Garlic mustard is an invasive Eurasian biennial that has spread throughout the eastern United States and southern Canada. Populations of this plant vary in their susceptibility to Erysiphe cruciferarum, a causal agent of powdery mildew disease in Brassicaceous plants. We examined whether there were biogeographic patterns in the distribution of resistance in invasive North American and native European populations of this plant. We grew plants from 78 invasive and 20 native populations and screened them for powdery mildew resistance in the greenhouse. We found that populations were mostly monomorphic for either resistance or susceptibility but that some polymorphic populations were found from both continents. The proportion of populations showing resistance versus susceptibility was similar in both Europe and North America. Within continents, the spatial distribution of resistant and susceptible populations did not deviate significantly from random. We also examined whether the possession of the resistance trait alter intraspecific competitive dynamics. In two trials, we competed plants from resistant and susceptible populations in a target-neighbor design in the presence and absence of powdery mildew inoculum and examined the growth of the target plant. Target plants from resistant populations were overall larger than target plants from susceptible populations. Target plants were overall larger when grown in competition with susceptible neighbors. Further, resistant target plants showed a greater degree of release from competition when grown with a susceptible neighbor versus a resistant neighbor than the degree of release shown by susceptible target plants. This suggests a benefit of possessing the resistance trait with little apparent costs which should promote selection for this trait within plant populations.

大蒜芥末是一种入侵性的欧亚双年度植物，已遍及美国东部和加拿大南部。这种植物的种群对十字花科植物的敏感性不同，是十字花科植物白粉病的病原体。我们检查了该植物的入侵北美和欧洲本土人群中抗药性分布中是否存在生物地理模式。我们从78个入侵种群和20个本地种群中种植了植物，并针对温室中的白粉病抗性进行了筛选。我们发现，无论是抗性还是敏感性，这些种群大多数都是单态的，但是在两大洲都发现了一些多态的种群。在欧洲和北美，显示出抗药性与药敏性的人口比例相似。在各大洲内，抗药性和易感种群的空间分布与随机分布没有明显差异。我们还检查了是否具有抗性性状会改变种内竞争动态。在两项试验中，我们在存在和不存在白粉病接种物的情况下，以目标邻居设计竞争了具有抗性和易感种群的植物，并研究了目标植物的生长。来自抗性种群的靶植物总体上大于来自易感种群的靶植物。与易感邻居竞争时，目标植物总体上更大。此外，当与易感邻居相比抗性邻居生长时，抗性靶植物表现出比竞争易感靶植物更大的竞争释放度。