Researchers in the U.K. College of Agriculture have a new method for sanitizing foam tobacco trays. In recent experiments, trays contaminated with plant disease pathogens were wetted, stacked, then placed in a special insulated chamber. An electric heating system raised chamber temperature to a high degree for varying lengths of time. After treatment, plants were grown in the trays for eight weeks then examined. Results were encouraging.

"Of three temperatures we tried - 145, 160, and 175 degrees Fahrenheit - and several different lengths of time, we found that 175 degrees had significant effect in sanitizing the trays," said George Duncan, Extension agricultural engineer. "The chamber system appears at this early stage of research to be a potentially promising alternative to other methods."

Why is tray sanitation necessary? Even with the best washing practices, some microscopic pathogens remain in cracks and crevices, awaiting next season's warm float bed environment to rejuvenate. There lies the problem - used trays often are contaminated with plant diseases. Proper cleaning is crucial.

"Until a cost effective substitute arrives, farmers often have no choice but to re-use trays," said Bill Nesmith, Extension plant pathologist. "That means disease pathogens must be removed before the next plants go in."

During the past few years, producers have used a number of different sanitizing methods. For example, several commercial operations have used steam. Although it's not considered convenient by many farmers, steam has proved to be effective. Some producers have favored fumigation with methyl bromide, an effective method with careful and safe use. But methyl bromide is being phased out under EPA regulations and is not considered a long-term solution.

Many farmers have used chlorine bleach. Bleach has performed nearly as well as fumigation or steam, and is widely used. But success with it has been highly variable because of the wide range of use patterns.

"The advantage of heat over bleach is that heat will be more reliable because it is applied more uniformly to trays," said Nesmith. "With bleach there's a lot of variation from tray to tray when washing and dipping. Bleach also presents safety concerns if not applied properly and with caution."

One of the challenges of developing a high-heat system for sanitizing trays is that foam trays will curl and even melt at temperatures above about 186-190 degrees Fahrenheit. The goal of Duncan, Nesmith, and research technician Billy Tapp was to find a temperature high enough to kill disease pathogens residing on or in the trays, but low enough not to deform the trays.

"No treatment will completely eliminate disease residues," said Duncan, "but in our heat chamber experiment we found that 175 degrees for at least 15 minutes was effective in reducing disease potential without damaging the trays. The 175 degree treatment for 60 minutes was consistently the most effective," said Duncan.

According to Duncan, UK agricultural engineers are working on a chamber design that will be cost effective for producers wanting another method for sanitizing tobacco trays.