Variation in the Latent Period of Bacterial Soft Rot in Tomato Fruit

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Abstract
The latent period (time between inoculation and water-soaking) of bacterial soft rot (BSR) in tomatoes wound inoculated with E. carotovora ssp. carotovora (Ecc) ranged from < 24 h to > 3 wk. This variation in latency was greater than could be accounted for by variation in the inoculation technique (needles previously dipped into inocula were thrust 2 mm into the flesh of a fruit). Wounds, inoculated with 5 .mu.l of bacterial suspension/5 mm3 wound by using a micropipet, had more uniform numbers of cells of Ecc than did wounds from needles previously dipped in inoculum. By 24 after inoculation the variance of numbers of bacteria in wounds in the 2 systems equaled each other and were often significantly larger than their respective 0-h values. The decrease in uniformity of the populations during the 24-h incubation seemed to be associated with differences in the suitability of the damaged tissue for growth of Ecc. Concentrations of 107 cells/ml led to populations in wounds 10-fold greater than in wounds inoculated with 106 cells/ml, not only immediately after inoculation, but also at 18, 24 and 36 h. When the number of cells approached 107/wound, growth of Ecc appeared restricted. Tenfold differences in the inoculum concentration (109 through 105 cells/ml) led to 2-fold differences in the incidence of BSR at 48 h. Disease progress curves were of the exponential or restricted growth shape whether the percentage of fruit with lesions after inoculation or the percentage of diseased wounds on individual fruit was plotted. The curves approached asymptotes whose levels varied according to the levels of initial disease; higher inoculum dosages generally led to higher levels of initial disease and higher asymptotes. Disease progress after inoculation with Pseudomonas marginalis was similar. Because most inoculated fruit succumbed to BSR during prolonged storage, the asymptotes not equal to 100% disease were interpreted as evidence for a host reaction that temporarily localized the bacterium. The temporary nature of the host reaction seemed responsible for the variable latency.

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