Producers of nursery and ornamental crops have relied on methyl bromide for many years to control weeds and soilborne pathogens in ground beds and mixes of potting media. For some diseases, such as Fusarium wilt of carnation, methyl bromide has been the primary means of control for decades. Development of alternatives for the diverse crops and horticultural practices inherent in the nursery and greenhouse industries is a significant challenge.

James MacDonald, a plant pathologist with the University of California (UC), Davis, and several colleagues have been experimenting with potential alternatives to methyl bromide for Fusarium wilt. This wilt, caused by the fungus Fusarium oxysporum f.sp. dianthl, is the most devastating disease of carnations. MacDonald's collaborators are Manuel Lagunas-Solar, UC-Davis; Steve Tjosvold, a farm adviser from Salinas, CA; Howard Ohr, UC-Riverside; and Ian Green Salinas Flower Grower's Co-Op, Salinas.

"We're investigating both chemical and nonchemical alternatives," MacDonald says. One approach of particular interest to MacDonald is soil heating. Known to be an effective, nonchemical means of killing soilborne pests and pathogens, heat "is an approach particularly well suited to ornamental production because of the high value of the growing media and the crops," says.

MacDonald and colleagues have been experimenting with steam and electronic processes for soil heating. "The nursery industry has a long history of using steam," he says. "It's a technology they're comfortable with."

However, steam doesn't heat ground beds deeply enough to eliminate Fusarium from carnation's root zone.

"Some growers buried steam manifolds a foot deep in the soil but still didn't achieve effective control. We experimented with buried manifolds, but found that this method doesn't uniformly heat the soil," MacDonald states.

Another method he is using is called ohmic heating. This involves passing electrical currents between an anode and a cathode in the soil, a process which generates heat by the soil's resistance to the flow of the current. MacDonald and colleagues use rows of steel rods, driven into the ground on either side of an empty bed, as anode and cathode arrays.

According to MacDonald, they have been able to heat the soil and kill Fusarium at a depth of 3 feet by driving these rods that depth into the soil.

In addition to soil heating, they've tested surface applications of methyl bromide (to represent current practice), methyl iodide, and Basamid. "In our experiments, methyl iodide provided better control than methyl bromide," MacDonald says. However, efficacy of all chemicals appeared limited to the upper foot of soil. "Ohmic heating was the only treatment that killed Fusarium spores to depths greater than 1 foot."

Some carnation growers are converting their operations to raised beds to escape the fungal spores surviving in the soil. While Fusarium wilt can occur in raised beds, the relatively shallow, small soil volumes are more easily treated by steam. But, MacDonald says that disease outbreaks in steamed beds are not uncommon. The pathogen probably gets reintroduced through workers or airborne spores. "I've recovered the pathogen from particles of dust carried on air currents within the greenhouse," he says.

One concern with steaming potting media or raised beds is that it's difficult to control temperatures with steam, so soils may be overheated, eliminating beneficial organisms as well as pathogens.

"The absence of competing organisms can allow pathogens to readily recolonize soils," notes MacDonald. For this reason, he is collaborating with James Locke, ARS, to evaluate potential biological controls. Locke is with the ARS Floral & Nursery Plants Research Unit in Beltsville, MD. MacDonald plans to incorporate Locke's "biologicals" into raised beds after steaming to try to prevent the fungus from re-establishing.

USDA, the California Cut Flower Commission, and the California Association of Nurserymen are funding this research. MacDonald and colleagues are just entering their second year of field trails and are still analyzing data from the first year of research. Preliminary results show the following:

• Ohmic heating was the only treatment that killed fungal spores to a significant depth in the soil.
  
  
• Surface-applied fumigants killed the fungus only in the uppermost parts of the soil.
  
  
• Methyl iodide appears more effective in surface applications than methyl bromide does.