Oospores of Phytophthora ramorum were produced from intraspecific pairings between a European A1 and European or American A2 strains. Their viability was evaluated through colouration with tetrazolium bromide. The distribution of oospores in the different classes of colouration was similar to that found in other Phytophthora species (homothallic and heterothallic): most of the oospores stained purple, which corresponds to spores in dormancy. In order to produce single-oospore cultures, a method was developed to separate oospores from mycelium and chlamydospores. Germination of oospores was observed in 110, 250, 350 and 500-d-old cultures at a low proportion. Microsatellite marker analyses on oospore progenies revealed that the oospores resulted from hybridisation. More than 50 oospore progenies were characterised in terms of mating type, aggressiveness on Rhododendron leaves, and growth rate on two different media. The results are discussed in the context of pest risk analysis.
"The identification of the new microsatellite primer pairs within the North American lineages allows higher resolution population monitoring to track evolution and patterns of spread of the pathogen. The primers also can be used to screen for sexual recombinants (Boutet et al. 2010, Vercauteren et al. 2011). "
[Show abstract][Hide abstract] ABSTRACT: Phytophthora ramorum is a recently introduced pathogen in Europe and North America consisting of three clonal lineages. Due to the limited intralineage genetic variation, only a few polymorphic markers are available for use in studies involving the epidemiology and evolution of P. ramorum. A total of 159 primer pairs for candidate polymorphic SSR loci were tested with universal labeling. Four polymorphic microsatellite loci were identified within the NA1 lineage and one within the NA2 lineage, demonstrating the power and flexibility of the screening technique. The markers may significantly increase the number of genotypes that can be identified and as such can help better characterize the North American lineages of P. ramorum.
"In 2003, a single isolate of the EU1 lineage with A2 mating type was detected in Belgium (Werres and De Merlier 2003). Both mating types co-existing at one site can lead to sexual recombination as the sexual cycle in P. ramorum is functional (Boutet et al. 2010). The occurrence of a sexual cycle in the population may have an important impact on the genetic diversity and pathogenic properties. "
[Show abstract][Hide abstract] ABSTRACT: Phytophthora ramorum, a recently described North American and European pathogen, has three clonal lineages. The NA1 and NA2 lineages are found
in North American forests and nurseries, while the EU1 lineage appears mainly in European nurseries. P. ramorum is heterothallic, having two mating types A1 and A2. All NA1 and NA2 isolates are of A2 mating type. When first collected,
all EU1 isolates were of A1 mating type, with the exception of one A2 isolate collected in Belgium in 2002. Screening 410
other Belgian isolates for mating type revealed two additional EU1-A2 isolates collected in 2002 and 2003. PCR-RFLP, AFLP
and SSR markers were used to determine the nature of the mating type change. The three isolates show no indications of sexual
recombination or mitotic crossing over, indicating that mutation or mitotic gene conversion is the most likely explanation
for the mating type change. We compared the pathogenicity and sporulation characteristics of the EU1-A2 isolates to those
of EU1-A1 and NA1-A2 isolates on four host plants. Despite small differences in pathogenicity on some hosts, the EU1-A2 isolates
were similarly aggressive to each other and to the EU1-A1 isolates and more aggressive than the NA1-A2 isolates. Sporulation
characteristics were also comparable among EU1-A2 isolates and between EU1-A1 and EU1-A2 isolates, except for EU1-A2 isolate
BBA 26/02. The limited genotypic and phenotypic differences between EU1-A2 isolates probably evolved after the mating type
change, which may have occurred several years before the isolates were detected. There are strong indications that the EU1-A2
population has been eradicated from Belgium.
KeywordsAggressiveness–Mating type–Oomycetes–Sporulation–Sudden Oak Death
European Journal of Plant Pathology 04/2011; 129(4):621-635. DOI:10.1007/s10658-010-9727-5 · 1.49 Impact Factor
"One explanation for the discrepancy between lack of necrosis and apparent oospore viability could be that the oospores became dormant at 30 o C. Phytophthora oospore dormancy is known, but not well understood (Boutet et al., 2010; Lippman et al., 1974; Lutz & Menge, 1986). Exposure to lower temperatures may be needed to break dormancy on certain species. "
[Show abstract][Hide abstract] ABSTRACT: Phytophthora kernoviae Brasier, Beales & Kirk, recently found in the UK and New Zealand, is a pathogen of more than 30 host species. It is not known to produce chlamydospores, but is homothallic and produces abundant oospores and sporangia. This study was conducted to examine long-term survival of oospores, sporangia, and mycelium buried in sand at different temperatures. Viability of oospores buried in sand kept at 4, 10, 20 or 30 o C was assessed by staining with tetrazolium bromide solution. After 1 year at these temperatures, 82, 81, 79, and 58% of oospores of a New Zealand isolate respectively had survived. Corresponding values for an English isolate were 86, 75, 82, and 78%. Necrosis was observed on Rhododendron L. leaf discs exposed to oospores that had been buried for 1 year at temperatures below 30 o C. Oospores exposed for 1 and 6 h at 50 o C and 24 h at 40 and 50 o C were less viable than controls and did not germinate. Sporangia or mycelium of two New Zealand and two English isolates introduced to moist sand and kept at different temperatures showed a population decline within 1 week. Numbers of colony-forming units then remained at a low but steady level over time. Sporangia and oospores were formed at 4, 10 and 20 o C but not at 30 o C. The ability of P. kernoviae to persist in sand for long periods of time at different temperatures is likely to be one of the factors determining the rate of spread of this pathogen.
New Zealand Journal of Forestry Science 01/2011; 41. · 0.45 Impact Factor
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