Methyl bromide is important for controlling verticillium wilt and other soil-borne pathogens that attack various crops in Israel, as well as other countries of the world. With the approaching ban on the chemical and the regulation to reduce consumption, Israeli scientists are seeking ways to reduce the amount of the fumigant needed and still maintain high crop yields.

A team of Israeli scientists has successfully controlled verticillium wilt of potato by using a reduced dose of methyl bromide under gas-impermeable films. The results were published in Plant Disease, May 1997.

These films retain higher concentrations of methyl bromide in the soil for a longer time than the commonly used low-density polyethylene films. As a result, the team cut the standard methyl bromide dose in half and still effectively controlled verticillium.

Potato yields in fumigated plots with these films were up to 69 percent higher than in nonfumigated plots, and tuber quality was also improved.

The members of the team are A. Gamliel and A. Grinstein, The Volcani Center; Y. Peretz, Hevel Maon; L. Klein, Bromine Compounds, Ltd.; A. Nachmias, L. Tsror, and L. Livescu, Gilat Experiment Station; and Jaacov Katan, Department of Plant Pathology, The Hebrew University of Jerusalem.

Verticillium wilt is a common disease of potatoes in Israel and in other regions of the world, including the United States. In severe cases, it causes plants to collapse before the end of the season, stunting the growth of potatoes and significantly decreasing yield. In some areas of Israel, two successive crops of potatoes are grown, while other areas rotate potatoes with peanuts, a crop that is also highly susceptible to Verticillium dahliae, the fungus that causes the wilt. This rotation practice tends to create a buildup of the pathogen in the soil.

Studies in the Netherlands had demonstrated that gas-impermeable films minimize the escape of methyl bromide from the soil, allowing dosage reduction without affecting pest control. The aim of this study was not only to evaluate the effectiveness of the gas-impermeable film with reduced amounts of methyl bromide to control verticillium in potato, but also to improve yield and quality in one or two successive seasons in experimental or commercial plots, under actual farmers' conditions.

The team conducted three field experiments in southern Israel from 1993 to 1995 in loessial—a loamy, sandy soil. A field with a long history of potato cropping and naturally infested with V. dahliae was selected for the first experiment in the fall of 1993. Both the second site, chosen in the spring of 1994, and the third site chosen the same year in the fall, had similar histories.

For the first and second experiments, at the end of August the team applied methyl bromide at
50 g/m² and 25 g/m², manually covering the soil with low density films with edges buried in trenches 20 cm deep, or at 25 g/m² with gas-impermeable films. They buried naturally produced V. dahliae microsclerotia from infected potato stems at 20 cm and 40 cm and assessed their control by fumigation.

Experiment three was carried out on a commercial scale in large plots to verify, under farmers' conditions, the effectiveness of reduced dosage of methyl bromide with gas-impermeable films. Using commercial equipment, a contractor mechanically laid plastic tarping and applied the fumigant. The plastic films were kept on all plots for 5 days and were then removed manually. Field preparation at each site included pre-irrigation, broadcast application of composted cow manure, and bed-shaping before fumigation—all common cultivation practices followed by commercial potato producers in this region of the country.

In experiments one and two, all methyl bromide treatments resulted in 100 percent control of
V. dahliae microsclerotia at both depths. First symptoms of wilt appeared in untreated plots 70 days after planting, followed shortly thereafter by early dying.

In all experiments, verticillium wilt was effectively controlled by methyl bromide at the commercial rate of 50 g/m² under low-density films and at 25 g/m² under gas-impermeable film. In comparison, fumigation at 25 g/m² under low-density films was less effective. Even methyl bromide applied at
15 g/m² under gas-impermeable film gave good control. Tuber yield in all fumigation treatments was higher than that in untreated plots.

Using gas-impermeable films and reduced amounts of methyl bromide demonstrated good pathogen control, maintained chlorophyll content of plants, reduced disease incidence and severity, and increased yields and quality. These effects were repeated for two consecutive seasons and results were verified at all test sites including the commercial plots, indicating a high reproducibility of the results.

These research results indicate that using gas-impermeable films can cut at least in half the amount of methyl bromide needed to be effective in controlling verticillium wilt in potato crops. Combining this with other control measures such as solarization, biocontrol agents, disease- resistant plant varieties, and cultural methods will help further reduce methyl bromide emissions without affecting pest control.

In Israel, Italy, Greece, and Spain, reduced methyl bromide dosage using gas-impermeable films has produced similar results in effectively controlling a variety of pathogens such as Fusarium, Phytophthora, and Pythium. This indicates that this approach has potential for wide applicability.