Deciphering the dynamics involved in past microbial invasions has proven difficult due to the inconspicuous nature of microbes and their still poorly known diversity and biogeography. Here we focus on powdery mildew, a common disease of oaks which emerged in Europe at the beginning of the twentieth century and for which three closely related Erysiphe species are mainly involved. The study of herbaria samples combined with an experimental approach of interactions between Erysiphe species led us to revisit the history of this multiple invasion. Contrary to what was previously thought, herbaria sample analyses very strongly suggested that the currently dominant species, E. alphitoides, was not the species which caused the first outbreaks and was described as a new species at that time. Instead, E. quercicola was shown to be present since the early dates of disease reports and to be widespread all over Europe in the beginning of the twentieth century. E. alphitoides spread and became progressively dominant during the second half of the twentieth century while E. quercicola was constrained to the southern part of its initial range, corresponding to its current distribution. A competition experiment provided a potential explanation of this over-invasion by demonstrating that E. alphitoides had a slight advantage over E. quercicola by its ability to infect leaves during a longer period during shoot development. Our study is exemplary of invasions with complexes of functionally similar species, emphasizing that subtle differences in the biology of the species, rather than strong competitive effects may explain patterns of over-invasion and niche contraction.

由于微生物的性质不明显以及它们仍不为人所知的多样性和生物地理学，事实证明，破译过去微生物入侵所涉及的动力学是困难的。在这里，我们关注白粉病，白粉病是一种常见的橡树病，在20世纪初出现在欧洲，并且主要涉及三种密切相关的Erysiphe物种。植物标本馆样品之间的相互作用的实验方法相结合的研究白粉病品种带领我们重新审视这个倍数侵略的历史。与以前的想法相反，对草标本的分析非常有力地表明，目前占主导地位的物种，E。alphitoides，不是引起首次爆发的物种，在当时被描述为新物种。取而代之的是，从疾病报告的早期开始就显示了quercicola大肠杆菌，并在二十世纪初遍布欧洲。在二十世纪下半叶，alphitoides传播并逐渐占主导地位，而quercicola大肠杆菌被限制在其初始范围的南部，这与它的当前分布相对应。一项竞争实验通过证明灭藻剂E. quercicola比E. quercicola略有优势，为这种过度入侵提供了可能的解释。能够在芽发育的较长时间内感染叶片。我们的研究是功能相似物种的复合体入侵的典范，强调了物种生物学上的细微差异，而不是强烈的竞争效应可能解释了过度入侵和生态位收缩的模式。