Apr 15, 2011

Molasses a replacement for methyl bromide?

As they search for soil treatments to replace methyl bromide, Agricultural Research Service (ARS) scientists in Fort Pierce, Fla., are using one technique that sounds as if it were borrowed from a family recipe: Taking molasses and heating it up.

Farmers have been using methyl bromide since the 1930s, but these days it is subject to strict environmental controls because it depletes Earth’s ozone layer. As part of an international agreement, growers worldwide are required to find a replacement, a tall order because methyl bromide eliminates a broad spectrum of pests, including weeds, nematodes and plant pathogens. The mandate is particularly challenging in Florida, where sandy soils limit organic alternatives and the mild winters serve as a safe harbor for many of nature’s most pernicious pests. Nutsedge is a particular weed problem in Florida and is a major reason methyl bromide is still used there.

“We don’t usually see the cold winter temperatures that you have in other places, so pests aren’t killed off here by Mother Nature the way they might be elsewhere,” said Erin Rosskopf, a microbiologist at the U.S. Horticultural Research Laboratory in Fort Pierce.
For Florida growers, the stakes are enormous. Fresh-market tomatoes and bell peppers, both raised in Florida with methyl bromide, netted growers there a combined $889 million in 2008. Caladiums, an ornamental also produced largely with methyl bromide, are a $15 million crop in Florida.

In their search for alternatives, ARS researchers are studying two approaches: a biologically based cropping system that builds on success overseas, and two recently developed fumigants. Similar approaches have had some success elsewhere, but the question is whether they will be effective in Florida with its unique soils, climate and pest pressures. Preliminary results are promising, but the researchers still need to evaluate whether the biologically based treatments can sufficiently control nutsedge.

Biological alternative

At the Fort Pierce lab, Rosskopf, ecologist Nancy Kokalis-Burelle and soil scientist David Butler are raising bell peppers followed by eggplant in a field to test a biological approach that uses a combination of composted broiler litter, molasses and anaerobic soil disinfestation (ASD). Lab colleagues Gregory T. McCollum and Joseph Albano are evaluating fruit quality and soil nutrients.

In ASD, a carbon source – in this case, molasses – is added to stimulate microbial activity, and the soil is covered with a clear plastic tarp. The topsoil is saturated with water and allowed to heat. The sun-drenched tarp “cooks” the weed seeds in the soil, and the carbon and water increase microbial activity and create anaerobic conditions conducive to pest control.
In this study, before heating the soil, the researchers treated plots with different levels of organic amendments: with and without poultry litter; with and without molasses; and with 2 inches of water per acre, 4 inches per acre or no water at all. The poultry litter increased soil moisture and added organic matter, and the molasses provided a readily available carbon source for soil microbes. The molasses they used is a waste product of the sugarcane processing industry. The researchers are also conducting studies to evaluate mustard meal, sorghum and cowpeas as possible alternatives to molasses.

They planted peppers in the fall and eggplant in the spring for two years, using the type of raised-bed production system common in Florida. Before planting, the researchers introduced Phytophthora capsici to the fields, an oomycete that causes crown rot and root rot, so they could track control rates. They buried packets of it 6 inches deep in the soil and retrieved them after three weeks of ASD to evaluate the effects. They also sampled the soil for nematodes, counted the number of nematodes extracted from crop roots, assessed weed populations and soil properties throughout the trials and measured crop yields.

They found that at depths of 6 inches, the ASD treatments heated the soil temperature to about 113˚ F, which was at or just below lethal levels for many soil pathogens. They also found that nematode populations were reduced when treated with molasses and poultry litter, that molasses and poultry litter controlled grass weeds just as well as methyl bromide did, and that the ASD treatments controlled P. capsici in the buried packets as well as methyl bromide did, regardless of how much water, poultry litter or molasses was applied.

“The nutsedge pressure at the test location wasn’t really adequate to show treatment differences, so we are still investigating that component,” Rosskopf said.

By Dennis O’Brien, Agricultural Research Service