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Differentiation of Races of Pseudomonas solanacearum by a Leaf Infiltration Technique. J. C. Lozano, Graduate Student, Department of Plant Pathology, University of Wisconsin, Madison 53706; Luis Sequeira, Professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 60:833-838. Accepted for publication 9 December 1969. DOI: 10.1094/Phyto-60-833.

The three known races of Pseudomonas solanacearum could be differentiated by the reaction of tobacco leaves following infiltration with bacterial suspensions containing at least 3.5 × 107 cells/ml. Race 1 isolates caused no visible symptoms on inoculated leaves after 24 hr, but a dark-brown necrotic lesion surrounded by a yellow halo appeared after 36 hr in the infiltrated area. By 60 hr, the bacteria had invaded the adjoining tissues and vascular elements. By 8 days after infiltration, there was extensive wilting, yellowing, and necrosis of the leaf tissues. In contrast, race 2 isolates induced a hypersensitive reaction (HR) by 10-12 hr after infiltration. At this time the infiltrated area was water-soaked, and slightly chlorotic, but became thin, white, and translucent by 60 hr. The reaction was limited to the infiltrated area. Race 3 isolates caused only a yellowish discoloration of the infiltrated area by 48 hr after inoculation. The populations of race 1 and race 3 isolates increased within the tobacco leaf tissues during the first 48 hr after infiltration, but declined by 72 hr. The populations of race 2 isolates, which induced the HR, remained unchanged during the first 6 hr after infiltration and decreased sharply thereafter; by 48 hr, no live bacteria could be detected. In leaves exposed to 900 ft-c for 48 hr, the HR was less intense than at 1,800 ft-c. In total darkness for 48 hr, the HR was not induced by race 2, but a spreading necrotic reaction, similar to that induced by race 1 in the light, was obtained. Increased periods of exposure to darkness were correlated with increased multiplication of the bacteria, and their spread into adjacent tissues. Sterile fluids extracted from tobacco leaves during the initial symptoms of the HR inhibited the growth of both incompatible and compatible isolates of P. solanacearum.