Comparison of <i>Phytophthora sojae</i> Populations in Iowa and Nebraska to Identify Effective <i>Rps</i> Genes for Phytophthora Stem and Root Rot Management

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Abstract

Phytophthora stem and root rot (PSRR) of soybean, caused by the oomycete Phytophthora sojae, is prevalent in Iowa and Nebraska. Reducing losses to PSRR primarily relies on growing cultivars with specific resistance (Rps) genes. Predominant genes used in commercial soybean cultivars include Rps 1a, Rps 1c, Rps 1k, and Rps 3a. Knowing which Rps gene to deploy depends on knowledge of which genes are effective against the pathogen. From 2016 to 2018, 326 isolates of P. sojae from were recovered from fields in Iowa and Nebraska and classified into pathotypes based on their virulence on 15 soybean genotypes. A total of 15 and 10 pathotypes were identified in Iowa and Nebraska, respectively. Almost all isolates were virulent on Rps 1a, and over 70% of isolates were virulent on Rps 1c and Rps 1k. Only 2.3% of isolates from Iowa were virulent on Rps 3a. Among commercial soybean cultivars tested in the Illinois Soybean Variety trials from 2010 to 2020, Rps 1c was always the most frequently reported gene, followed by Rps 1k. In contrast, Rps 1a and Rps 3a were present in less than 10% and less than 5% of the cultivars tested, respectively. Because many of the P. sojae isolates in our study were virulent on Rps 1a, Rps 1c, and Rps 1k, soybean cultivars with these genes are unlikely to provide protection against PSRR unless they have a high level of partial resistance.

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A molecular phylogeny of Phytophthora and related oomycetes.

D.E.L. Cooke, A Drenth, J.M. Duncan … (2000)

Phylogenetic relationships among 50 Phytophthora species and between Phytophthora and other oomycetes were examined on the basis of the ITS sequences of genomic rDNA. Phytophthora grouped with Pythium, Peronospora, and Halophytophthora, distant from genera in the Saprolegniales. Albugo was intermediate between these two groups. Unlike Pythium, Phytophthora was essentially monophyletic, all but three species forming a cluster of eight clades. Two clades contained only species with nonpapillate sporangia. The other six clades included either papillate and semipapillate, or semipapillate and nonpapillate types, transcending traditional morphological groupings, which are evidently not natural assemblages. Peronospora was related to P. megakarya and P. palmivora and appears to be derived from a Phytophthora that has both lost the ability to produce zoospores and become an obligate biotroph. Three other Phytophthoras located some distance from the main Phytophthora-Peronospora cluster probably represent one or more additional genera.