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The monophyletic origin of host-specific taxa in the plant-pathogenic Fusarium oxysporum complex was tested by constructing nuclear and mitochondrial gene genealogies and amplified fragment length polymorphism (AFLP)-based phylogenies for 89 strains representing the known genetic and pathogenic diversity in 8 formae speciales associated with wilt diseases and root and bulb rot. We included strains from clonal lineages of F. oxysporum f. spp. asparagi, dianthi, gladioli, lilii, lini, opuntiarum, spinaciae, and tulipae. Putatively nonpathogenic strains from carnation and lily were included and a reference strain from each of the three main clades identified previously in the F. oxysporum complex; sequences from related species were used as outgroups. DNA sequences from the nuclear translation elongation factor 1alpha and the mitochondrial small subunit (mtSSU) ribosomal RNA genes were combined for phylogenetic analysis. Strains in vegetative compatibility groups (VCGs) shared identical sequences and AFLP profiles, supporting the monophyly of the two single-VCG formae speciales, lilii and tulipae. Identical genotypes were also found for the three VCGs in F. oxysporum f. sp. spinaciae. In contrast, multiple evolutionary origins were apparent for F. oxysporum f. spp. asparagi, dianthi, gladioli, lini, and opuntiarum, although different VCGs within each of these formae speciales often clustered close together or shared identical EF-1alpha and mtSSU rDNA haplotypes. Kishino-Hasegawa analyses of constraints forcing the monophyly of these formae speciales supported the exclusive origin of F. oxysporum f. sp. opuntiarum but not the monophyly of F. oxysporum f. spp. asparagi, dianthi, gladioli, and lini. Most of the putatively nonpathogenic strains from carnation and lily, representing unique VCGs, were unrelated to F. oxysporum f. spp. dianthi and lilii, respectively. Putatively nonpathogenic or rot-inducing strains did not form exclusive groups within the molecular phylogeny. Parsimony analyses of AFLP fingerprint data supported the gene genealogy-based phylogram; however, AFLP-based phylogenies were considerably more homoplasious than the gene genealogies. The predictive value of the forma specialis naming system within the F. oxysporum complex is questioned.

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The population structure of Guignardia citricarpa sensu lato (anamorph: Phyllosticta citricarpa), a fungus of which strains pathogenic to citrus are subject to phytosanitary legislation in the European Union and the United States, was investigated. Internal transcribed spacer sequences revealed two phylogenetically distinct groups in G. citricarpa. This distinction was supported by amplified fragment length polymorphism analysis that also supported the exclusion of two isolates that had apparently been misclassified as G. citricarpa. On cherry decoction agar, but not on other media, growth rates of group I isolates were lower than those of group II isolates. Conidial dimensions were similar, but group I isolates formed conidia with barely visible mucoid sheaths, whereas those of group II formed conidia with thick sheaths. Cultures of isolates belonging to group I produced rare infertile perithecia, whereas fertile perithecia were formed by most isolates of group II. Colonies of isolates belonging to group I were less dark than those of group II, with a wider translucent outer zone and a lobate rather than entire margin. On oatmeal agar, exclusively group I isolates formed a yellow pigment. Group I harbored strains from citrus fruits with classical black spot lesions (1 to 10 mm in diameter) usually containing pycnidia. Group II harbored endophytic strains from a wide range of host species, as well as strains from symptomless citrus fruits or fruits with minute spots (<2-mm diameter) without pycnidia. These observations support the historic distinction between slowly growing pathogenic isolates and morphologically similar fast-growing, nonpathogenic isolates of G. citricarpa. The latter proved to belong to G. mangiferae (P. capitalensis), a ubiquitous endophyte of woody plants with numerous probable synonyms including G. endophyllicola, G. psidii, P. anacardiacearum, and P. theacearum. G. mangiferae occurs in the European Union and the United States on many host species including citrus, and does not cause symptoms of citrus black spot, justifying its exclusion from quarantine measures.

The colonization processes of the xylem in the susceptible carnation cv. Early Sam and the resistant cv. Novada were studied ultrastructurally following inoculation with Fusarium oxysporum f.sp. dianthi. Samples from 1 to 3 cm above the incision were collected over 5 weeks and processed following conventional procedures as well as with probes for cellulose, N-acetyl-glucosamine, and pectin. The fungus grew profusely in the vessel lumina of the susceptible cultivar. Some of the colonized vessels were lined with coating material connected to the fungal cell wall and extending into the host cell wall through microfilamentous-like structures. Coatings did not label for pectin or cellulose. The pathogen crossed from one vessel element to another (and at times to parenchyma cells) usually directly through pit membranes; often the invading structures of the fungus appeared to be either only membrane-bound or formed solely of microfilamentous-like entities. The fungus subsequently invaded extensively, generally by means of microhyphae, the vessel intercalary walls from the pit membranes and vessel wall junctures. Microhyphae had thin or imperceptible walls and contained only some of the normal cytoplasmic components. Initially, the invading hyphae dislocated the host cell walls, apparently mechanically more than by lysis; however, more pronounced lysis occurred following general tissue invasion. Host parenchyma cells seemed relatively unaffected, even after the surrounding walls had undergone severe degradation. Colonization of resistant plants was restricted. Degradation of tissues did not occur and microhyphae were not observed. Inoculated vessel elements in the 'Novada' plants contained numerous fungal cells and little occluding material, whereas the surrounding vessels were almost completely occluded. The initially invaded xylem became tangentially compartmentalized by parenchyma cell wall thickenings and by hyperplastic parenchyma. Occasionally, hyperplastic tissues were slightly re-invaded, forming secondary invasion pockets. Vessel-occluding material varied in structure and opacity, not only from vessel to vessel but also within the same vessel, and contained microfilamentous-like structures and other types of fine fibrillar material. Some vessel elements in or near the secondary invasion pockets contained only the finer fibrils that reacted strongly with an antibody specific for pectin. Vessel elements rarely contained tyloses.Key words: cellulose, chitin, Dianthus caryophyllus, Fusarium wilt, gels and gums, host wall degradation, microhyphae, pectin, tyloses.