| Summary: | The increasing movement of exotic pathogens calls for systematic surveillance so that newly introduced pathogens can be recognized and dealt with early. A resource crucial for recognizing such pathogens is knowledge about the spatial and temporal diversity of endemic pathogens. Here, we report an effort to build this resource for Pennsylvania (PA) by characterizing the identity and distribution of <i>Phytophthora</i> species isolated from diverse plant species in PA nurseries and greenhouses. We identified 1137 <i>Phytophthora</i> isolates cultured from clinical samples of >150 plant species submitted to the PA Department of Agriculture for diagnosis from 1975 to 2019 using sequences of one or more loci and morphological characteristics. The three most commonly received plants were <i>Abies</i>, <i>Rhododendron</i>, and <i>Pseudotsuga</i>. Thirty-six <i>Phytophthora</i> species identified represent all clades, except 3 and 10, and included a distinct subgroup of a known species and a prospective new species. Prominent pathogenic species such as <i>P. cactorum</i>, <i>P. cinnamomi</i>, <i>P. nicotianae</i>, <i>P. drechsleri</i>, <i>P. pini</i>, <i>P. plurivora</i>, and <i>P.</i> sp. <i>kelmania</i> have been found consistently since 1975. One isolate cultured from <i>Juniperus horizontalis</i> roots did not correspond to any known species, and several other isolates also show considerable genetic variation from any authentic species or isolate. Some species were isolated from never-before-documented plants, suggesting that their host range is larger than previously thought. This survey only provides a coarse picture of historical patterns of <i>Phytophthora</i> encounters in PA nurseries and greenhouses because the isolation of <i>Phytophthora</i> was not designed for a systematic survey. However, its extensive temporal and plant coverage offers a unique insight into the association of <i>Phytophthora</i> with diverse plants in nurseries and greenhouses.
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