Formation, production and viability of oospores of Phytophthora infestans from potato and Solanum demissum in the Toluca Valley, central Mexico

Aspects of the ecology of oospores of Phytophthora infestans were studied in the highlands of central Mexico. From an investigation of a random sample of strains, it was found that isolates differed in their average capability to form oospores when engaged in compatible pairings. Most crosses produced large numbers of oospores but a few yielded none and some yielded only a few oospores. The results reveal that oospore production and fecundity is dependent on both isolates and the combining ability of a specific combination of parental strains. On average, 14% of the oospores produced were viable as determined by the plasmolysis method. Viability ranged from a low 1% in one cross to a high of 29% in another cross. Oospores were found in 10-20% of naturally infected Solanum demissum leaves from two different collections, and leaflets with two lesions per leaflet produced more oospores than did leaflets with 3-5 lesions per leaflet. There was no consistent trend for preferential mating between isolates from the same location or host

The biology of Phytophthora infestans at its center of origin

The central highlands of Mexico are considered to be a center of genetic diversity for both the potato late blight pathogen and for tuber-bearing Solanum spp. Recent work conducted in Mexico and South America sheds new light on the biology and evolution of Phytophthora infestans and other related Phytophthora pathogens. It now appears that Mexican Solanum species, which coevolved with P. infestans and were previously known for providing a source of R-genes, also provide a source of quantitative, rate-reducing resistance that is highly effective, stable, and durable. It is now apparent that Mexico is the center of origin not only of the potato late blight pathogen P. infestans, but also of several related Phytophthora species including P. mirabilis, P. ipomoeae, and possibly P. phaseoli. We close with the hypothesis that these Phytophthora species evolved sympatrically from one ancestral host through adaptive radiation onto their respective four host familie

Short-term cover crop decomposition inorganic and conventional soils : Soil microbial and nutrient cycling indicator variables associated with different levels of soil suppressiveness to Pythium aphanidermatum

Stages of oat–vetch cover crop decomposition were characterized over time in terms of carbon and nitrogen cycling, microbial activity and community dynamics in organically and conventionally managed soils in a field experiment and a laboratory incubation experiment. We subsequently determined which variables describing soil microbial community dynamics, C and N cycling could be used as predictors of Pythium aphanidermatum damping-off incidence and relative growth. Disease incidence and relative growth of P. aphanidermatum were measured in growth chamber assays and in vitro growth tests. No significant differences were detected between the conventional and organic farming system with respect to either relative growth or disease incidence. Stepwise discriminant analysis on three classes of disease incidence or relative growth led to selection of qualitatively similar variables. Only one soil microbial variable, total biomass of actinomycetes, was selected. Total C and N content of debris extracted from soil as well as NH4-N content of soil were selected most consistently and show promise for assessment of potential damping-off incidence by P. aphanidermatum for young seedlings

Short-term cover crop decomposition in organic and conventional soils : Characterization of soil C, N, microbial and plant pathogen dynamics

Stages of oat–vetch cover crop decomposition were characterized over time in terms of carbon and nitrogen cycling, microbial activity and damping-off pathogen dynamics in organically and conventionally managed soils in a field and a controlled incubation experiment. A measurement of relative growth consisting of radial growth of a fungal colony over non-sterilized soil divided by that over sterilized soil was used as an assay of suppressiveness. No differences in relative growth of Pythium aphanidermatum and Rhizoctonia solani were detected between organic and conventionally managed soils amended with cover crop residue. Significant effects of cover crop decomposition stage on the relative growth of both pathogens were obtained. Relative growth of P. aphanidermatum was highest just after incorporation and decreased 3 weeks after incorporation. Relative growth of R. solani was highest about 20 days after incorporation, and decreased 2 weeks later in the organic system, but continued to increase in the conventional system. In both experiments, the N or C content, C : N ratio or dry weight of retrieved debris were significantly correlated with relative growth of P. aphanidermatum. Relative growth of R. solani was significantly correlated with the C :N ratio of soil or the C or N content of debris. Microbial activity was not consistently associated with relative growth of either pathogen

Population genetic structure and cryptic species of Plasmopara viticola in Australia

Downy mildew of grape caused by Plasmopara viticola is a global pathogen of economic importance to commercial viticulture. In contrast to populations in the northern hemisphere, few studies have investigated the population biology, genetic diversity, and origin of the pathogen in Australian production systems. DNA was extracted from 381 P. viticola samples from Vitis vinifera and alternate hosts collected via fresh and herbarium leaves from populations within Australia and Whatman FTA cards from North America, Brazil, and Uruguay. A total of 32 DNA samples were provided from a French population. The populations were genotyped using 16 polymorphic microsatellite markers. Representative samples from within Australia, Brazil, and Uruguay were also genotyped to determine which of the cryptic species (clades) within the P. viticola species complex were present. Our findings suggest the Australian and South American populations of P. viticola are more closely related to the European population than the North American population, the reported source of origin of the pathogen. The Western Australian population had similarities to the South Australian population, and the tight clustering of samples suggests a single introduction into Western Australia. P. viticola clade aestivalis was the only clade detected in Australian and South American populations. Analysis of the Western Australian population suggests that it is reproducing clonally, but additional research is required to determine the mechanism as to how this is occurring

Short-term cover crop decomposition in organic and conventional soils : Characterization of soil C, N, microbial and plant pathogen dynamics

Stages of oat–vetch cover crop decomposition were characterized over time in terms of carbon and nitrogen cycling, microbial activity and damping-off pathogen dynamics in organically and conventionally managed soils in a field and a controlled incubation experiment. A measurement of relative growth consisting of radial growth of a fungal colony over non-sterilized soil divided by that over sterilized soil was used as an assay of suppressiveness. No differences in relative growth of Pythium aphanidermatum and Rhizoctonia solani were detected between organic and conventionally managed soils amended with cover crop residue. Significant effects of cover crop decomposition stage on the relative growth of both pathogens were obtained. Relative growth of P. aphanidermatum was highest just after incorporation and decreased 3 weeks after incorporation. Relative growth of R. solani was highest about 20 days after incorporation, and decreased 2 weeks later in the organic system, but continued to increase in the conventional system. In both experiments, the N or C content, C : N ratio or dry weight of retrieved debris were significantly correlated with relative growth of P. aphanidermatum. Relative growth of R. solani was significantly correlated with the C :N ratio of soil or the C or N content of debris. Microbial activity was not consistently associated with relative growth of either pathogen

Short-term cover crop decomposition inorganic and conventional soils : Soil microbial and nutrient cycling indicator variables associated with different levels of soil suppressiveness to Pythium aphanidermatum

Stages of oat–vetch cover crop decomposition were characterized over time in terms of carbon and nitrogen cycling, microbial activity and community dynamics in organically and conventionally managed soils in a field experiment and a laboratory incubation experiment. We subsequently determined which variables describing soil microbial community dynamics, C and N cycling could be used as predictors of Pythium aphanidermatum damping-off incidence and relative growth. Disease incidence and relative growth of P. aphanidermatum were measured in growth chamber assays and in vitro growth tests. No significant differences were detected between the conventional and organic farming system with respect to either relative growth or disease incidence. Stepwise discriminant analysis on three classes of disease incidence or relative growth led to selection of qualitatively similar variables. Only one soil microbial variable, total biomass of actinomycetes, was selected. Total C and N content of debris extracted from soil as well as NH4-N content of soil were selected most consistently and show promise for assessment of potential damping-off incidence by P. aphanidermatum for young seedlings