Bacterial Leaf Spot of Lettuce: Relationship of Temperature to Infection and Potential Host Range of Xanthomonas campestris pv. vitians.

Epidemiological aspects, including optimum temperature for infection and host range of Xanthomonas campestris pv. vitians, causal organism of bacterial leaf spot (BLS) of lettuce, were investigated. The optimum temperature for infection was determined to be 22.7°C based on growth chamber studies. Internal populations were monitored over time in lettuce, tomato, pepper, parsley, cilantro, and beet. Each plant species was infiltrated with the bacterium at 105CFU/ml. Highest populations developed in lettuce (108CFU/cm2) followed by pepper with 106CFU/cm2, whereas the other plant species harbored much lower populations (105 to 103CFU/cm2). Infectivity titration endpoints were similar in pepper and lettuce (103 to 104CFU/ml). For other plant species tested, infectivity titration endpoints were 106 to 107 CFU/ml. Electrolyte leakage data and corresponding internal population data support the conclusion that fresh-market tomato is not a host of X. campestris pv. vitians but, instead, interacts in an incompatible response. Electrolyte leakage from cells of tomato plants inoculated with X. campestris pv. vitians or a pepper strain of X. axonopodis pv. vesicatoria peaked at 48 h, suggesting that tomato is not a host for the BLS pathogen. Both electrolyte leakage and population dynamics results point to pepper as a potential host of X. campestris pv. vitians.

Trends of PCB, hexachlorobenzene, and beta-benzene hexachloride levels in the adipose tissue of the U.S. population.

Results are presented on the levels of selected toxic chemicals in the adipose tissue of the U.S. population. Estimates of baseline levels and time trends are presented for the chemicals beta-benzene hexachloride (beta-BHC), hexachlorobenzene, and polychlorinated biphenyls (PCBs). The statistical analyses are based on chemical data obtained from analysis of tissue specimens collected from a nationally representative sample of autopsied cadavers and surgical patients by EPA's National Human Adipose Tissue Survey during the period 1970 through 1983. The results show that while nearly the entire U.S. population has detectable levels of these chemicals, the actual concentration levels of beta-BHC and PCBs are steadily decreasing and the levels of HCB are remaining fairly constant. Comparisons of levels across demographic subpopulations show that there are some differences in concentration levels across geographic regions, age groups, and race groups.

An evaluation of hexachlorobenzene body-burden levels in the general population of the USA.

Hexachlorobenzene (HCB) is a chemical of concern to the US Environmental Protection Agency. Although the use of HCB as a fungicide has virtually been eliminated, detectable levels of HCB are still found in nearly all people in the USA. This paper describes an examination of HCB-residue levels from the US National Human Adipose Tissue Survey, which is an annual programme to collect a nationwide sample of adipose-tissue specimens and analyse them for the presence of toxic compounds. The specimens are collected from surgical patients and autopsied cadavers according to a multistage statistical survey design that allows identification of geographic and demographic subpopulations with elevated HCB levels, and thus may help to indicate sources of human exposure to this chemical. Data on HCB body-burden levels are available for the years 1974-1983, excluding 1980 and 1982. The estimated average residue level of 6115 specimens is 0.053 ppm, the ninetieth percentile value is 0.09 ppm and the range is 'not detected' to 4.33 ppm. The analysis reveals that while the percentage of individuals with detectable levels is increasing, the annual average level is decreasing. A large percentage of specimens with elevated HCB levels comes from the Pacific census division and the oldest age group. In comparison, world-wide detectable levels are approaching 100% and average residue levels tend to increase with age. World-wide HCB levels are higher in females, which contrasts with findings from the National Human Adipose Tissue Survey.

Development, evaluation and use of a pharmacokinetic model for hexachlorobenzene.

This study discusses the background, biological basis, development characteristics, application and evaluation of six physiologically based multicompartment models that describe the absorption, distribution and elimination of hexachlorobenzene (HCB) in growing rats and growing humans. The models for rats and for humans have similar structures but differ in specific physiological parameters. The goal of the modelling effort was to obtain toxicological information about HCB based on its pharmacokinetics. Comparisons were made between estimated tissue concentrations based on the rat models and observed tissue distributions based on pharmacokinetic animal studies with HCB, using both chronic and single-dose studies. The estimates from the female model agreed reasonably well with experimental results, and estimated a long half-life of approximately 180 days in all tissues; it did not, however, duplicate the biphasic tissue efflux of HCB that has been reported in the literature. The male model estimated a half-life of approximately 55 days in all tissues, shorter than that observed experimentally. The estimated yield of metabolites agreed well with values reported in the literature. A pregnancy and offspring model predicted minimal transfer of HCB to the fetal compartment during gestation and extensive mobilization of HCB to the offspring during lactation. This agrees with results reported in the literature. Correlations were obtained between experimentally observed liver toxicity and estimated yield of metabolites; between experimentally observed effects on the central nervous system and estimated HCB concentrations in the brain; and between experimentally observed offspring mortality and estimated extent of lactational HCB transfer. This paper also discusses the effects on tissue concentrations and half-lives of trapping HCB in the intestines by sequestering a large portion of it there. Various characteristics of the human models are discussed. The female model has a biphasic elimination pattern, the second elimination phase having a half-life of greater than 200 days. The pregnancy and offspring model projects extensive transfer of HCB via the milk to the nursing offspring, which correlates with the greater mortality of nursing infants whose mothers were exposed to HCB in Turkey.