Abstract: Coast live oak (Quercus agrifolia) is a known host for Phytophthora ramorum, the casual agent of sudden oak death in California, with symptoms expressed as necrotic stem cankers. In the forest, leaves on two saplings in California were found to be infected with P. ramorum and these were associated with infected bay laurel (Umbellularia californica) trees. Coast live oak leaves supported sporulation and produced chlamydospores in vitro. This is the first report to identify foliage of coast live oak as a source of infection of P. ramorum in the forest and its confirmation in in vitro inoculations.
Abstract: An unknown Phytophthora species was recovered in southwestern Oregon from rhododendron and tanoak leaf baits used for monitoring streams and soils for the presence of Phytophthora ramorum, from a blighted shoot of myrtlewood and from tanoak bark cankers. Isolates of this species yielded ITS-DNA sequences that differed substantially from other Phytophthora sequences in GenBank. Morphological features also differed from available descriptions of known Phytophthora species. Based on the combination of unique morphology and unique ITS sequences a new species is proposed. The new species, Phytophthora siskiyouensis, is homothallic with globose to subglobose oogonia, which may be terminal, sessile or laterally intercalary. Antheridia are capitate and mostly paragynous but sometimes amphigynous. Oospores are mostly aplerotic. Sporangia are variable but commonly ovoid to reniform, with apical, subapical or lateral semipapillae (occasionally more than one). Sporangia are terminal, subterminal or occasionally intercalary on unbranched sporangiophores, with basal, subbasal or lateral attachment. Sporangia are weakly deciduous, with variable length pedicels. This combination of characters clearly separates Phytophthora siskiyouensis from other known Phytophthora species.
Abstract: Fifty genotypes of black pepper (Piper nigrum L.) including released varieties and cultures were screened against Phytophthora capsici foot rot disease in nursery to identify resistant materials. None of the genotypes screened were immune to P. capsici. ‘Kalluvally II’, ‘Panniyur 5’, and ‘Kalluvally IV’, however, showed less than 60% leaf infection and were statistically at par with ‘Balankotta’, ‘Cheriyakaniakadan’, and ‘Shimoga’. ‘Kalluvally II’ also suffered relatively lower mortality rates (26.67%), implying some tolerance against P. capsici. ‘Panniyur 5’ was moderately susceptible while the remaining cultivars were clearly susceptible to the disease, as they showed higher leaf infection and mortality.
Abstract:Phytophthora ramorum, the cause of sudden oak death in the western USA and a damaging pathogen in Europe, is a biosecurity threat of unknown magnitude to New Zealand and Australasia because of its presence in traded ornamental plants. Knowledge of potential hosts acting as carriers and of symptoms caused by the pathogen on such hosts will strengthen precautionary quarantine regulations to prevent inadvertent introductions of P. ramorum into the region. Also, the identification of potential hosts will permit determination of areas at risk within countries that do not have P. ramorum. Susceptibility of New Zealand plants, including 17 endemic and three commercial species (Eucalyptus globulus, Pinus radiata and Acacia melanoxylon), as well as two known Rhododendron cultivar hosts, was determined by analysing the size of lesions on inoculated excised leaves and branches, while infectivity was determined by counting sporangia produced on leaves. In order to identify extremely susceptible hosts, seven species were inoculated using three concentrations of zoospores ranging from low (1 × 102 zoospores/mL) to high (5 × 103 zoospores/mL). In branch inoculations, P. radiata and Nothofagus fusca were as susceptible as the Rhododendron cultivars. Pseudopanax arboreus, Fuchsia excorticata and one Rhododendron cultivar were equally susceptible in leaf inoculations. However, F. excorticata was the only species with 100% infected leaves, high foliar sporulation and was highly susceptible at all three zoospore concentrations. Leptospermum scoparium was the only asymptomatic foliar host that had high reisolations of the pathogen. F. excorticata, P. radiata, N. fusca, P. arboreus and L. scoparium should be added to the potential host list for ,i>P. ramorum and monitored for symptoms and sporulation in gardens and nurseries in the USA and Europe. As part of a precautionary strategy, these species are suitable candidates for targeted surveillance programs in high-risk incursion areas of New Zealand. Furthermore, the sympatry of foliar hosts with high infectivity and of highly susceptible stem hosts was identified: these areas may be at risk for the development of a forest epidemic.
Abstract: A recent study to determine the cause of collar and root rot disease outbreaks of cold tolerant Eucalyptus species in South Africa resulted in the isolation of two putative new Phytophthora species. Based on phylogenetic comparisons using the ITS and β-tubulin gene regions, these species were shown to be distinct from known species. These differences were also supported by robust morphological characteristics. The names, Phytophthora frigida sp. nov. and Phytophthora alticola sp. nov. are thus provided for these taxa, which are phylogenetically closely related to species within the ITS clade 2 (P. citricola, P. tropicali and P.multivesiculata) and 4 (P. arecae and P. megakarya), respectively. Phytophthora frigida is heterothallic, and produces stellate to rosaceous growth patterns on growth medium, corraloid hyphae, sporangia with a variety of distorted shapes and has the ability to grow at low temperatures. Phytophthora alticola is homothallic and has a slower growth rate in culture. Both P. frigida and P. alticola are pathogenic to Eucalyptus dunnii. In pathogenicity tests, they were, however, less pathogenic than P. cinnamomi, which is a well-known pathogen of Eucalyptus in South Africa.
Abstract: The objective of this study was to evaluate the potential of recurrent selection for improving seed yield in a soybean population. Seventeen parents, chosen for their high yield potential in the sub-tropical environments of Southern and Central Queensland were intermated using a diallel cross mating system. F1 plants were intercrossed in pairs for two more generations and then allowed to self for two generations to produce the base population. The selection criterion was the seed yield of S1 lines grown at Hermitage Research Station. The selection intensity was approximately 10%. After five cycles of recurrent selection the mean yield of the population and the mean yield of the lines selected for recombination had improved by 17% and 54% respectively, in comparison with one of the original parents, Davis. Progress from recurrent selection was evaluated using fifty randomly selected lines from the base population and from each of the first three cycles of selection. The average gain per cycle in seed yield, averaged over five sites was 128 kg ha-1 or 5.4% of the yield of the base population. Greater progress (9.8% per cycle) was measured at the testing site for selection (Hermitage Research Station) which was heavily infested with phytophthora root and stem rot than at three other sites (2.7% per cycle) which has nil or low levels of disease. Broad sense heritabilities for the base population and for cycles 1, 2 and 3 in the evaluation trial were 0.34, 0.25, 0.13 and 0.14 respectively compared with a mean heritability of 0-64 for the first five cycles of the selection experiment. Expected gain from further cycles of selection was estimated at 102 kg ha-1 per cycle for three replicates at a single site. In the evaluation trial correlated responses to selection for yield were found in a number of other traits. Days to flowering was reduced by 0.8 days/cycle while days to maturity increased by 1 day/cycle. Seed shattering score was significantly reduced from 0.98 to 0-69 while there were only minor changes in plant height, lodging and seed coat quality score. Resistance to root and stem rot and seed coat mottling, due to primarily to soybean mosaic virus, were also significantly increased. The improvement in phytophthora resistance was expected as the field used for S1 yield testing had become infested with the disease. A covariate analysis indicated that about half the improvement in seed yield at the Hermitage site was due to the correlated response of increased resistance to phytophthora root and stem rot.
Abstract: Pathogenicity tests were carried out on the bark of Alnus glutinosa with 19 isolates of the standard (near-tetraploid) hybrid alder phytophthora, nine isolates representing its known heteroploid variants and 11 isolates of P. cambivora, a probable parent species of the hybrid. Over a 4-year period, 12 experiments were conducted on living alder logs incubated at 20°C. Most isolates of the standard hybrid and those of the 'Dutch variant' were highly aggressive to alder bark. Isolates of the 'Swedish', 'UK' and 'German variants', and of P. cambivora, were only weakly pathogenic. Also, isolates of P. fragariae, P. cinnamomi, P. sp. 'O-group', P. cryptogea, P. megasperma, P. gonapodyides and P. citricola were either weakly or nonpathogenic. Rates of lesion development were greatest on logs cut during July–October, slower on logs cut between November and March and zero on logs cut during April, indicating a strong seasonal effect. Other evidence indicated that lesion development was subject to critical thresholds of host resistance. The standard hybrid was nonpathogenic to the bark of four other hardwood and two conifer species, indicating that it is relatively host specific. In contrast, P. cambivora was an aggressive pathogen on live bark of Quercus and Castanea. The significance of these results is discussed.
Abstract: Lesions of Phytophthora infestans were found on woody nightshade (Solanum dulcamara), black nightshade (S. nigrum) and S. sisymbriifolium during a nationwide late blight survey in the Netherlands in 1999 and 2000. Pathogenicity and spore production of P. infestans isolates collected from potato (S. tuberosum), S. nigrum, S. dulcamara and S. sisymbriifolium were determined on several host plant species, and oospore formation in naturally infected and inoculated foliage of hosts was quantified. The present population of P. infestans in the Netherlands is pathogenic on S. nigrum, S. dulcamara and S. sisymbriifolium. Oospores were produced in leaves of ,i>S. nigrum, S. dulcamara and S. sisymbriifolium following infection with A1 and A2 isolates. Therefore these plant species should be regarded as alternative hosts for the late blight pathogen. In the case of S. nigrum and S. dulcamara infection was a relatively rare event, suggesting that diseased plants do not significantly contribute to the overall late blight disease pressure present in potato-production areas. Oospore production in ageing S. nigrum and S. dulcamara plants in autumn, however, may generate a considerable source of (auto) infections in following years. Considerable numbers of sporangia and oospores were produced on S. sisymbriifolium following infection with P. infestans. Additional field infection data are needed to evaluate the epidemiological consequences of a commercial introduction of S. sisymbriifolium as a potato cyst nematode trap crop.
Abstract: The pathogenicity of five species of Phytophthora to English walnut was studied in a greenhouse experiment. Phytophthora cinnamomi was the most aggressive species, causing severe root rot and seedling mortality. The other species tested, P. cambivora, P. citricola, P. cactorum and P. cryptogea, did not induce visible crown symptoms on seedlings 2 months after inoculation. Some strains of P. cambivora and P. cactorum also caused taproot damage to seedlings. All except one of the tested isolates caused significant necrosis of fine roots and a significant reduction of root weight compared with noninoculated seedlings. Reduction of above-ground plant development was not statistically significant. While P. cinnamomi is well known as an aggressive primary pathogen of English walnut, the other species of Phytophthora may act as predisposing factors to walnut decline, affecting root system development and increasing host vulnerability to environmental stress.
Abstract:Pyracantha spp. (firethorn) are grown mainly for their display of red, orange or yellow fruit in the autumn and winter and is also used as a hedge plant. In 2001 and 2002, two samples of berries of Pyracantha (both orange berry varieties) from Hampshire (RHS/10715/01) and from North Yorkshire (RHS/13308/02), showing signs of rotting, were received at the advisory service of the Royal Horticultural Society. The symptoms observed were brown/black spots that enlarged, with the whole berry eventually turning black and shrivelled. Whole clusters of berries were affected but the symptoms were only
observed on berries produced on the lower branches of the Pyracantha shrubs.
Abstract