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Harold A.A. Gibbs

Xanthomonas campestris pv. vesicatoria (Xcv) recovered from Commelina benghalensis L., caused bacterial spot disease in cultivars of pepper and tomato susceptible to the pathogen. This is the first reported case of a dicot-infecting Xc pathovar infecting a monocot plant, represented here by a member of the Family Commelinaceae. Laboratory strains of the pathogen that included 81-23, 81-23M13, 82:4, 2595, and P6AD4, known to be pathogenic to pepper and tomato, promoted bacterial spot symptoms on leaves of C. benghalensis L. Of the 63 field isolates recovered from infected C. benghalensis L., 30 gave biochemical and physiological reactions consistent with Xcv pathogens, whereas 10 of the latter promoted bacterial spot disease in the test cultivars resulting in the identification of seven pathogenic races, including P2, P5, P6, P5T1, P5T2, P6T2, and P6T3. Bacterial spot disease symptoms developed on stems only when C. benghalensis L. was spray-inoculated with strains 81-23, 81-23M13, and P6AD4. Bacterial concentration increased in planta by as much as 103 per lesion of the leaf, whereas growth of the same strains was restricted in the stem of this weed. Growth of these three strains was, however, significantly (P ≤ 0.05) lower on NYGA amended with C. benghalensis L. stem extract than on NYGA amended with leaf extract. The ability of the bacterial spot pathogen to infect the stem of C. benghalensis L. has serious implications for management of bacterial spot disease in fields populated with this weed since stems of this plant infected with the pathogen continue to grow vegetatively and disperse throughout all fields in which it is found.

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Harold A.A. Gibbs and Leonard W. O'Garro

Colorimetric and chromatographic methods were used to assess capsaicinoid levels in a pungent Caribbean-grown pepper collection comprising 28 accessions of Capsicum chinense and one each of C. annuum and C. frutescens. Two colorimetric methods, one commonly used and attributed to Bajaj (1980) and a modification of the Bajaj method were also compared for congruity and ease of use. Capsaicin content of the cultivars ranged from 37.6 to 497.0 mg/100 g in ripe fruits and 27.8 to 404.5 mg/100 g in green fruit, as determined by Bajaj's method. The corresponding Scoville units of pungency varied from 15,000 to 300,000 for ripe fruit and 7,500 to 270,000 for green fruit. Levels of capsaicin assessed by the modified Bajaj method varied from 15.0 to 402.4 mg/100 g and 13.7 to 356.4 mg/100 g in ripe and green fruit, respectively. On the basis of capsaicin levels assessed by each colorimetric method, the pepper cultivars were differentiated into seven distinct pungency groups. For each method, similar groupings of cultivars were observed for ripe and green fruit and groups of the same numerical designation were mainly comprised of common assessions. These results indicate that the two colorimetric methods generally agree. In contrast, the modified colorimetric method was more efficient than Bajaj's procedure, which required pretreatment of pepper extracts to remove the extracting solvent by evaporation and interfering chromogenic pigments by column chromatography. Phase separation of capsaicin and interfering pigments in pepper extracts by use of dilute acid was the only pretreatment required in the modified Bajaj method before colorimetry. High performance liquid chromatography performed on fruit extracts of the cultivars revealed the presence of the capsaicinoids capsaicin, homocapsaicin, dihydrocapsaicin, nordihydrocapsaicin, and homodihydrocapsaicin. Capsaicin and homocapsaicin were detected in greater abundance than dihydrocapsaicin and nordihydrocapsaicin in fruit of all cultivars. Homodihydrocapsaicin was the least abundant of the capsaicinoids and was generally absent in ripe fruit.