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Page number:1164 
Remarks (internal):Phytophthora ramorum can easily be identified morphologically by its unique combination of semipapillate, deciduous sporangia with short pedicels and high length:width ratio, large chlamydospores, relatively slow growth and low cardinal temperatures for growth. P. palmivora, which it resembles superficially, has smaller chlamydospores, papillate sporangia with a lower length:width ratio and higher cardinal temperatures. The closest known relative, P. lateralis, has mainly non-papillate sporangia, with a wider pore, and chlamydospores which are mainly lateral, whereas in P. ramorum sporangia are semi-papillate with a narrow pore and chlamydospores are mainly terminal and intercalary. Very few Phytophthora species have sporangia that are both semipapillate and deciduous. Of these species, only P. colocasiae produces chlamydospores, however, its sporangia are ovoid and have a much lower length:width ratio than those of P. ramorum. Phytophthora ramorum was isolated from Rhododendron and Viburnum. Artificial inoculation experiments with isolates from both host plants confirmed that this species causes twig blight in Rhododendron, identical to the symptoms with which the isolates were initially associated. Reisolation of the fungus confirmed the identity of the pathogen morphologically, thus fulfilling Koch's postulates. On old Rhododendron plants, sometimes young shoots sprout from bases of branches attacked by dieback ( J. de Gruyter, pers. comm.). Apparently the plant is sometimes able to arrest the infection or it dies out naturally.
The fact that fourteen isolates, isolated in different years and localities in Germany and The Netherlands only represent the A1 mating type is striking. This suggests that the investigated isolates may constitute a clonal lineage, introduced into western Europe in recent times. However, the intraspecific variation as re¯ected by the differences in AFLP fingerprints and lactate dehydrogenase profiles (W. A. Man in` t Veld, unpubl.) suggests that the population is more variable, and therefore more ancient. Alternatively, P. ramorum may have been introduced on several occasions, or a single introduction occured of an already variable population. A wider population-genetic study will be necessary to elucidate this.
 
Description type:Original description 
Description:Phytophthora ramorum Werres, De Cock & Man in't Veld, sp. nov.
Colonies on CPA, V8, and CMA submerged, showing concentric rings; on CHA with appressed aerial mycelium and an indistinct rosette pattern. Main hyphae up to 8 µm wide. Chlamydospores abundant on agar, commonly intercalary and terminal, occasionally lateral, (sub-)globose, 22-53,8-72 µm diam. Sporangia abundantly produced on agar as well as on hemp seeds in soil extract, single or in sympodia, terminal or lateral after proliferation of the subtending hypha, ellipsoid, spindle-shaped or elongate-ovoid, with rounded or occasionally tapering base, with a single, narrow and indistinct papilla, 20-32 x 40-80 µm, average 52 x 24 µm, average length: width ratio 2,16, but on hemp seeds in soil extract lower; caducous with a short pedicel, occasionally without pedicel. Zoospores produced in water at temperatures below 20°. Heterothallic. Oogonia only developed in dual cultures with a complementary strain, subglobose, 28-31,2-38 µm diam, wall smooth, colourless, up to 2 µm thick. Antheridia single, terminal, diclinous, amphigynous, spherical to barrelshaped, approx. 10-18x14-16 µm . Oospores plerotic. Cardinal temperatures for growth: minimum 2°, optimum 20°, maximum 26°. Daily radial growth on CPA at 20° 2-8 mm.
 
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