Mycorrhizal fungi in relation to control of tobacco stunt disease with soil fumigants

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Fumigation of soil with 67% methyl bromide-33% chloropicrin (MBC) was more effective than fumigation with 98% methyl bromide-2% chloropicrin (MB) in increasing growth and accelerating maturity of tobacco in three fields in Central Kentucky infested with the tobacco stunt pathogen. However, both fumigants had similar effects on yields. Two fields had more species of endogonaceous fungi and higher populations of endogonaceous spores than the third, in which the disease was more severe. In all three fields, Glomus macrocarpum was prevalent, reproduced on tobacco, and was controlled by fumigation. For plants growing in nonfumigated soil, height at harvest time was correlated negatively with populations of spores of G. macrocarpum, and less strongly with G. microcarpum and Acaulospora scrobiculata. No such correlations occurred with plants growing in fumigated soil. Roots of plants growing in both fumigated and nonfumigated soil were colonized by mycorrhizal fungi throughout the growing season, but by harvest time, roots of plants growing in nonfumigated soil were colonized 10-fold more than those of plants growing in fumigated soil. However, stunting was not correlated with colonization. These experiments generally support inoculation experiments indicating that G. macrocarpum causes tobacco stunt disease.