Forest and Plantation Soil Microbiomes Differ in Their Capacity to Suppress Feedback Between Geosmithia morbida and Rhizosphere Pathogens of Juglans nigra Seedlings

Abstract

Thousand cankers disease (TCD) is hypothesized to have a greater impact on eastern black walnut (Juglans nigra) in urban forests and plantations compared with natural forest stands. Along with other factors such as resource availability, the phytobiome could partly account for observed differences in disease severity across management regimes. We investigated the extent to which J. nigra-associated soil microbiomes from plantations and natural forests modulate (i) the amount of necrosis caused by Geosmithia morbida in 1-year-old seedlings, and (ii) relative abundance of rhizosphere endophytes and opportunistic pathogens Fusarium and Rhizoctonia in response to aboveground inoculation with G. morbida. Our results suggest that the microbiome from natural forest soil in central Indiana suppresses Fusarium spp. and is indirectly suppressive of G. morbida. Natural forest soil had a greater ability to reduce the size of necrotic area caused by G. morbida compared with steam-treated soil. Inoculating stems of seedlings with G. morbida induced a shift in fungal community composition in the rhizosphere, including Fusarium and Rhizoctonia spp.; however, the direction and magnitude of the shift depended on whether seedlings were amended with forest, plantation, or steam-treated soil. In a companion experiment, necrotic area in G. morbida-inoculated seedlings was twice as high in seedlings grown from seeds that were treated with Fusarium solani relative to those grown from seed treated with water. Our findings support the hypothesis that TCD severity can be modulated by host-mediated feedback between above- and belowground pathogens, as well as by microbial interactions in the rhizosphere.