Phytophthora Species in Rivers and Streams of the Southwestern United States

Abstract

Phytophthora species were isolated from rivers and streams in the southwestern United States by leaf baiting and identified by sequence analysis of internal transcribed spacer (ITS) ribosomal DNA (rDNA). The major waterways examined included the Rio Grande River, Gila River, Colorado River, and San Juan River. The most prevalent species identified in rivers and streams were Phytophthora lacustris and P. riparia, both members of Phytophthora ITS clade 6. P. gonapodyides, P. cinnamomi, and an uncharacterized Phytophthora species in clade 9 were also recovered. In addition, six isolates recovered from the Rio Grande River were shown to be hybrids of P. lacustris ؋ P. riparia. Pathogenicity assays using P. riparia and P. lacustris failed to produce any disease symptoms on commonly grown crops in the southwestern United States. Inoculation of Capsicum annuum with P. riparia was shown to inhibit disease symptom development when subsequently challenged with P. capsici, a pathogenic Phytophthora species. IMPORTANCEMany Phytophthora species are significant plant pathogens causing disease on a large variety of crops worldwide. Closer examinations of streams, rivers, and forest soils have also identified numerous Phytophthora species that do not appear to be phytopathogens and likely act as early saprophytes in aquatic and saturated environments. To date, the Phytophthora species composition in rivers and streams of the southwestern United States has not been evaluated. This article details a study to determine the identity and prevalence of Phytophthora species in rivers and streams located in New Mexico, Arizona, Colorado, Utah, and Texas. Isolated species were evaluated for pathogenicity on crop plants and for their potential to act as biological control agents.

Diversity and Distribution of Phytophthora Species in Protected Natural Areas in Sicily

“…In accord with previous studies in other areas of the world [38,39,41,50], Clade 6 species prevailed in rivers, indicating their adaptation to aquatic environments. Interestingly, two mainly aquatic opportunistic pathogens from Clade 6, P. gonapodyides and P. lacustris, which often co-occur in river systems in temperate regions of North America, Europe, and Asia [40,[77][78][79], showed opposite distribution patterns in Sicily.…”

“…In recent years, great attention has been paid to surface water as a source of Phytophthora inoculum in natural ecosystems. Surveys of rivers, streams, and riparian ecosystems in several continents have revealed a huge diversity of Phytophthora species, including primarily aquatic species which are considered as opportunistic pathogens, but also soilborne and airborne primary pathogens [8,20,21,[37][38][39][40][41]. However, all Phytophthora species have the potential to be disturbance factors in natural ecosystems, in particular, those of exotic origin, provided that the environmental conditions are conducive to disease development [9,10,32].…”

Phytophthora ramorum and Phytophthora gonapodyides Differently Colonize and Contribute to the Decomposition of Green and Senesced Umbellularia californica Leaves in a Simulated Stream Environment

“…It is well-established that California bay leaves are an optimal substrate for P. ramorum, and though clade 6 Phytophthora species are known from a great variety of vegetative litter, it is uncertain how conducive other vegetative litter would be to survival and sporulation of either species. Stamler et al [20] recovered primarily clade 6 and 9 Phytophthora species from rivers in the southwestern USA using leaves of Salix and Populus species, common as riparian trees, as bait. It would be expected that natural leaf litter in such ecosystems would also harbor these organisms.…”

“…Incidental to this research has been the discovery that many species of Phytophthora are abundant in natural surface waters, especially in streams. Many such species are so widespread and regularly encountered that they are now considered resident, if not endemic, and characteristic of such environments [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. Nevertheless, isolates of well-known plant pathogenic species or species complexes are also regularly recovered, often without discernible symptoms or signs of disease on the vegetation [7,9,15,16,21,[23][24][25][26].…”

Draft genome assemblies for tree pathogensPhytophthora pseudosyringaeandPhytophthora boehmeriae

“…Among the most commonly detected Phytophthora species in the United Kingdom in diseased plants and soil is Phytophthora pseudosyringae (Clade 3) ( Riddell et al 2019 ). P. pseudosyringae is also present in many other countries ( Linzer et al 2009 ; Scanu et al 2015 ; Stamler et al 2016 ; Hansen et al 2017 ; Khaliq et al 2019 ). A draft genome sequence was recently reported for P. pseudosyringae ( McGowan et al 2020 ), which provided the first view of the pathogenicity arsenal in this species.…”