The introduction, spread, and impact of fungal plant pathogens is a critical concern in ecological systems. In this study, we were motivated by the rather sudden appearance of Acer macrophyllum heavily infected with powdery mildew. We used morphological and genetic analyses to confirm the pathogen causing the epidemic was Sawadaea bicornis. In subsequent field studies, this pathogen was found in several locations in western North America, and in greenhouse studies, A. macrophyllum was found to be significantly more susceptible to S. bicornis than nine other Acer species tested. A genetic analysis of 178 specimens of powdery mildew from freshly collected and old herbarium specimens from 15 countries revealed seven different haplotypes. The high diversity of haplotypes found in Europe coupled with sequence results from a specimen from 1864 provides evidence that S. bicornis has a European origin. Furthermore, sequence data from a specimen from 1938 in Canada show that the pathogen has been present in North America for at least 82 years revealing a considerable lag time between the introduction and current epidemic. This study used old herbarium specimens to genetically hypothesize the origin, the native host, and the invasion time of a detrimental fungal plant pathogen.

真菌植物病原体的引入、传播和影响是生态系统中的一个关键问题。在这项研究中，我们受到了严重感染白粉病的大叶A cer macrophyllum 的突然出现的启发。我们使用形态学和遗传学分析来确认引起流行的病原体是Sawadaea bicornis。在随后的实地研究中，在北美西部的几个地方发现了这种病原体，在温室研究中，发现A. macrophyllum比其他九种Acer对S. bicornis明显更敏感测试的物种。对来自 15 个国家/地区的新鲜收集的和旧的植物标本馆标本的 178 个白粉病标本进行的遗传分析揭示了七种不同的单倍型。在欧洲发现的高度多样性的单倍型加上 1864 年标本的序列结果提供了双角链球菌起源于欧洲的证据。此外，来自加拿大 1938 年标本的序列数据表明，该病原体在北美已经存在至少 82 年，这表明从传入到当前流行之间存在相当长的滞后时间。本研究使用古老的植物标本馆标本对有害真菌植物病原体的起源、本地宿主和入侵时间进行了遗传假设。