First Report of Phytophthora europaea in Oak Forests in the Eastern and North-Central United States

Metabarcoding reveals a high diversity of woody host-associated Phytophthora spp. in soils at public gardens and amenity woodlands in Britain

Forests and woodlands worldwide are being severely impacted by invasive Phytophthora species, with initial outbreaks in some cases occurring on host trees located in public parks and gardens. These highly disturbed sites with diverse planting practices may indeed act as harbours for invasive Phytophthora pathogens which are particularly well adapted to surviving in soil. High throughput Illumina sequencing was used to analyse Phytophthora species diversity in soil samples collected from 14 public garden/amenity woodland sites in northern Britain. Bioinformatic analyses revealed some limitations to using internal transcribed spacer as the barcode region; namely reporting of false positives and ambiguous species matches. Taking this into account, 35 distinct sequences were amplified across the sites, corresponding to 23 known Phytophthora species as well as twelve oomycete sequences with no match to any known Phytophthora species. Phytophthora pseudosyringae and P. austrocedri, both of which cause serious damage to trees and are regarded as fairly recent introductions to Britain, were the two most abundant Phytophthora species detected. There was no evidence that any of the detected Phytophthora species were more associated with any one type of host, healthy or otherwise. This study has demonstrated the ubiquity and diversity of Phytophthora species endemic in highly managed, extensively planted soil environments in Britain. Suggested improvements to the methodology and the practical implications of the findings in terms of mitigating Phytophthora spread and impact are discussed.

Phytophthora Species Detected in Two Ozark Forests with Unusual Patterns of White Oak Mortality

Widespread decline and mortality of white oaks (Quercus alba) occurred in Missouri Ozark forests between 2011 and 2017. Symptoms included rapid crown death with bronzing of leaves, retention of dead leaves, crown dieback and thinning, and loss of large limbs within one year of death. Decline and mortality were associated with hillside drainages and fit descriptions of European oak forests predisposed to decline by pathogenic Phytophthora species. A survey was performed at two locations in 2014 and 2015 to assess the distribution of dead and declining white oaks, and the occurrence and distribution of Phytophthora species. Multiple Phytophthora species were detected, including P. cinnamomi, P. cactorum, P. europaea, and P. pini. P. cinnamomi was the most common and widely distributed species among plots at both locations. The detection of P. cinnamomi at the base of white oaks was not associated with poor crown vigor. However, more quantitative survey techniques are necessary to clearly evaluate this relationship. P. cinnamomi kills fine roots of white and red oaks in North America and has been associated with the decline of white oaks in the United States (Ohio) and other countries. Further studies are needed to determine the importance of P. cinnamomi in oak decline within the Ozark highlands.

Phytophthora spp. Associated with Forest Soils in Eastern and North-Central U.S. Oak Ecosystems

A survey of soils associated with oak species was conducted in 2003 and 2004 in Indiana, Illinois, Maryland, Michigan, Minnesota, Pennsylvania, Ohio, West Virginia, and Wisconsin to investigate the occurrence of Phytophthora spp. Soils taken from around the base of healthy and declining oak trees were flooded with H₂O and Quercus robur leaflets were used as bait for Phytophthora spp. From 829 soil samples collected near trees, 21% were positive for Phytophthora spp., with 55% of the 125 sites surveyed yielding a Phytophthora sp. Phytophthora cinnamomi was the most frequently isolated species, representing 69.4% of the Phytophthora-infested sites surveyed. Other species, in decreasing order of isolation frequency were Phytophthora sp. 2, P. citricola, P. europaea, P. cambivora, P. quercina-like isolates, and Phytophthora sp. 1. No significant association was found between the presence of Phytophthora organisms and site characteristics such as latitude, elevation, soil pH, or the crown condition of the trees. However, in P. cinnamomi-infested sites, a significant association was found with the deteriorating crown status of Q. alba and the presence of P. cinnamomi. The absence of P. cinnamomi above the 40°N latitude range also was noteworthy.