Bill B. Brodie, USDA, ARS, Department of Plant Pathology, Cornell University, Ithaca, NY 14853.

Since the establishment of the golden nematode regulatory program in 1944, methyl bromide applied at 240 g/m³ for 24 hours under polyethylene has been the treatment of choice for disinfesting equipment and other articles to free them of the golden nematode. This treatment is used routinely to decontaminate articles that are moved from golden nematode-regulated areas to nonregulated areas. Because of the effectiveness of methyl bromide, there has been essentially no effort to develop other types of treatments. However, the impending phaseout of methyl bromide has forced the development of a more environmentally compatible treatment to ensure the integrity of the golden nematode quarantine.

High temperature has the greatest potential as an environmentally safe alternative to methyl bromide for disinfesting items contaminated with the golden nematode. We have found that golden nematode eggs in cysts that had been presoaked in water for 24 hours are killed when exposed to 55 °C for as little as 30 seconds. In contrast, eggs in dry cysts tolerated temperatures as high as 75 °C for brief periods.

Steam is a common source of heat used to sterilize soil and other items but it has not been used as a treatment to disinfest equipment contaminated with the golden nematode. However, high steam is used to clean soil from golden nematode-contaminated equipment, but the direct effect of this procedure on survival of nematode eggs is not known. In initial studies to develop an alternative treatment to methyl bromide, solarization with or without supplemental heat looked promising as a lethal treatment against the golden nematode. We found that when cysts were soaked in water prior to the solarization treatment, only 4 percent of the eggs survived. Furthermore, the juveniles from these eggs failed to infect potato plants. When supplemental steam heat was added, only 1 percent of the eggs survived, and their resultant juveniles failed to infect potato plants.

In further experiments, we tested different types of heat treatments against the golden nematode. The heat treatments consisted of solarization (sealed in clear polyethylene for 28 hours and exposed to direct sunlight), solarization plus supplemental steam heat, and solarization plus supplemental dry heat. Soil containing 20 golden nematode cysts in nylon sackettes was placed in small crevices of tillage equipment. The equipment was either left dry or washed with high pressure water to wet the cysts and increase the humidity. The equipment was sealed in clear polyethylene for 28 hours and supplemental heat was applied for 6 hours on each of 2 days. Methyl bromide applied at 240 g/m³ under black polyethylene for 28 hours served as a control. The check consisted of cysts contained in nylon sackettes that were not subjected to treatment.

After the treatments were completed, the sackettes were retrieved and the cysts were subjected to a hatching test. The hatching test consisted of soaking the cysts in water for 5 days then placing them in potato root exudate for 3 weeks. The number of juveniles that emerged was counted weekly and fresh exudate was added. An average of 78 juveniles/cyst hatched from the nontreated cysts. Hatch from cysts subjected to the standard methyl bromide treatment averaged 0.6 juveniles/cyst. Hatch from cysts subjected to supplemental steam heat averaged 0.07 juveniles/cyst when the equipment was not prewashed with high pressure water and 0.18 juveniles/cyst when the equipment was prewashed to wet the cysts. Hatch from cysts subjected to supplemental dry heat averaged 69 juveniles/cyst when the equipment was not prewashed and 25 juveniles/cyst when the equipment was prewashed. Hatch from cysts subjected to solarization without supplemental heat averaged 56 juveniles/cyst when the equipment was not prewashed and 25 juveniles/cyst when the equipment was prewashed.

In all treatments, some nematode eggs appeared to survive, but they did not hatch in response to potato root exudates. The numbers of viable eggs that did not hatch included 47 eggs/cyst from the untreated control, 0.7 eggs/cyst from the methyl bromide treatment, 1.2 eggs/cyst from the steam heat treatment, 31 eggs/cyst from the dry heat treatment, and 30 eggs/cyst from the solarization treatment. These eggs have been placed around the roots of potato plants to determine the infectivity of the resultant juveniles.

These tests indicate that solarization treatment alone in the northeastern United States is not sufficient to disinfest equipment contaminated with the golden nematode. Although prewashing equipment with high-pressure water increased the sensitivity of golden nematode eggs to high temperatures, it was not enough to achieve the desired amount of nematode mortality. Lethal temperatures were achieved with supplemental dry heat under polyethylene but because the eggs were desiccated, nematode mortality was minimal. Nematode mortality with supplemental steam heat equaled that achieved with methyl bromide, suggesting that steam heat is a suitable alternative to methyl bromide for disinfesting equipment contaminated with the golden nematode.

Last Updated: January 27, 1997