Screening Triploid Hybrids of 'Lakeland' Limequat for Resistance to Citrus Canker.

Resistance of citrus genotypes to Xanthomonas axonopodis pv. citri, the cause of Asiatic citrus canker (ACC), was evaluated by injection infiltration of 103 and 104 CFU/ml through stomates on the abaxial surface of immature leaves. Citrus genotypes for screening comprised two autotetraploids and nine triploid hybrids of 'Lakeland' limequat (Citrus aurantifolia × Fortunella japonica) and their progenitors ('Lakeland' limequat, the autotetraploids 'Femminello' lemon (Citrus limon) and 'Giant Key' lime (C. aurantifolia), and the somatic hybrids 'Key' [also known as 'Mexican'] lime + 'Valencia' orange and 'Hamlin' orange + 'Femminello' lemon). 'Meiwa' kumquat (Fortunella crassifolia) and 'Pineapple' sweet orange (C. sinensis) were used as known resistant and susceptible standards, respectively. Lesion number per inoculation site and bacterial population per lesion were recorded 15 to 19 days after inoculation. The assay was performed four times during a spring-summer-fall period under greenhouse conditions. Canker lesions were consistently produced by stomatal inoculation with 104 but not 103 CFU/ml. Susceptible and resistant genotypes were separated based on lesion number per inoculation site and bacterial population per lesion. Spearman's rank correlation analysis for lesion numbers on 15 genotypes common to all four assays showed significant correlations among the genotype rankings. Genotype rankings were also significantly correlated between the two bacterial population assays. Lesion number per inoculation site is sufficient for assessment of resistance of citrus genotypes to ACC without the necessity of conducting bacterial population assays. 'Lakeland' limequat is a promising seed parent for breeding acid citrus fruit that is resistant to ACC.

Characterization of Phytophthora spp. Causing Outbreaks of Citrus Brown Rot in Florida.

ABSTRACT Epidemics of citrus brown rot from 1994 to 1997 in the south-central and east-coast citrus areas of Florida were characterized and the causal Phytophthora spp. identified. Two species of Phytophthora, P. palmivora and P. nicotianae, were consistently associated with brown rot. Epidemics caused by P. palmivora appeared to be initiated on immature fruit dropped on the orchard floor. The soilborne fungus infected and sporulated on these fruit and was then disseminated to fruit above 1 m in the canopy. In contrast, infection by P. nicotianae, the common cause of root rot, was confined to the lowest 1 m of the canopy. Fruit infected by P. palmivora produced large amounts of ellipsoidal sporangia available for splash dispersal, whereas those infected by P. nicotianae produced far fewer spherical sporangia. Isolates from brown rot epidemics were compared with P. nicotianae from citrus in Florida and Texas, P. citrophthora in California, P. palmivora, and selected Phytophthora spp. from other hosts. Brown rot symptoms produced by the different pathogenic citrus isolates on inoculated fruit were indistinguishable. Morphology, mating behavior, and isozyme patterns of brown rot isolates from 1988 to 1997 matched P. palmivora from citrus roots, other host plants, and other locations, but were different from characterized isolates of P. citrophthora in California and P. nicotianae in Florida and Texas. Cellulose acetate electrophoresis of the isozyme glucose-6-phosphate isomerase rapidly identified the causal citrus pathogen from infected fruit and soil isolation plates. Although P. palmivora is an aggressive pathogen of citrus roots, bark, and fruit, populations in orchard soils were low compared with P. nicotianae.

Carbon economy of sour orange in response to different Glomus spp.

Vesicular-arbuscular mycorrhizal (M) fungal colonization, growth, and nonstructural carbohydrate status of sour orange (Citrus aurantium L.) seedlings were compared at low- and high-phosphorus (P) supply following inoculation with four Glomus isolates: G. intraradices (Gi, FL208), G. etunicatum (Ge, UT316), G. claroideum (Gc, SC186), and Glomus sp. (G329, FL906). Nonmycorrhizal (NM) seedlings served as controls. At low-P supply, increases in incidence of M colonization, vesicles and accumulation of fungal fatty acid 16:1omega(5)C in roots were most rapid for G329-inoculated seedlings, followed closely by Gi- and Gc-inoculated seedlings. Glomus etunicatum was a less aggressive colonizer and produced lower rates of fungal fatty acid accumulation in seedling roots than the other Glomus species. Nonmycorrhizal and Ge-inoculated seedlings had lower P status and growth rates than seedlings inoculated with Gi or G329. Glomus claroideum increased seedling P status, but growth rate was lower than for seedlings colonized by Gi or G329, suggesting higher belowground costs for Gc colonization. In P-sufficient roots colonized by Gi, Gc, or G329, starch and ketone sugar concentrations were lower than in P-deficient NM and Ge-inoculated plants. Under conditions of high-P supply where mycorrhizae provided no P benefit to the seedlings, colonization by Gc, Gi, and G329 was delayed and reduced compared to that at low-P supply; however, the relative colonization rates among Glomus spp. were similar. Colonization by Ge was not detected in roots until 64 days after inoculation. Compared to NM seedlings, growth rates of mycorrhizal seedlings were reduced by the three aggressive fungi but not by the less aggressive Ge. After 64 days, starch and ketone sugar concentrations were lower in fibrous roots colonized by Gc, Gi, and G329 than in NM roots, indicating greater utilization of nonstructural carbohydrates in roots colonized by the aggressive fungi. After 49 days, colonization by the aggressive fungi increased root biomass allocation which may have contributed to the lower growth rate of mycorrhizal seedlings compared to NM seedlings. Thus, Glomus spp. that were aggressive colonizers of roots at low-P supply were also aggressive colonizers at high-P supply, resulting in greater belowground C costs and growth depression compared with the less aggressive colonizer, Ge.

Distinctions among pathogenic human mycoplasmas.

Cytadsorption by Mycoplasma pneumoniae requires dense clustering of the surface membrane protein, P1, at the extreme end of the mycoplasma tip-like organelle. M. pneumoniae mutants incapable of cytadsorption either lack P1 or cannot mobilize and cluster P1 at the terminus. Specific cytadsorption-associated proteins in addition to P1 have been shown by mutant and revertant analysis to be essential for cytadsorption. Using monoclonal antibody probes and surface iodination techniques, additional chemical differences were observed between wild-type and mutant M. pneumoniae. M. genitalium, the recently identified new species, possesses structural and antigenic properties that appear similar to M. pneumoniae. Studies were initiated to establish the relatedness between M. pneumoniae and M. genitalium in terms of cytadsorption and membrane proteins.

Absence of Mycoplasma pneumoniae cytadsorption protein P1 in Mycoplasma genitalium and Mycoplasma gallisepticum.

Polyclonal and monoclonal antibodies to Mycoplasma pneumoniae protein P1 were nonreactive with whole-cell or soluble preparations of M. genitalium and M. gallisepticum. However, radioimmunoprecipitation performed with hyperimmune rabbit sera raised against each mycoplasma species indicated antigenic cross-reactivity between M. pneumoniae and M. genitalium.

Lymphocyte sorting on albuminated CIBA blue dextran-staphylococcal protein A-conjugated sepharose 6MB affinity columns.

Modification of Sepharose 6MB affinity beads by conjugation with CIBA blue dextran and albumination together with siliconization and modification of the bed supports of glass chromatography columns permitted the construction of affinity columns with low non-specific binding characteristics. When used with staphylococcal protein A as the affinity ligand to fractionate antibody-coated lymphocytes, non-specific adherence was reduced to 2--3% of the viable input cells and essentially the entire input cell number could be recovered in viable, functional form. The capacity of the columns permitted quantitative separation of T and B lymphocytes in 250 microliter amounts at cell densities up to 5 X 10(8)/ml on 1.2 ml columns. Once used, the columns may be regenerated and reused numerous times without alteration of their characteristics. The method thus appears useful for preparative separation without loss of large numbers of highly purified cells having normal in vitro reactivity.