Boxwood blight caused by Cps is an emerging disease that has had devastating impacts on Buxus spp... more Boxwood blight caused by Cps is an emerging disease that has had devastating impacts on Buxus spp. in the horticultural sector, landscapes, and native ecosystems. In this study, we produced a process-based climatic suitability model in the CLIMEX program and combined outputs of four different correlative modeling algorithms to generate an ensemble correlative model. All models were fit and validated using a presence record dataset comprised of Cps detections across its en-tire known invaded range. Evaluations of model performance provided validation of good model fit for all models. A consensus map of CLIMEX and ensemble correlative model predictions in-dicated that not-yet-invaded areas in eastern and southern Europe and in the southeastern, midwestern, and Pacific coast regions of North America are climatically suitable for Cps estab-lishment. Most regions of the world where Buxus and its congeners are native are also at risk of establishment. These findings provide the first insights into Cps global invasion threat, suggesting that this invasive pathogen has the potential to significantly expand its range.
A comprehensive phylogeny representing 142 described and 43 provisionally named Phytophthora spec... more A comprehensive phylogeny representing 142 described and 43 provisionally named Phytophthora species is reported here for this rapidly expanding genus. This phylogeny features signature sequences of 114 ex-types and numerous authentic isolates that were designated as representative isolates by the originators of the respective species. Multiple new subclades were assigned in clades 2, 6, 7, and 9. A single species P. lilii was placed basal to clades 1 to 5, and 7. Phytophthora stricta was placed basal to other clade 8 species, P. asparagi to clade 6 and P. intercalaris to clade 10. On the basis of this phylogeny and ancestral state reconstructions, new hypotheses were proposed for the evolutionary history of sporangial papillation of Phytophthora species. Non-papillate ancestral Phytophthora species were inferred to evolve through separate evolutionary paths to either papillate or semi-papillate species.
We have developed a web-based knowledge center for water and nutrient management and conservation... more We have developed a web-based knowledge center for water and nutrient management and conservation, for the nursery and greenhouse industries. Increasing environmental concerns and legislation in the Unites States and other countries require that we take a more comprehensive approach to improving and integrating cultural best management practices, since these operations are typically intensive users of resources which are applied to relatively small land areas. The website (http://www.waternut.org) provides the general public with an overview of the project and a link to the knowledge center (http://www.waternut.org/moodle/), which provides access to more than twenty intensive
. Fluctuations of Phytophthora and Pythium spp. in components of a recycling irrigation system. P... more . Fluctuations of Phytophthora and Pythium spp. in components of a recycling irrigation system. Plant Dis. 87:1500-1506. Stringent standards of water quality have prompted many horticultural enterprises to limit pollutant discharge associated with nutrient and pesticide applications. Collecting and recycling effluent is a method that has been implemented by many operations to contain pollutants; however, plant pathogens may be spread through recycled effluent. In this study, Phytophthora and Pythium spp. present in a water-recycling irrigation system at a perennial container nursery in southwestern Virginia were characterized using filtering and baiting techniques with two selective media. Members of Phytophthora were identified to species, whereas Pythium spp. were identified to genus only. Pythium spp. were recovered more frequently and in greater numbers than Phytophthora spp. Phytophthora capsici, P. citricola, P. citrophthora, P. cryptogea, P. drechsleri, and P. nicotianae were recovered in filtering assays. Only P. cryptogea and P. drechsleri were identified from baits placed on the surface of the irrigation reservoir, whereas P. cactorum, P. capsici, P. citricola, P. citrophthora, P. cryptogea, and P. drechsleri were recovered at depths, specifically at 1 and 1.5 m. This research provides data for development of detection technology and management practices for plant pathogens in irrigation water and may lead to improvements in conventional assay protocols.
Isolates from avocado tree cankers have been recognized as a distinct subgroup within the P. citr... more Isolates from avocado tree cankers have been recognized as a distinct subgroup within the P. citricola complex since 1974, both morphologically and molecularly (isozyme and amplified fragment length polymorphism [AFLP] analyses). This subgroup is formally separated from P. citricola after comparative DNA fingerprinting and sequence analyses of the ITS region, as well as by morphological examinations. This new taxon is homothallic, produces plerotic oospores with paragynous antheridia and noncaducous semipapillate sporangia. Morphologically it differs from other species of Waterhouse group III by producing many large bizarre-shaped sporangia and smaller oogonia with asymmetric capitate antheridia. It belongs to clade 2 and is phylogenetically closer to P. siskiyouensis, P. capsici and P. tropicalis than to P. citricola. P. mengei can be easily differentiated from its relatives in the same clade and other species of this morpho-group by DNA fingerprints and sequence analysis. This new...
A novel Phytophthora species was frequently recovered from irrigation reservoirs at several ornam... more A novel Phytophthora species was frequently recovered from irrigation reservoirs at several ornamental plant production facilities in eastern Virginia. Initial sequencing of the internal transcribed spacer (ITS) region of this species generated unreadable sequences due to continual polymorphic positions. Cloning and sequencing the ITS region as well as sequencing the mitochondrially encoded cytochrome c oxidase 1 and beta-tubulin genes revealed that it is a hybrid between P. taxon PgChlamydo as its paternal parent and an unknown species genetically close to P. mississippiae as its maternal parent. This hybrid has some diagnostic morphological features of P. taxon PgChlamydo and P. mississippiae. It produces catenulate hyphal swellings, characteristic of P. mississippiae, and chlamydospores, typical of P. taxon PgChlamydo. It also produces both ornamented and relatively smooth-walled oogonia. Ornamented oogonia are another important diagnostic character of P. mississippiae. The relatively smooth-walled oogonia may be indicative of oogonial character of P. taxon PgChlamydo. The new hybrid is described here as Phytophthora 6stagnum.
Mefenoxam is one of the most commonly used fungicides for managing diseases caused by Phytophthor... more Mefenoxam is one of the most commonly used fungicides for managing diseases caused by Phytophthora spp. on ornamentals. The objectives of this study were to determine whether Phytophthora nicotianae , a destructive pathogen of numerous herbaceous annual and perennial plant species in nurseries, has developed resistance to mefenoxam, and to evaluate the fitness of mefenoxam-resistant isolates. Ninety-five isolates of P. nicotianae were screened for sensitivity to mefenoxam on 20% clarified V8 agar at 100 a.i. μ g mL -1 . Twenty-five isolates were highly resistant to this compound with EC 50 values ranging from 235·2 to 466·3 μ g mL -1 and four were intermediately resistant with EC 50 values ranging from 1·6 to 2·9 μ g mL -1 . Sixty-six isolates were sensitive with EC 50 values less than 0·04 μ g mL -1 . Nine resistant and seven sensitive isolates were tested for mefenoxam sensitivity on Pelargonium × hortorum cv. White Orbit. Mefenoxam provided good protection of pelargonium seedlings from colonization by sensitive isolates, but not by any resistant isolates. Four resistant and four sensitive isolates were compared for fitness components and their relative competitive ability on Lupinus Russell Hybrids in the absence of mefenoxam. Resistant isolates outcompeted sensitive ones within 3 to 6 sporulation cycles on lupin seedlings, regardless of their initial proportions in mixed zoospore inoculum. Resistant isolates exhibited greater infection rate and higher sporulation ability than sensitive ones when they were applied separately onto lupins. These results suggest that fungicide resistance may pose a serious challenge to the continued effectiveness of mefenoxam as a control option for nursery growers.
In controlled environment experiments, when oilseed rape pods or leaves were inoculated with spor... more In controlled environment experiments, when oilseed rape pods or leaves were inoculated with spore suspensions of Alternaria brassicae, the maximum disease incidence (proportion of pods or leaves diseased) increased as wetness period after inoculation increased from 4 to 24 h and as temperature increased to 208C. There was a clear relationship between disease incidence on pods and incidence on leaves with the same wetness/temperature conditions. Logistic equations described the effects of wetness period after inoculation on disease incidence (number of pods or leaves infected) or disease severity (number of lesions on pods or leaves) using temperature-dependent and tissue-dependent parameters. The time from inoculation to the appearance of the first lesions was shorter on pods than on leaves at temperatures 158C and wetness periods 12 h. Two-dimensional response surface equations or simple interpolations from one-dimensional equations were used to develop contour maps of expected disease incidence and severity, respectively, on leaves or pods to estimate the effects of different combinations of wetness period during infection and temperature on disease development.
A new species of Phytophthora, previously referred to as taxon Dre II, is named Phytophthora hydr... more A new species of Phytophthora, previously referred to as taxon Dre II, is named Phytophthora hydropathica. It is heterothallic, but all isolates recovered to date are of the A1 compatibility type. Plerotic oospores are produced. Its sporangia are usually obpyriform and are nonpapillate and noncaducous. Isolates of P. hydropathica had nearly identical single-strand conformation polymorphism (SSCP)-based DNA fingerprints that are distinct from those of all existing species. Their closest relatives are P. parsiana and P. irrigata. This new species is able to grow at relatively high temperatures, with an optimum of 30°C and a maximum of 40°C. It was frequently isolated from irrigation water during warm summers. This species caused leaf necrosis and shoot blight of Rhododendron catawbiense and collar rot of Kalmia latifolia at two nurseries where irrigation reservoirs yielded P. hydropathica. Its potential impact on other horticultural crops is discussed.
Hong, C. X., Holtz, B. A., Morgan, D. P., and Michailides, T. J. 1997. Significance of thinned fr... more Hong, C. X., Holtz, B. A., Morgan, D. P., and Michailides, T. J. 1997. Significance of thinned fruit as a source of the secondary inoculum of Monilinia fructicola in California nectarine orchards. Plant Dis. 81:519-524.
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