In Australia, Phytophthora cinnamomi is the only species reported as the causal agent of stem canker and root rot in macadamia. In other countries, five Phytophthora species have been reported to cause diseases in macadamia, which led us to question if more than one Phytophthora species is responsible for poor tree health in macadamia orchards in Australia. To investigate this, samples were collected from the rhizosphere, stem, and root tissues of trees with and without symptoms, nurseries, and water sources from 70 commercial macadamia orchards in Australia. Phytophthora isolates were identified based on morphological characteristics and DNA sequencing. P. cinnamomi was the most predominant and widely distributed species, and was obtained from the different types of samples including symptomless root tissues. In addition to P. cinnamomi, only P. multivora was isolated from diseased tissue (stem canker) samples. Six other Phytophthora species were obtained from the rhizosphere samples: P. pseudocryptogea, P. citrophthora, P. nicotianae, P. gondwanense, P. sojae, and a new Phytophthora taxon. Only P. cinnamomi was obtained from macadamia nursery samples, while five Phytophthora species were obtained from water sources. Of the heterothallic Phytophthora species, mating type A2 isolates were dominant in P. cinnamomi isolates, whereas only mating type A1 isolates were obtained for P. nicotianae, P. pseudocryptogea, and P. citrophthora. Pathogenicity assays revealed that P. cinnamomi and P. multivora caused significantly larger stem and leaf lesions than P. citrophthora, P. nicotianae, and P. pseudocryptogea. Phytophthora sp. and P. sojae were nonpathogenic towards leaves and stems.

中文翻译：

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澳大利亚澳洲坚果果园中疫霉属物种的流行及其引起茎溃疡病的能力

在澳大利亚，Phytophthora cinnamomi 是唯一被报道为澳洲坚果茎溃疡病和根腐病病原体的物种。在其他国家，据报道有五种疫霉属会导致澳洲坚果疾病，这让我们怀疑是否有不止一种疫霉属物种导致澳大利亚澳洲坚果果园的树木健康状况不佳。为了对此进行调查，我们从澳大利亚 70 个商业澳洲坚果果园的有和无症状树木的根际、茎和根组织、苗圃和水源中收集了样本。基于形态特征和 DNA 测序鉴定了疫霉属分离株。P. cinnamomi 是最主要和分布最广的物种，它是从不同类型的样本中获得的，包括无症状的根组织。除了P. cinnamomi，只有P. multivora 是从患病组织（茎溃疡病）样本中分离出来的。从根际样本中获得了其他六种疫霉属物种：假隐孢霉属、柠檬酸疫霉属、烟草疫霉属、冈瓦氏疫霉属、大豆疫霉属和一个新的疫霉属类群。只有 P. cinnamomi 是从澳洲坚果苗圃样品中获得的，而五种 Phytophthora 物种是从水源中获得的。在异种疫霉属物种中，交配型 A2 分离株在肉桂假单胞菌分离株中占优势，而仅在 P. nicotianae、P.pseudocryptogea 和 P. citrophthora 中获得交配型 A1 分离株。致病性分析表明，肉桂假单胞菌和多食假单胞菌比 P. citrophthora、P.nicotianae 和 P.pseudocryptogea 引起明显更大的茎叶损伤。疫霉属 和 P. sojae 对叶和茎无致病性。