Phytophthora Species Are Common on Nursery Stock Grown for Restoration and Revegetation Purposes in California

Abstract

Phytophthora tentaculata was detected for the first time in North America in 2012 in a nursery on sticky monkeyflower plant (Diplacus aurantiacus) and again in 2014 on outplanted native plants. At that time, this species was listed as a federally actionable and reportable pathogen by the USDA. As a result of these detections, California native plant nurseries were surveyed to determine the prevalence of Phytophthora species on native plant nursery stock. A total of 402 samples were collected from 26 different native plant nurseries in California between 2014 and 2016. Sampling focused on plants with symptoms of root and crown rot. Symptomatic tissue was collected and tested by immunoassay, culture, and molecular techniques (PCR). Identifications were made using sequences from the internal transcribed spacer (ITS) rDNA region, a portion of the trnM-trnP-trnM, or the atp9-nad9 mitochondrial regions. Phytophthora was confirmed from 149 of the 402 samples (37%), and from plants in 22 different host families. P. tentaculata was the most frequently detected species in our survey, followed by P. cactorum and members of the P. cryptogea complex. Other species include P. cambivora, P. cinnamomi, P. citricola, P. hedraiandra, P. megasperma, P. multivora, P. nicotianae, P. niederhauserii, P. parvispora, P. pini, P. plurivora, and P. riparia. A few Phytophthora sequences generated from mitochondrial regions could not be assigned to a species. Although this survey was limited to a relatively small number of California native plant nurseries, Phytophthora species were detected from three quarters of them (77%). In addition to sticky monkeyflower, P. tentaculata was detected from seven other hosts, expanding the number of associated hosts. During this survey, P. parvispora was detected for the first time in North America from symptomatic crowns and roots of the nonnative Mexican orange blossom (Choisya ternata). Pathogenicity of P. parvispora and P. nicotianae was confirmed on this host. These findings document the widespread occurrence of Phytophthora spp. in native plant nurseries and highlight the potential risks associated with outplanting infested nursery-grown stock into residential gardens and wildlands.

Phytophthora Introductions in Restoration Areas: Responding to Protect California Native Flora from Human-Assisted Pathogen Spread

“…Nursery stock outplanted into wildland settings presents an opportunity for pathogen infections acquired in nurseries to be moved into new locations along with the plants, which may inadvertently result in lasting environmental damage to surrounding natural communities and habitats [18]. In California, native plant nursery stock outplanted for restoration has served as a pathway of introduction for a diverse array of Phytophthora species into natural areas and critical habitats [12]. Starting with healthy, disease-free stock gives restoration projects the best chance of meeting established performance targets.…”

“…Species detected included P. cactorum, P. xcambivora, P. crassamura, P. syringae, and P. sp. kelmania, all species which have been detected in nursery stock [12,25,26]. No Phytophthora was detected from 34 samples of C. ferrisiae and other plants located upslope or beyond the drainage area below the 1993 planting.…”

“…In another first U.S. detection, P. quercina T. Jung was found in a restoration area on a planted valley oak (Quercus lobata Née) in San Jose (Santa Clara County) [10]. Further sampling in restoration planting sites and native plant nurseries resulted in the detection of numerous new Phytophthora taxa and dozens of known Phytophthora species [10][11][12]. Phytophthora species are a common problem in horticultural nurseries [13][14][15][16], but Phytophthora species in California native plant or restoration nurseries were largely overlooked before the restoration site detection of P. tentaculata in 2014 [17].…”

Phytophthora Diversity in Pennsylvania Nurseries and Greenhouses Inferred from Clinical Samples Collected over Four Decades

“…P. parvispora was only intercepted once, from a greenhouse in 2000 ( Figure S19 ). This species was also detected in California nursery in 2016 causing dieback on a Choisya ternata [ 34 ]. This is the first report of P. parvispora to be associated with Lavandula sp.…”

“…This study does report many new host associations across multiple species (marked with * in the Supplementary Tables for each clade). Some species, such as P. pini [ 24 , 26 , 34 , 44 ], P. sp. kelmania [ 26 , 45 ], P. chlamydospora [ 46 , 47 , 48 ], and P. drechsleri [ 24 , 49 , 50 ], are reported from mostly never-before documented hosts ( Tables S3, S5, and S7 ), suggesting that their host ranges are much larger than previously thought.…”