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Twenty isolates of Phytophthora infestans from potato and twenty-two from tomato, collected in Uganda and Kenya in 1995, were compared for dilocus allozyme genotype, mitochondrial DNA (mtDNA) haplotype, mating type and restriction fragment length polymorphism (RFLP) fingerprint using probe RG57. Based on RFLP fingerprint and mtDNA haplotype, all isolates were classified in the US21 clonal lineage. Nonetheless, isolates from potato differed from isolates from tomato in several characteristics. Isolates from potato had the 86/100 glucose-6-phosphate isomerase (Gpi) genotype, while those from tomato were 100/100, which represents a variant of US21 that had been identified previously as US21.7. Furthermore, while pure cultures of the pathogen were acquired from infected potato leaflets by first growing the isolates on potato tuber slices, this approach failed with infected tomato tissue because the isolates grew poorly on this medium. Tomato isolates were eventually purified using a selective medium. Six isolates from each host were compared for the diameter of lesions they produced on three tomato and three potato cultivars in one or two detached-leaf assays (four isolates from the first test were repeated in the second). On potato leaflets, isolates from potato caused larger lesions than isolates from tomato. On tomato leaflets, isolates from that host caused larger lesions than did isolates from potato, but the difference was significant in only one test. The interaction between source of inoculum (potato or tomato) and inoculated host (potato or tomato) was significant in both tests. Isolates from tomato were highly biotrophic on tomato leaflets, producing little or no necrosis during the seven days following infection, even though abundant sporulation could be seen. In contrast, isolates from potato sporulated less abundantly on tomato leaflets and produced darkly pigmented lesions that were most visible on the adaxial side of the leaflets. Nonetheless, all isolates infected and sporulated on both hosts, indicating that host adaptation is not determined by an ability to cause disease but rather by quantitative differences in pathogenic fitness. Assessment of Gpi banding patterns, mtDNA haplotype and RFLP fingerprint of 39 isolates from potato collected in Uganda and Kenya in 1997 indicated that the population had not changed on this host. The population of P. infestans from Kenya and Uganda provides an interesting model for the study of quantitative host adaptation.

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Twenty-six isolates of a Phytophthora population from two wild solanaceous species, Solanum tetrapetalum (n 11) and S. brevifolium (n = 15), were characterized morphologically, with genetic and phenotypic markers, and for pathogenicity on potato and tomato. Based on morphology, ribosomal internal transcribed spacer region 2 (ITS2) sequence, and pathogenicity, all isolates closely resembled P. infestans and were tentatively placed in that species. Nonetheless, this population of Phytophthora is novel. Its primary host is neither potato nor tomato, and all isolates had three restriction fragment length polymorphism (RFLP) bands (probe RG57) and a mitochondrial DNA haplotype that have not been reported for P. infestans. All the isolates were the A2 mating type when tested with a P. infestans A1 isolate. The A2 mating type has not been found among isolates of P. infestans from potato or tomato in Ecuador. Geographical substructing of the Ecuadorian A2 population was detected. The three isolates from the village of Nono, identical to the others in all other aspects, differed by three RFLP bands; those from Nono lacked bands 10 and 16, but possessed band 19. Most of the Ecuadorian A2 isolates were nonpathogenic on potato and tomato, but a few caused very small lesions with sparse sporulation on necrotic tissue. Cluster analysis of multilocus genotypes (RFLP, mating type, and two allozymes) dissociated this A2 population from genotypes representing clonally propagated populations of P. infestans worldwide. The current hypotheses for the historical global movements of P. infestans do not satisfactorily explain the origin or possible time of introduction into Ecuador of this A2 population. Assuming the population is P. infestans, its presence in Ecuador suggests either a hitherto unreported migration of the pathogen or an indigenous population that had not previously been detected.

A purified wall fraction was prepared from the mycelium of Agaricus bisporus. The isolated wall consisted of 43",, (wiw) chitin, 1 4 "~) KOH-soluble glucan, 27Oh /I-glucan, 1 6 O , protein and ~-5~) , , lipid. Traces of mannose and xylose were detected by gas chromatography. The architecture of the wall was investigated with sequential enzyme digestion and electron microscopy. The outer wall surface is covered by a mucilage that is removcd by the isolation procedure. The wall fraction could be completely degraded by extraction in warm I M-KOH followed by digestion with a mixed /,'-glucanase and then chitinase. The outer layer of KOHsoluble glucan is amorphous and or variable thickness. Incubation with glucanase did not change the dimensions of the wall but was necessary before the inner wall was susceptible to attack by chitinase, indicating that />'-glucan does not constitute a separate wall layer but is a matrix associated with the fibrillar chitin. The inner layer presents an even, compact. fibrillar side as the inside surface of the wall, but is looser and uneven outwardly where it interdigitates with the irregular inner surface of the KOH-soluble glucan. Silver hexamide staining showed cystine-containing protein throughout the wall.The fruit body of the Agaricales is massive, yet mainly consists of relatively undifferentiated hyphal elements similar to the vegetative mycelium, and thus provides an interesting system for studying the mechanisms of hyphal aggregation i n morphogenesis. A number of striking correlations between fungal cell form and wall composition, particularly in such phenomena as hyphal-yeast cell type dimorphism, emphasize the importance that wall composition may have in determining dif'ferentiation and rnorphogenesis (Smith & Galbraith, 1971). The composition and architecture of the hyphal wall is now known in a variety of fungi, but a more or less complete picture is available for only one near relative of the mushrooms, Schizopliyllrrm co~iiniuno, a member of the Aphyllophorales. We present here a study of the hyphal architecture 01' the common mushroom, Agirricus bisyorus, and compare and relate it to S. coyIztizunc and the existing fragmentary picture in the Agaricales itself.

Host–parasite interfaces in potato tubers (Solanum tuberosum) of a resistant (Eba) and a susceptible (Bintje) cultivar inoculated with Phytophthora infestans were studied with transmission electron microscopy. In the resistant host the fungal haustoria are typically small and surrounded by an electron-dense extrahaustorial matrix and electron-transparent wall appositions, normally in form of complete encasements. Wall appositions are generally lacking in the susceptible host or occasionally are found as a collar at the base of the rather large haustorium, which is surround by a well developed extrahaustorial matrix. Since wall appositions probably contain largely a callose-like material, potential roles of fungal glucanases in this host–parasite system are discussed.

Host–parasite interfaces in leaves from a resistant (Eba) and a susceptible (Bintje) potato cultivar inoculated with Phytophthora infestans were studied using light and electron microscopy. Host penetration occurs similarly in both cultivars either through stomata or directly through the epidermis. In the susceptible host the fungus spreads throughout the tissue intercellularly and transcellularly, whereas in the resistant host it remains confined to the focus of infection. Yet in both cases live, normal-appearing hyphae can be observed even after 5 days. The appearance of haustoria in both cultivars is similar: they are either surrounded by an extrahaustorial matrix alone or in combination with wall appositions (collars or encasements). Structures with cytological features between those of haustoria and transcellular hyphae were also recorded, indicating a variety of host–parasite interactions within leaf tissue. Sporulation has been observed on the susceptible cultivar but not on the resistant one. The results suggest that cellular reactions against the intruding fungal hypha are qualitatively similar in both cultivars but that at the tissue level the intercellular spread of the pathogen and its sporulation are prevented in the resistant host but not in the susceptible host.