Stems of susceptible and resistant cassava plants have been cytologically investigated for their defense reactions to an aggressive strain of Xanthomonas campestris pv. manihotis. Histochemistry, in conjunction with gold cytochemistry, revealed that in susceptible and resistant plants, phloem and xylem parenchyma cells displayed a wide range of responses that limited the bacterial growth within the infected plants. Lignification and suberization associated with callose deposition were effective mechanisms that reinforced host barriers in the phloem. In the infected xylem, vessels were plugged by a material of pectic and (or) lignin-like origin. Flavonoids have been seen to be incorporated in secondary cell wall coatings. These reactions occurred at a higher intensity in the resistant plants. The number of phoem and xylem cells producing autofluorescent compounds was higher in infected resistant plants than in susceptible plants. Reactions have been observed in the resistant variety only, such as secretion of phenol-like molecules by tyloses and hyperplasic activity of phloem cells that compartmentalized bacterial lysis pockets, which are potent secondary inoculum sources.Key words: lignin, suberin, callose, phenol, tylose, flavonoid, pectin.

Asiatic citrus canker (ACC) is a severe disease of several citrus species and hybrids in many tropical and subtropical areas. Populations of Xanthomonas axonopodis pv. citri in leaf and twig lesions are the most important inoculum source for secondary infections. In areas with a marked winter season (e.g., Argentina and Japan), low temperatures induce a decrease of 10(2) to 10(4) in population sizes in lesions, thus creating a discontinuity in the X. axonopodis pv. citri life cycle. The purpose of this study was to evaluate the dynamics of X. axonopodis pv. citri populations in leaf lesions exposed to the mild winter temperatures prevailing in a tropical environment. Internal X. axonopodis pv. citri population levels in Mexican lime leaf lesions reached 10(6) to 10(7) CFU lesion(-1) whatever the lesion size. These densities, however, were not strongly negatively affected by winter temperatures prevailing under experimental conditions. The estimated decrease in internal X. axonopodis pv. citri population sizes was approximately 10-fold. When exposed to 35 mm h(-1) of simulated rainfall, internal population sizes decreased over time by approximately 1 log unit for lesions 1 and 2 months old, but did not for older lesions. A microscopic examination indicated that lignin-like compounds are present in lesions more than 6 months old. The slow decrease over time of X. axonopodis pv. citri population sizes in leaf lesions may be the balanced result of defense reactions by the host at late stages of disease development, and the concomitant multiplication of the pathogen at the margin of old lesions. We conclude that the epidemiological significance of overwintered leaf lesions in the tropics is higher than that reported in other areas.

campestris ~ ~~Three-hundred and twenty-six strains of Xanthomonas campestris pv. manihotis from 22 countries were studied to detect and assess genetic and evolutionary relationships within the pathovar. A range of techniques was used for this study including restriction fragment length polymorphism (RFLP) analysis. The probes used for the RFLP analysis were 16 + 23s rRNA genes from Escherichia coli and three restriction fragments from the chromosomal and plasmid DNA of X. campestris pv. manihotis. Analysis of the rRNA probe data showed five RFLP groups whilst the other three probes were used to further sub-divide these groupings. Genetic variability of X. campestris pv. manihotis was pronounced in strains from South America where the host plant originated but was limited in strains from other regions. The results obtained confirm the hypothesis that the pathogen has been introduced only recently to Africa and suggests that African strains have not as yet diversified significantly at the chromosomal level. Our results indicate that rRNA and DNA probes are useful tools for epidemiological studies and in following the genetic evolution of strains.

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We thank C. Breuil (University of B.C., Canada), K. Roberts (J. Innes Institute, England), and L. R. .Haaheim (University of Bergen, Norway) for kindly providing the purified ,ß-1,4-exoglucanase, the JIM 5 antipectin antibodies, and the antibacterial EPS antibodies respectively.