Involvement of Soilborne <i>Phytophthora</i> Species in Northeast Illinois Rapid Oak Decline and the Effect of Site Factors on the Disease

Abstract Background Mature oaks across Northeastern Illinois declined unusually rapidly, and many died, starting in 2019 and 2020. Investigation into the cause was initiated in 2021. In the absence of signs of disease or insect infestations aboveground, site soil characteristics and precipitation history that could facilitate root problems were investigated. Methods Eighteen sites with rapidly declining oaks were identified. Root samples were collected and tested for the presence of Phytophthora spp. using DAS-ELISA. An investigation of soil conditions was conducted on one of the sites with declining and healthy trees growing on lower and slightly elevated ground, respectively. Results DAS-ELISA results were positive in 87% of the samples. All sites had fine textured soils classified as moderately to poorly drained, with high spring water tables. Topography was always very flat. The water table was near the surface for most of the spring on poorly drained sites with declining trees, and significantly higher than in the slightly elevated sites with better drainage and healthy trees. From 2018 to 2020, the region experienced the highest average spring rainfall for any 3 consecutive years in the last 70 years. This created soil conditions conducive to root disease development. Conclusions Predominantly positive ELISA results on fine roots indicated that Phytophthora species are present in root systems when growing in moderately to poorly drained, fine textured soils across the Chicago region. The consecutive years of above-normal rainfall and poor soil drainage appears to have facilitated the development of root disease leading to rapid decline of oaks in the region.