SINGLE CELL COLONIES OF XANTHOMONAS-MALVACEARUM IN SUSCEPTIBLE AND IMMUNE COTTON LEAVES AND THE LOCAL RESISTANT RESPONSE TO COLONIES IN IMMUNE LEAVES

In leaves of susceptible upland cotton (Gossypium hirsutum L.), Xanthomonas malvacearum (E. F. Sm.) Dows. produced expanding water-soaked areas, whereas in leaves of an immune cotton line, small clusters of dark brown host cells developed. The number of water-soaked areas or of brown cell clusters was proportional to the number of bacteria administered, when the initial bacterial population density was within the range 20 to 2500 bacteria per cm2 of leaf area. In paradermal sections of infected immune leaves, the brown host cells stained intensely with Azure B. Bacteria were found in the intercellular spaces adjacent to 64% of these darkly stained cell clusters and adjacent to only 7% of symptomless host cells. Compact masses of bacteria were observed on the surfaces of some darkly stained host cells. We conclude that in both susceptible and immune cotton leaves infection can be initiated by a single cell of X. malvacearum which grows into a colony. Bacterial growth was logarithmic in susceptible and immune leaves, but stopped at lower population densities in immune leaves. Any initial bacterial population density less than 850 cells per cm2 in immune leaves gave a growth yield of approximately 2000 per initial bacterium. This suggests that the immune leaf responds locally to each bacterial colony. © 1979.