Fundamentals of fumigation

Soil fumigation is the standard control measure for apple replant disease in South Africa. By Anna Mouton.

Fumigation is tightly regulated, as incorrect practices pose a safety risk, and growers have to employ one of the several licensed contractors to fumigate their soils for apple replant. Fresh Quarterly spoke to Cole Stanton, managing director of specialist soil-fumigation company Draslovka Services, to learn more about this key control measure.

Fumigant formulations typically contain chloropicrin and 1,3-dichloropropene. Chloropicrin is a broad-spectrum antimicrobial and fungicide. It also acts against some nematodes and some insects. The primary role of chloropicrin in soil fumigation is to eliminate disease-causing fungi and oomycetes.

Chloropicrin is broken down rapidly by soil organisms, and is destroyed by sunlight.

Chloropicrin has limited activity against nematodes — it does not offer effective stand-alone control of nematodes. Nematode control is achieved by including 1,3-dichloropropene in soil fumigants. It acts against some insects, but has limited efficacy against disease-causing fungi and oomycetes, and weeds. Like chloropicrin, 1,3-dichloropropene is short-lived in the environment, and is destroyed by sunlight.

“The ratio of chloropicrin to 1,3-dichloropropene that we use depends on the pathogen control that you need,” says Stanton. He recommends a product that contains both chloropicrin and 1,3-dichloropropene when nematode infestations are severe, and a product that contains chloropicrin only for sites where testing has shown that nematodes are not a problem.

The fumigant is injected along the planting row with the aid of a ripper, and immediately covered with plastic to keep the chemicals in the soil. “You cover it for seven days, for safety reasons,” says Stanton, “but the product has effectively done its work within 48 hours.” After seven days, people can operate on the site, but trees should not be introduced until 21 days after fumigation.

Correct soil preparation is critical for the success of fumigation, stresses Stanton. “The biggest issue is plant material left in the soil. We understand that you’re not going to have 100% removal of roots, and big clods of soil, but it’s important to remove as much as possible, or the fumigation won’t work as well as it should.”

Stanton warns against planting too soon after removing an old orchard. “The ideal would be to pull out the old orchard, clean as much as possible, and wait at least a year, so that whatever organic material is left in the soil can break down before fumigating.”

The efficacy of fumigation is also affected by soil moisture and temperature, explains Stanton. Soil moisture must be between 40%–60% field capacity, with less moisture needed in clay than in sandy soils, and temperatures must be above 8 °C.

Stanton emphasises that fumigation doesn’t provide permanent protection against apple replant disease. “We guarantee clean soils for a maximum of six months. We can’t guarantee clean soils for the lifetime of the new orchard, because growers are re-introducing pathogens.”

Image supplied by Cole Stanton | Draslovka Services.