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Bacterial Blight of Cassava in Colombia: Etiology. J. C. Lozano, Research Assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706; Luis Sequeira, Professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 64:74-82. Accepted for publication 19 July 1973. DOI: 10.1094/Phyto-64-74.

The bacterial blight of cassava (Manihot esculenta) has increased in severity in Colombia during the past 5 yr. Symptoms on susceptible cultivars include leaf spotting, wilting, die-back, and gum exudation on young shoots, and vascular discoloration in mature stems. The bacterium (CBB) penetrates via the stomata or through wounds in epidermal tissues. It invades the vascular tissues of leaves and young shoots, resulting in extensive breakdown of parenchymatous tissues. In highly lignified tissues of old stems or roots, the bacterium remains restricted to the vascular strands. These symptoms are similar to those reportedly induced by Xanthomonas manihotis, but the isolates of CBB differ from the former in cell size, motility, production of H2S, utilization of nitrate, hydrolysis of starch, and in several serological characteristics. CBB is a gram-negative, motile, slender rod, with a single polar flagellum. It is aerobic, fast-growing, and forms no pigments on carbohydrate-containing media. It hydrolyzes starch and gelatin, and reduces litmus milk. It produces levan, catalase, arginine dihydrolase, and lipase, but not H2S, indole, urease, tyrosinase, or phenylalanine deaminase. It grows in ordinary media plus NaCl or tetrazolium chloride at a maximum concentration of 2.5 and 0.2%, respectively. It utilizes nitrate and ammonium as sources of nitrogen, and most of the simple sugars as sources of carbon, but acid is not produced; various amino acids and other organic acids are readily utilized. Isolates of CBB from distinct geographical areas induced similar symptoms on cassava, but belonged to two different serological groups, each separable into two additional groups on the basis of their ability to utilize sucrose, cellobiose, and trehalose as carbon sources. However, these groupings were not correlated with geographical origin of the isolates. CBB was separated by serological- and phage-typing methods from three species of Erwinia, two of Pseudomonas, and ten of Xanthomonas, including X. manihotis. A Bdellovibrio sp. caused lysis of CBB specifically and was used to separate CBB from other plant pathogenic bacteria.