Abstract

                                Black arm or angular leaf spot or bacterial blight of cotton caused by Xanthomonas malvacearum (Smith) Dowson, has been prevalent in the Madras Stato for over thirty years (Ramakrishnan 1950). Primary infection is mainly through the seeds. Rain combined with heavy winds (rainstorms) helps in the spread of the disease (Andrews 1936, Brown 1941). Control of primary infection of black arm by seed treatment has been attempted in soveral countries with varying dogroes of success. Seed disinfection with formalin (1: 100) has boon found to be vory effective in the U.S.S.R. (Verdorovski 1937). Ceresan materially increased the seedling emergence and yield of cotton in various localities in Toxas, at the same time reducing the incidence of angular leaf spot (Smith et al. 1936). Massey. (1937) has found that the use of mercurial dusts like Abavit B, ethyl mercury phosphate, ethyl mercury iodide, phenyl mercury acetate and sovoral others gave satisfactory control of black arm in Africa. Seed treatment with mercuric iodide had resulted in the control of black arm in the Sudan (Clouston and Andrews 1938). Macdonald et al (1947) have stated that delinting with sulphuric acid and the removal of floaters have resulted not only in the reduction of primary infection of black arm but also improved the germination and vigour of the seedlings. Ramakrishnan (1950) usod formalin as steep and dust, Ceresan and Agrosan GN as seed dressings and observed that all these gave protection from primary infection. Tarr (1953 a, b, c) after the use of several seed dressing agents found that mercuric iodido gave botter protection as a steep than as a dry dressing. Tarr has further observed that slurry methods were most effective when applied to acid delinted seeds and that soluble organomercury compounds were preferable for slurry methods. Acid delinted seeds wore much more susceptible to mercury injury than fuzzy ones. Cuprous oxide was also found to be a promising seed dressing for the control of black arm. (Tarr 1953 a).

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