Fungicides that can be applied to tree fruit in storage by a process called thermofogging have been registered in the United States, and the registrant, Pace International, LLC, of Seattle, Washington, is testing them in commercial settings. Thermofogging (the aerosol application of chemicals) has a number of advantages over drenching as a method of applying antioxidants or fungicides, Dr. Peter Sanderson, Pace’s director of technical services, explained during the Washington State Horticultural Association’s convention. Typically, fruit is drenched upon arrival at the packing house, before it is put into storage. Thermofogging is rapid, there is no waste solution to dispose of, and it avoids the problem of pathogens building up in the drenching solution. Another advantage is that chemicals can be reapplied while the fruit is in storage.

Thermofogging was developed more than 20 years ago by the French company Xeda International S.A. The technology involves heating droplets of the chemicals and shearing them into small particles less than a micron in diameter that diffuse throughout the storage room and bins.

Initially, thermofogging was used to apply antioxidants, such as diphenylamine, to control superficial scald. Now, the method is used also to apply fungicides for decay control. It is widely used in Italy and France.

Trials

Xeda conducted the first trials in the United States with its thermofogging system in 1989 in New York. Sanderson said it worked well for applying DPA and ethoxyquin. In 2002, Pace began trials in Washington State and in Chile on apples and pears, using the Xeda system. It found that antioxidants applied by thermofogging provided as good control of scald as drenching.

In 2005, Pace received a registration from the U.S. Environmental Protection Agency for the Xeda product Xedamine A, containing 10 percent DPA, and changed the name to EcoFOG100. That year, the company treated 12 storage rooms (22,000 bins of fruit) in Washington State and smaller volumes in New York and Ontario, Canada. The following year, 30 rooms in Washington (61,000 bins) and 11 rooms in the Northeast were treated. In the Washington trials, fruit in six of the rooms showed symptoms of phytotoxicity.

In 2007–2008, Pace expanded its treatments in the eastern United States, where the method had proven successful, but decided to retrench in the West and find out what was causing the problems there.

The company took several steps to improve the process. It modified the equipment to produce smaller particles that would act more like a fog and disperse more easily, and, thinking that condensation on the fruit might be part of the problem, the company also developed tools to sense wetness on top of the bin stacks.

Sanderson said it also became apparent there was a problem of scale. Most storage rooms that had been treated in Europe and the eastern United States contained fewer than 875 bins, whereas storage rooms in Washington were much larger. The surface-to-volume ratio of a large room is significantly lower than in a small room, but more chemical is applied in larger rooms because the amount is based on the volume of fruit to be treated. Higher residues on the fruit could be a cause of phytotoxicity.

The particles of chemical applied by the thermofogging machine are warmer than the ambient temperature in storage and tend to rise to the top of the room and then disperse, Sanderson explained. However, some larger particles can fall onto the fruit in the top bins. Last year, Pace conducted experiments in large storage rooms in Chile to test the effect of covering the bins with tarps. The company now feels strongly that covers are necessary to protect the fruit on the top layer, he said.

Pace, which has a technology-sharing agreement with Xeda, received registration in the United States last June for Xedaquin A, a formulation of ethoxyquin designed to be applied through thermofogging. Pace is renaming the product ecoFOG180.

In September, it received registration for the fungicide Xedathan A, containing pyrimethanil (Penbotec), which it is naming ecoFOG160, for control of postharvest diseases including gray mold and botrytis. Five rooms were treated in Washington last fall. Sanderson said the amount of decay would be analyzed when the rooms are opened up.

"We want to continue to improve the process," he said. "We’re hoping to look for new fungicides for disease management to cover the disease spectrum and are interested in looking at new chemistries."