A rapid and efficientin vivoinoculation method for introducing tree stem canker pathogens onto leaves: Suitable for large-scale assessment of resistance in poplar breeding progeny

Abstract

AbstractHybrid breeding, a direct and efficient strategy for disease control and management in tree species, is currently limited by the selection method of resist clones: the “in vitrostem segment inoculation method.” This method, constrained by the availability of inoculating materials, cannot rapidly, efficiently, and cost-effectively screen the resistance of all hybrid clones. To overcome these limitations, we introduce a novel pathogen inoculation method for the resistance assessment of hybrid clones in the poplar-Valsa sordidapathosystem. This method involves inoculating the stem canker pathogen on the host leaf, a unique and promising approach we have successfully validated.Results showed that stem canker pathogenV. sordidainduced the extended necrotic lesion and even induced the formation of pycnidium structure and conidia on the leaf surface five days after mycelium inoculation; 1) the upper 5-7thleaves exhibited higher resistance than the middle 18-20thleaves; 2) the shading conditions induced more severe symptoms on the leaves than lighting conditions; 3) the juvenile mycelium inoculums (4-day-cultured) were more susceptible to poplar leaves than the old ones (7-day-cultured). Our results demonstrate the robustness of the “in vivoleaf inoculation method” in revealing the resistance differentiation in poplar hybrid clones. According to the leaf necrotic area disease index, we divided these poplar clones into seven different resistance groups. The resistance assessed by leaf assessment was validated in 15 selected poplar clones using the “in vitrostem segment inoculation method.” Results showed that the effectiveness of these two methods was consistent. Moreover, the leaf inoculation method can be used to detect the pathogenicity diversity of the pathogen population of tree species. Compared to the conventional “in vitrostem segment inoculation method,” the leaf method has the advantages of abundant inoculation materials, easy operation, rapid disease onset, and almost no adverse effect on the host. It is particularly suitable for the resistance screening of all progeny and the early (seedling) phenotypic selection of resistant poplar clones in poplar stem disease resistance breeding. The “in vivoleaf inoculation method” holds significant promise in poplar breeding, tree pathology, and molecular biology research on tree stem diseases.