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Phytophthora species introduced to Southern California restoration plantings on the Angeles National Forest threaten rare species habitat

Susan Frankel: USDA Forest Service, Pacific Southwest Research Station

<div>Over the past several years, more than 40 <em>Phytophthora</em> species have been recovered from native plant nursery stock in northern California habitat restoration plantings. These unintentional introductions of <em>P. tentaculata</em>, <em>P. cactorum,</em> and other species, which were extensive in several cases, potentially undermine the very purpose of the restoration projects because they degrade rather than enhance habitat. In 2016 – 2017, the Angeles National Forest in Southern California conducted an assessment to determine if <em>Phytophthora</em> species were present in restoration areas planted to mitigate habitat loss along utility corridors. Baiting with green pears was used to detect <em>Phytophthora</em> from sampled roots of symptomatic (dead, declining, stunted, or off-color) nursery stock planted at fifteen restoration sites. All locations had plants associated with <em>Phytophthora</em>. Numerous <em>Phytophthora</em> species, including two likely novel or hybrid species, were identified via DNA sequencing. At four source native plant nurseries, groups of container plants grown for restoration were tested by baiting irrigation leachate with green pears. Four <em>Phytophthora</em> species were recovered, three of which were also detected in the field sites. Some of the <em>Phytophthora</em> recoveries were from nursery plants without obvious shoot symptoms. Restoration areas are conservation investments, and these results confirm that infested nursery stock has the potential not only to interfere with restoration success, but to introduce plant pathogens into the wildlands of southern California. Preventive measures are needed to sustain plant health, including production of healthy nursery stock using strict phytosanitary measures, or direct seeding and other changes to restoration practices.</div>