one might guess, the untreated control had the highest nematode count at harvest, and also the lowest yield, with production at just below 30,000 pounds of marketable sweetpotatoes per acre. The treated plots produced from close to 50,000 pounds to 65,000 pounds per acre.

The biggest surprise from the trial were the metam products. “The question was whether they could be efficacious and yes, they are efficacious,” he said. “To be able to just go and shank them in as you do Telone is a great improvement in boosting application efficiency as compared to flood or sprinklers.”

The best treatment of all, with just more than 65,000 pounds of marketable yield per acre, was Telone combined with metam sodium. Not only were the results excellent, but it’s a very grower-friendly process. “You can get a lot of acreage done in a short period of time,” says Stoddard. “And avoiding all the attendant potential safety problems with other application methods is great.”

Hotbeds Can Be Cool

For three years now, Scott Stoddard has been conducting a trial funded by USDA-Agricultural Research Service exploring methyl bromide alternatives for sweetpotato hotbeds. One consequence of the research was it required the use of cold-bed production techniques so as not to disturb the integrity of the fumigation treatments. In the process, the University of California Cooperative Extension farm advisor wondered whether hotbeds, which essentially serve as the growers’ nurseries to raise transplants, really need to have warm soil.
Hotbeds are normally constructed by digging a trench and laying in cotton gin waste or some other compost, which produces heat in decomposition. This warms the seed roots from below, which improves early plant growth — an important consideration because hotbeds are normally installed in the very early spring. But often the transplants need to be trim-med back before they are transplanted because they have grown too large.
Stoddard wondered if by skipping the initial step and simply producing transplants in cold beds, growers could have viable plants in time for transplanting. His trials proved they could. “Cold beds are much simpler and quicker to install, so they are more economical while still giving excellent plant production,” he says.
In California, the seed has traditionally been planted in early February, so the additional heat was needed to warm up the cool soil. But if growers simply waited until late February or early March when the soil was warmer, they could plant into a cold bed. Transplant production and size are often just as good, and there could be a reduction in trimming. “A lot of things are done in life simply because we’ve always done it that way,” he concludes. “A few hotbeds, with the majority cold, would be a nice, economical compromise.”
His hotbed research also revealed that when it comes to pests, weeds are the real concern; insects and nematodes are not really a threat to the young plants. “It’s a time factor,” he explains. “In spring in the hotbed there either isn’t the time for nematodes to infect the roots, or the numbers are too low by the time the plants are cut. Even if nematodes hit the roots they are late in coming to the game, so to speak.”

 

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