Pathogenicity to alder of Phytophthora species from riparian ecosystems in western Oregon

Abstract

Described as one of the most destructive pathogens of agricultural crops and forest trees, Phytophthora is a genus of microorganisms containing over 100 known species. Phytophthora alni has caused collar and root disease in alders throughout Europe, and a subspecies has recently been isolated in North America. Reports of canopy dieback in red alder, Alnus rubra, prompted a survey of their overall health and to determine whether P. alni was present in western Oregon riparian ecosystems. Over 1100 Phytophthora isolates were recovered, representing 20 species and 2 taxa. Phytophthora-type cankers were observed in many trees, and their incidence was positively correlated with canopy dieback. High levels of mortality for red alder were not observed, which suggests these Phytophthora species are not aggressive pathogens. To test this hypothesis, three stem wound inoculations and one root dip were conducted on red alder seedlings using 13 Phytophthora species recovered from the riparian survey. Ten of the 13 Phytophthora species produced significant lesions in at least one pathogenicity test. Phytophthora siskiyouensis produced the largest lesions on red alder from the two stem wound inoculation tests conducted under summer conditions, while P. taxon Pgchlamydo caused the largest lesions during the winter stem wound inoculation test. Phytophthora gonapodyides, P. taxon Pgchlamydo and P. siskiyouensis have previously been found associated with necrotic alder roots and bole cankers in the field, and with the pathogenicity results reported here, we have established these species as causes of Phytophthora root disease and Phytophthora bole canker of alder in Oregon. While none of the Phytophthora species were especially aggressive towards red alder in the pathogenicity tests, they did cause localized disease symptoms. By weakening the root systems or boles of alders, the Phytophthoras could be leaving alders more susceptible to other insects and pathogens.

Pathogenicity of Phytophthora alni complex and P. plurivora in Alnus glutinosa seedlings

“…Other studies involving pathogenicity tests with these pathogens used different methods and the tests were not carried out simultaneously. Therefore, interstudy comparison of isolates’ aggressiveness should be regarded with caution because pathogenicity varies depending on the growing season in which inoculation was carried out (Brasier & Kirk, ; Chandelier, Husson, Druart, & Marçais, ; Navarro, Sims, & Hansen, ) as well as the provenance or age of inoculated seedlings (Balci, Balci, MacDonald, & Gottschalk, ; Jung & Blaschke, ).…”

“…Moreover, Navarro () and Navarro et al. () demonstrated that both P. uniformis and P. plurivora caused root damage when soil infestation and zoospore root dip methods of inoculation were used.…”

“…Fleischmann, Göttlein, Rodenkirchen, Lütz, and Oßwald (2004) regarded a different pattern of spread after soil inoculation: in particular, they found that belowground biomass of beech seedlings infected with P. plurivora (referred to as P. citricola) was lower than in control plants, whereas aboveground biomass was not different. Moreover, Aboveground biomass (g) Navarro (2014) and Navarro et al (2015) demonstrated that both P. uniformis and P. plurivora caused root damage when soil infestation and zoospore root dip methods of inoculation were used.…”

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

“…Parasitism is considered an early characteristic in the evolution of oomycetes, [67], but the possible evolution of a saprotrophic lifestyle from parasitic precursors has been considered for fungi and oomycetes [67,68]. Clade 6 Phytophthora are known to be opportunistic pathogens [2,[69][70][71]. Stradling saprotrophic and parasitic lifestyles, stream-resident Phytophthora may play an important role in the early breakdown of leaves and vegetative matter that still contain living cells.…”

Susceptibility to the rare Phytophthora tentaculata and to the widespread Phytophthora cactorum is consistent with host ecology and history

“…The inoculation method used was similar to the one presented in Navarro, Sims, and Hansen (, Test 3). A 6‐mm‐diameter agar disc, containing mycelium and grown on V8 agar, was placed over a pinpricked stem approximately 10 cm above the soil line.…”