REDUCED DOSAGE OF METHYL-BROMIDE FOR CONTROLLING VERTICILLIUM WILT OFPOTATO IN EXPERIMENTAL AND COMMERCIAL PLOTS

The use of gas-impermeable films to reduce the dosage of methyl bromid e (MB) required to control Verticillium wilt in potatoes was examined in field experiments, conducted in soils naturally infested with Verti cillium dahliae. The incidence and severity of Verticillium wilt were significantly reduced (by 74 to 94%) by fumigation with MB at 50 g/m(2 ) under standard low density polyethylene (LDPE) or at 25 g/m(2) under gas-impermeable films. Fumigation at 25 g/m(2) under LDPE was less ef fective. Disease severity was inversely correlated (r(2) = 0.89 to 0.9 1) with chlorophyll content in the leaves. Fumigation also reduced (by 89 to 100%) stem colonization by the pathogen. Potato yield in the fu migated plots was significantly higher (26 to 69%), than in their nonf umigated counterparts, and was inversely correlated with disease index (r(2) = 0.69 to 0.9). The percentage of high-value tubers (above 45 g ) was 52 to 56% of total yield in the fumigated plots as compared with 32 to 40% in the nonfumigated controls. Thus, fumigation also improve d the commercial quality of tuber yield. Effective control of V. dahli ae and yield increases following MB fumigation at the recommended dosa ge or at a reduced dosage with gas-impermeable films was also observed in a consecutive crop. These results were Verified in a large-scale f ield experiment using commercial applications, further demonstrating t he feasibility of reducing MB dosages under farm conditions, without r educing its effectiveness in terms of disease control and yield improv ement.