Elsevier

Mycological Research

Volume 110, Issue 11, November 2006, Pages 1309-1322
Mycological Research

Phytophthora foliorum sp. nov., a new species causing leaf blight of azalea

https://doi.org/10.1016/j.mycres.2006.07.017Get rights and content

Abstract

A previously unknown Phytophthora was recovered more than 60 times from evergreen hybrid azalea leaves collected during surveys for the sudden oak death pathogen Phytophthora ramorum in California and Tennessee. The novel Phytophthora was discovered when genomic DNA from this species cross-reacted with the ITS-based diagnostic PCR primers used to screen plants for the presence of P. ramorum. This species had caducous, semi-papillate sporangia, was homothallic with both paragynous and amphigynous antheridia, and was pathogenic on both wounded and intact azalea leaves. Nuclear and mitochondrial sequence data indicate that this species is related to, but distinct from, P. ramorum. AFLP analysis indicates that the isolates of this species have limited genotypic diversity and share no markers with P. ramorum. This paper presents the formal description of P. foliorum as a new species and underscores the need for caution when relying solely on DNA-based diagnostic tools.

Introduction

Commonly called water moulds, more than 60 Phytophthora species have currently been described and are classified in the kingdom Chromista (Yoon et al., 2002, Erwin and Ribeiro, 1996). Most Phytophthora species are considered plant pathogens (Cooke et al., 2000, Erwin and Ribeiro, 1996). P. ramorum, a recently described species (Werres et al. 2001), has been found to cause oak death in 14 counties of coastal forest in California and in a single county in southern Oregon in the USA. The disease has been detected in nurseries and garden centres in the western USA (Washington, Oregon and California), British Columbia, Canada, as well as in Europe (Hansen et al. 2005). P. ramorum is found on several genera of woody nursery stock in Europe (Tooley et al. 2004). In response to the concern that P. ramorum will move on nursery stock and infect the red oak woodlands of the eastern USA, the United States Department of Agriculture-Animal and Plant Health Protection Service–Plant Protection and Quarantine (USDA-APHIS-PPQ) funded a nationwide survey of nursery stock in 2004 and 2005. New species of Phytophthora have been described as a result of this survey and forest survey efforts (Hansen et al., 2003, Jung et al., 2003). The USDA mandated that a single ITS-based nested PCR test be used the detection of P. ramorum in the US national survey. P. hibernalis, a close relative of P. ramorum, has been previously shown to yield false-positives in this P. ramorum detection assay (Blomquist et al. 2005). As a result of intensive survey efforts in California and Tennessee in 2004 and 2005, a previously undescribed Phytophthora species with distinctly different morphology from P. ramorum was identified. This previously undescribed Phytophthora species produced a false-positive in the P. ramorum detection assay. This paper presents the morphological description of P. foliorum as a new pathogen of azalea, the phylogenetic relationship to other Phytophthora species based on nuclear and mitochondrial DNA sequences, and the intraspecific variation within the species P. foliorum as assessed by AFLP.

Section snippets

Cultures

All isolates used in this study are listed in Table 1. Cultures were obtained from leaf samples submitted during the national survey for the sudden oak death pathogen Phytophthora ramorum. Leaf tissue from the edge of foliar lesions was plated on corn meal agar, (CMA; Sigma, St Louis, MO) amended with PARP (25 ppm pimaricin, 100 ppm ampicillin, 25 ppm rifampicin and 25 ppm pentachloronitrobenzene) (Erwin & Ribeiro 1996). Cultures were subsequently hyphal-tipped to ensure single isolates.

Isolates and pathogenicity

More than 60 isolates of Phytophthora foliorum were recovered from leaf samples of azalea collected in California and Tennessee during P. ramorum surveys in the summer of 2004 and winter of 2005. P. foliorum was isolated from the margin of brown leaf spots and healthy tissue. Inoculation experiments to complete Koch's postulates with ‘Pink Ruffles’ and P. foliorum isolate 192 resulted in lesions similar to those seen on naturally-infected plants. Artificially-wounded leaves developed lesions at

Taxonomy

Phytophthora foliorum Donahoo & Lamour, sp. nov.

Etym.: foliorum refers to pathogenicity on leaves.

Species homothallica, oosporas in cultura procerans; oogoniis in medio 33 μm; antheridias paragynis. Sporangiis semipapillatis, in medio 51 × 34 μm. Sporangiis saepe deciduis, cum pedicellulo brevi (<5–20 μm). Temperaturae optima 18–22 °C. Incrementum diurnum radiale 3 mm in agaro V8 ad 20 °C.

Typus: USA: Tennessee: isol. ex Azalea ‘Pink Ruffles’ in nursery, May 2004, K. Lamour 192 (ATCC MYA-3638 —

Discussion

This is the first report of Phytophthora foliorum sp. nov., a pathogen of azalea. P. foliorum belongs to group III of the Waterhouse classification based on morphological characteristics (Waterhouse, 1963, Erwin and Ribeiro, 1996). Morphologically, P. foliorum is distinct from its sister taxa (P. ramorum, P. lateralis, and P. hibernalis). P. foliorum differs from P. ramorum in that it is homothallic and rarely if ever produces sporangia in culture. P. foliorum differs from P. lateralis in that

Acknowledgements

We thank Frank Martin for excellent advice and use of his data for phylogenetic analysis and Mark Windham for providing plants for inoculation tests. We would like to acknowledge Ledare Finley and Marinell Soriano for their excellent technical assistance and Anni Self from the Tennessee Department of Agriculture for managing the collection and pre-screening of nursery samples in Tennessee.

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