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Aerial Strands of Erwinia amylovora: Structure and Enhanced Production by Pesticide Oil. H. L. Keil, Research Plant Pathologist, Plant Science Research Division, ARS, USDA, Beltsville, Maryland 20705; T. van der Zwet, Research Plant Pathologist, Plant Science Research Division, ARS, USDA, Beltsville, Maryland 20705. Phytopathology 62:355-361. Accepted for publication 22 October 1971. DOI: 10.1094/Phyto-62-355.

Sprays of Superior 70 oil enhanced production of aerial bacterial strands from the blighted shoots of pear and apple trees infected with Erwinia amylovora. Strand production was greater on pears than on apples, and was increased over that for the water control even at the lowest (0.125%) oil concentration. Proportionately more strands were produced at each higher concentration. Oil increased aerial strand production even when applied 10 days before or after inoculation. Strands were produced at relative humidities as high as 94%, but not at ca. 100%. Apparently, the oil clogs some of the natural openings of the epidermis, and thereby effects in the shoots an increase in their internal pressure that results in extrusion of rapidly proliferating bacteria in the form of strands. Bacteria in the strands were virulent, and caused more infection in injured than in noninjured plants. Examination of the strands with a scanning electron microscope revealed their variation in morphological structure and the approximate ratio of bacterial cells to matrix. Disintegration of the strands in water did not result in complete separation of the cells. Instead, many individual cells and clumps of cells remained held together by a cobweblike network. The possible importance of the strands in the dissemination of E. amylovora is discussed.

Additional keywords: Pyrus communis, Malus sylvestris, fire blight.