Phytophthora Species and Their Associations with Chaparral and Oak Woodland Vegetation in Southern California

Abstract

Evidence of unintended introductions of Phytophthora species into native habitats has become increasingly prevalent in California. If not managed adequately, Phytophthora species can become devastating agricultural and forest plant pathogens. Additionally, California's natural areas, characterized by a Mediterranean climate and dominated by chaparral (evergreen, drought-tolerant shrubs) and oak woodlands, lack sufficient baseline knowledge on Phytophthora biology and ecology, hindering effective management efforts. From 2018 to 2021, soil samples were collected from Angeles National Forest lands (Los Angeles County) with the objective of better understanding the diversity and distribution of Phytophthora species in Southern California. Forty sites were surveyed, and soil samples were taken from plant rhizospheres, riverbeds, and off-road vehicle tracks in chaparral and oak woodland areas. From these surveys, fourteen species of Phytophthora were detected, including P. cactorum (subclade 1a), P. multivora (subclade 2c), P. sp. cadmea (subclade 7a), P. taxon 'oakpath' (subclade 8e, first reported in this study), and several clade-6 species, including P. crassamura. Phytophthora species detected in rhizosphere soil were found underneath both symptomatic and asymptomatic plants and were most frequently associated with Salvia mellifera, Quercus agrifolia, and Salix sp. Phytophthora species were present in both chaparral and oak woodland areas and primarily in riparian areas, including detections in off-road tracks, trails, and riverbeds. Although these Mediterranean ecosystems are among the driest and most fire-prone areas in the United States, they harbor a large diversity of Phytophthora species, indicating a potential risk for disease for native Californian vegetation.

Keywords: Mediterranean vegetation; oomycete; plant pathogen.

Figure 1

Examples of areas of the Angeles National Forest from which soil samples were taken to determine the presence of Phytophthora species. These include transition areas between chaparral and oak woodland areas (A), montane chaparral areas (B), rip riparian areas with dry and wet riverbeds (C), and oak woodlands (D). Adenostoma fasciculatum, Eriodictyon crassifolium, Salvia mellifera, and Quercus agrifolia were the most sampled native plant species in these areas.

Figure 2

Location of the 40 sites sampled in the Angeles National Forest (ANF). Three main areas were sampled: the southwestern (SWA, B) area, with six sites; the northwestern (NWA, C) area, with two sites; and the northeastern (NEA, E) area, with 32 sites. Red dots indicate Phytophthora-positive sites, and yellow indicate Phytophthora-negative sites (B,C,E). Map (A) displays the location of the ANF in Southern California, with black box indicating Los Angeles County, and map (D) shows the general location of the sampling areas.

Figure 3

Diversity and frequency of Phytophthora taxa isolated from surveys of burnt areas of the Angeles National Forest from 2018 through 2021. Upland areas and streams beds were sampled in chaparral and oak woodlands, including restoration areas. Multiple isolates of a Phytophthora taxon from the same sample were considered as one record.

Figure 4

Maximum likelihood tree of Phytophthora subclade 7a. The tree is inferred with IQ-TREE 2 from single-locus ITS rDNA alignment. Support values above the branches are ultrafast bootstrap approximations ≥ 50, and those below are posterior probabilities ≥ 0.90, according to an analysis with MrBayes v2.3.7a. The isolates in bold are from this study.

Figure 5

Maximum likelihood tree of Phytophthora subclade 7a. The tree is inferred with IQ-TREE 2 from a mitochondrial cox1 alignment. Support values above the branches are ultrafast bootstrap approximations ≥ 50, and those below are posterior probabilities ≥ 0.90, according to an analysis with MrBayes v2.3.7a. Isolates in bold are from this study. Isolates PDA2194 and PDA1795 were uploaded as P. abietivora and are included in the original description of the species.

Figure 6

Maximum likelihood tree of Phytophthora clade 8. The tree is inferred with IQ-TREE 2 from single-locus ITS rDNA alignment. Support values above the branches are ultrafast bootstrap approximations ≥ 50, and those below are posterior probabilities ≥ 0.90, according to an analysis with MrBayes. The isolate in bold is from this study.

Figure 7

Maximum likelihood tree of Phytophthora clade 8. The tree is inferred with IQ-TREE 2 from a mitochondrial cox1 alignment. Support values above branches are ultrafast bootstrap approximations ≥ 50, and those below are posterior probabilities ≥ 0.90, according to an analysis with MrBayes v2.3.7a. The isolate in bold is from this study.

Figure 8

Phytophthora-positive plants and streams in fire areas of the Angeles National Forest: (A) Eriodictyon crassifolium positive for P. cactorum; (B) white arrow indicates dead Quercus agrifolia restoration plant positive for P. gonapodyides; (C) dead Salvia mellifera positive for P. multivora; (D) E. crassifolium with mild symptoms of crown thinning, close to a P. crassamura-positive stream; (E) thinning Adenostoma fasciculatum from which P. gonapodyides was isolated; (F–I) sites positive for P. crassamura and other clade-6 Phytophthora species.