Methyl Iodide as an alternative to Methyl Bromide in Tobacco Seedbeds Nematodes control

Abstract

Plant parasitic nematodes cause an estimated 120 billion US dollars in crop losses globally per annum, of which $50 billion is accredited to tobacco. A need to control these notorious plant pests (nematodes) arose early in the last century and Methyl Bromide (MeBr) proved to be the right remedy that can be used. In addition to nematicidal properties, Methyl Bromide has also been used as an insecticide, fungicide, acaricide, herbicide and rodenticide. However, research has shown that Methyl Bromide, a chemical that has been used and trusted by farmers and crop protectionists for so many decades; causes so 'riiany environmental problems of which the greatest is the depletion of the Stratospheric Ozone Layer. Ozone layer depletion property of Methyl Bromide has led to the banning and total phase-out of the chemical under the Montreal Protocol of the United Nations. In 2015, Methyl Bromide must be totally phased out in Zimbabwe. The phase out of the much trusted MeBr has forced the science community and ' (j-. t \ (agriculturalists to coin new methods of nematode control that have a reduced impact on the environment and the stratospheric ozone. One of the potential chemical substitutes that have been proposed to have the same cadillac fumigant properties as Methyl Bromide but with less toxicity and ozone layer depletion potential is Methyl Iodide, another halogenated methane. In this study, Iodomethane, as Methyl Iodide is sometimes known, was tested and evaluated as a possible substitute to Methyl Bromide in conventional tobacco seedbeds fumigation. It was evaluated against weeds and nematodes in tobacco seedbeds. Phytotoxicity of Methyl Iodide was also evaluated in tobacco seeds and seedlings. Methyl bromide was applied at its standard rate of 35g/m2 and the Iodomethane was applied at 3 different rates of 10g/m2, 20g/m2 and 30g/m2. Methyl Iodide is a liquid at ambient temperature and therefore, it was applied using the hot gas system and all chemicals were applied under a 125 gauge polyvinylchloride plastic sheet. The plastics were removed 4 days after chemical application. Tobacco seeds of two Zimbabwean cultivars (T66 and KM 10) were sown 7 days and 14 days after chemical applications. The seedbeds were mulched and watered every day, for nearly 3 months, using a hose pipe with a fine rose as recommended by the tobacco growing manual. Phytotoxicity tests, germination counts, weeds counts, seedling survival counts, gall ratings and tobacco stalk heights were some of the measured parameters. The results at 28 days after sowing indicated that Methyl Iodide effectively controlled narrow leaved weeds and broad leaved weeds (p<0.05) as all the three x Methyl Iodide treatments were comparable to the standard Methyl Bromide when applied 7 and 14 days before sowing (p<0.001). Statistically, no significant interaction was found between cultivars (T66 and KM10) and nematicide applied (Methyl Iodide and Methyl Bromide) (p<0.05). Root-knot galling assessments from bioassays which contained the soil samples taken from the treated plots at final pulling indicated that there was significant interaction between the tobacco cultivars and nematicide treatments (p<0.05). The test product (Mel) managed to reduce root-knot nematode numbers better than the untreated control and it was comparable to Methyl Bromide at all the three tested rates (p<0.001); LSD =0.34). Summarily, the results above show that Methyl Iodide is as effective as Methyl Bromide in controlling weeds and nematodes, even at its lowest tested rate of 1 Og/m.