Abstract: An experiment, focusing on the effects of chronically enhanced O3 regimes on young beech (Fagus sylvatica) and on the microbial rhizosphere community structure, was conducted from November 2002 to August 2006 in eight field lysimeters at the Helmholtz Zentrum München. The instrumentations of the lysimeters enabled the establishment of the water balance in the unsaturated zone and the assessment of the water uptake by plants. Further, the containment provided by the lysimeters made it possible to apply a root rot pathogen infection without contaminating the surrounding soil. A free-air fumigation system allowed to double the O3 concentration in the air above four lysimeters relative to the ambient air. To avoid damage of the leaves the maximum O3 concentration was limited to 150 nL L-1. For nearly 70% of the time the set-point concentration was reached within 10%. In the final harvest the whole soil column was retrieved and a nearly complete data-set of above-ground and below-ground parameters became available.
Abstract: Isolates belonging to an undescribed Phytophthora species were frequently recovered during an oak forest soil survey of Phytophthora species in eastern and north-central USA in 2004. The species was isolated using an oak leaf baiting method from rhizosphere soil samples collected from Quercus rubra, Q. macrocarpa, and Q. phellos. This species is formally described as P. quercetorum. It is homothallic and has aplerotic oogonia and paragynous antheridia. It produces papillate sporangia (occasionally bipapillate) of ovoid-elongated shapes. Its temperature optimum for growth is ca 22.5C with the upper limit of ca 32.5C. P. quercetorum differs from the morphologically related P. quercina in producing distinct submerged colony-patterns, different growth-temperature requirements, and oogonial shapes and sizes. Phylogenetic analyses using seven nuclear loci supported P. quercetorum as a novel species within clade 4, closely related to P. arecae, P. palmivora, P. megakarya, and P. quercina.
Abstract: A simple in-vitro ‘wet-plate’ method for mass-producing Phytophthora nicotianae zoospores at ≥ 1.0 × 106 zoospores/ml is described. Temperature critically affected zoospore production; 22 °C was optimum, while 36 °C was completely inhibitory. Zoospores being the most important propagule of P. nicotianae, temperature of recycled irrigation water may be manipulated to reduce diseases in irrigated nursery crops.
Abstract: Considerable losses of citrus trees have been observed in the major citrus-growing areas of Spain. Samples were collected from 132 orchards, and isolations and pathogencity tests were conducted to determine the aetiology of a serious canker disease. Affected trees showed cankers on the scion that frequently began on the branches. Three Phytophthora species were identified based on their morphological, cultural, physiological and molecular profiles. Phytophthora citrophthora was the main species associated with this new syndrome in 114 orchards. Phytophthora nicotianae (syn. P. parasitica) was isolated from nine orchards as the sole Phytophthora species and in coinfection with P. citrophthora from another nine orchards. Phytophthora citricola was isolated only from one orchard. In stem-inoculation studies conducted under greenhouse conditions, clementine mandarin cv. Hernandina and sweet orange cv. Navel Late were more susceptible to P. citrophthora than sour orange and Carrizo citrange rootstocks. Clementine cv. Hernandina was also highly susceptible in field inoculation experiments. In agreement with field surveys, clementine mandarin cultivars were the most affected, their rootstocks remaining healthy. Phytophthora citrophthora was found to be the predominant species in orchard soils; however, P. nicotianae was also isolated. This information changes the scenario of diseases caused by Phytophthora spp. in Spain and consequently, the present knowledge of epidemiology and the effectiveness of the current control measures should be reassessed.
Abstract: The aetiology and frequency of Phytophthora spp. in discoloured xylem tissue beneath phloem lesions was investigated in a range of broadleaved trees infected with P. ramorum, P. kernoviae, P. cambivora, P. citricola and other species. Isolation was attempted from the inner surface of 81 sterilized discoloured wood panels (6 × 4 cm) from 53 trees. Discolouration mostly extended 1–5 mm into the xylem (75%), but incursions of 6–10 mm (10%) and 10–25 mm (15%) were frequent. Of the wood panels, 81% yielded Phytophthora spp. In 66 cases, both a wood panel and an overlying phloem panel were sampled. In 56% of these, a Phytophthora sp. was isolated from both the wood and the phloem panel. In 23% the Phytophthora sp. was isolated from the wood panel only and in 8% from the phloem panel only. Small 'island' phloem lesions, often in linear arrays adjacent to main lesions, were a common feature of Fagus sylvatica and Quercus spp. trees infected with P. ramorum or P. kernoviae. Island lesions were often connected by underlying strips or intermittent pits of discoloured xylem in line with the wood grain. Phytophthora ramorum, P. kernoviae and other Phytophthora spp. were successfully isolated from these connecting xylem features with P. ramorum and P. kernoviae also recovered from discoloured tissue 5–25 mm below exposed xylem surfaces 24–27 months after the overlying phloem was removed. These results show that these pathogens commonly occupy xylem beneath phloem lesions; that they can perennate in xylem tissue; that they can spread in xylem tissue ahead of phloem lesions; and indicate that they may initiate new phloem lesions in this way. Such colonization must lead to at least local xylem dysfunction. It is recommended that, if xylem discoloration is present, isolation of the Phytophthora sp. should be attempted from the xylem as well as the bark; also, that removal of infected outer sapwood should be undertaken during excision of bleeding lesions for disease control and in protocols aimed at preventing national or international spread of these tree stem pathogens.
Abstract:Phytophthora ramorum, recently found in the US, is causing concern for hardwood forests and the nursery industry. In an effort to identify some of the environmental limitations to growth and sporulation we undertook a laboratory study of four US and three European (EU) isolates. On V8 media, isolates grew when incubated at 2–28 C and produced chlamydospores at 8–28 C. Sporangia were produced at all temperatures tested: 10–30 C for US isolates and 6–26 C for EU isolates. Optimal temperatures were 16–26 C for growth, 14–26 C for chlamydospore production and 16–22 C for sporangia production. US isolates grew less and produced fewer spores when exposed to increasing doses of near-UV radiation (50–300 µW/cm2) and visible radiation (250–1500 µW/cm2). EU isolates were exposed to 300 µW/cm2 near-UV only, which significantly reduced growth of one of three isolates and had no significant effect on spore production. In our studies P. ramorum tolerated a broad range of temperature and light conditions, which suggests that it is capable of establishment in a wide geographic area.
Abstract: A survey was taken up during crop seasons of 2003–2004 and 2004–2005 in Baghpat, Muzaffarnagar, Meerut and Ghaziabad districts of western U.P. for assessing late blight severity. In general, late blight severity was high during 2004–2005 in most of the localities and ranged from 4.3–81%. During 2003–2004, disease severity ranged from 2.5 to 68.8%. Ghaziabad and Baghpat districts were worst affected. A1 mating type (old strain) was dominant in all the four districts. Most of the isolates collected from the four districts were metalaxyl sensitive. Race complexity of 7 and 8 genes were detected from Meerut while race complexicity of 5, 8, 9; 7, 8; and 5, 7, 9 genes were recorded from Baghpat, Muzaffarnagar and Ghaziabad districts, respectively.
Abstract: Two two-compound mixtures of candidate structures for the Phytophthora α1 mating hormone have been synthesized. The mixtures were designed to have differing configurations at C3, but proved to be identical. This suggests that epimerization occurred at C3, and comparison with the data for the natural samples suggest that this is also a mixture of isomers.
Abstract:PnPMA1, a gene encoding a putative P-type plasma membrane H+-ATPase, has been isolated by differential screening of a Phytophthora nicotianae germinated cyst cDNA library. PnPMA1 is differentially expressed during pathogen asexual development with a more than 10-fold increase in expression in germinated cysts, the stage at which plant infection is initiated, compared to vegetative or sporulating hyphae or motile zoospores. PnPMA1 proteins are encoded by two closely linked genes that have no introns and encode identical proteins having 1068 amino acid residues and a molecular mass of 116.3 kDa. PnPMA1 shows moderate identity (30–50%) to plant and fungal plasma membrane H+-ATPases and weak identity to other P-type cation-transporting ATPases. PnPMA1 contains all the catalytic domains characteristic of H+-ATPases but also has a distinct domain of not, vert, similar155 amino acids that forms a putative cytoplasmic loop between transmembrane domains 8 and 9, a feature that is not present in PMA1 proteins from other organisms. Polyclonal antibodies raised against the 155 residue domain were shown by immunogold labelling to react with a protein in the plasma membrane of P. nicotianae germinated cysts but not with the plasma membrane of motile zoospores. Genetic complementation experiments demonstrated that the P. nicotianae PnPMA1 is functional in yeast, Saccharomyces cerevisiae.
Abstract: Isolation, detection with diagnostic polymerase chain reaction (PCR), and microscopy demonstrated the presence of Phytophthora ramorum in the sapwood of mature, naturally infected tanoak (Lithocarpus densiflorus) trees. The pathogen was strongly associated with discolored sapwood (P < 0.001), and was recovered or detected from 83% of discolored sapwood tissue samples. Hyphae were abundant in the xylem vessels, ray parenchyma, and fiber tracheids. Chlamydospores were observed in the vessels. Studies of log inoculation indicated that P. ramorum readily colonized sapwood from inoculum placed in the bark, cambium, or sapwood. After 8 weeks, radial spread of P. ramorum in sapwood averaged 3.0 to 3.3 cm and axial spread averaged 12.4 to 18.8 cm. A field study was conducted to determine if trees with infected xylem had reduced sap flux and reduced specific conductivity relative to noninfected control trees. Sap flux was monitored with heat-diffusion sensors and tissue samples near the sensors were subsequently tested for P. ramorum. Adjacent wood sections were excised and specific conductivity measured. Both midday sap flux and specific conductivity were significantly reduced in infected trees versus noninfected control trees. Vessel diameter distributions did not differ significantly among the two treatments, but tyloses were more abundant in infected than in noninfected trees. Implications for pathogenesis, symptomology, and epidemiology are discussed.
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