Abstract
A previously unknown Phytophthora species was isolated from irrigation water in Virginia, USA. This novel species produces abundant noncaducous and nonpapillate sporangia in soil water extract solution. It sometimes produces chlamydospores and hyphal swellings in aged cultures and in Petri’s solution. This species has optimum vegetative growth at 30 C and grows well at 35 C. The lowest and highest temperatures for growth are 5 and 40 C. All isolates examined in this study are compatibility type A1 and produce mostly plerotic oospores when paired with an A2 mating-type tester of P. cinnamomi. Sequence analyses of the rDNA internal transcribed spacer (ITS) regions and the mitochondrially encoded cytochrome c oxidase 1 (cox 1) gene placed this species in clade 9 of the genus Phytophthora. These characteristics support the description of this taxon as a new species for which we propose the name P. hydrogena sp. nov. Further phylogenetic and physiological investigations of clade 9 species revealed a high-temperature tolerant cluster including P. hydrogena, P. aquimorbida, P. hydropathica, P. irrigata, P. chrysanthemi, P. insolita, P. polonica and P. parsiana. These species all grow well at 35 C. The monophyly of the species in this heat-tolerant cluster except P. insolita and P. polonica is highly supported by the maximum-likelihood analyses of the ITS and cox 1 sequences.
Highlights
Many species in the genus Phytophthora have been considered destructive plant pathogens
Traditional taxonomy of Phytophthora species was based on morphological characteristics, as exemplified by the classic morphological key by Waterhouse (1963), which separated the genus into six groups and is still widely used today
This study described a novel species, Phytophthora hydrogena, based on its distinct phylogenetic position, morphology and physiology, and revealed a cluster of high-temperature tolerant species within Phytophthora clade 9
Summary
Many species in the genus Phytophthora have been considered destructive plant pathogens. The SOD disaster reminded the science community and the public of the constant plant biosecurity threat posed by Phytophthora species and invigorated research in these pathogens (Gruenwald et al 2012). Traditional taxonomy of Phytophthora species was based on morphological characteristics, as exemplified by the classic morphological key by Waterhouse (1963), which separated the genus into six groups and is still widely used today. Due to the plasticity and overlapping of morphological characteristics among species, taxonomy based on morphology cannot be used to reliably distinguish some species within the genus (Erwin and Ribeiro 1996, Naher et al 2011)
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