Genetic resources resistant to black spot (Alternaria alternate) identified from Chrysanthemum-related genera and potential underlying mechanisms

Chrysanthemum black spot disease caused by Alternaria alternate infestation is a widespread and extremely destructive foliar disease of chrysanthemums. We compared the resistance of 14 chrysanthemum relatives to chrysanthemum black spot disease, and identified the main indicators for the evaluation and screening of chrysanthemum disease resistance, which is of great significance in laying the foundation for a larger-scale screening of chrysanthemum relatives for disease resistance and the breeding of new disease-resistant cultivars. After artificial inoculation and identification, two disease-resistant germplasm resources, 11 moderately resistant materials, and one sensitive material were obtained. In both resistant and susceptible species, we found that the trichome density and leaf wax content of the resistant material were significantly higher than that of the sensitive material, while the stomata size was smaller than that of the sensitive material. In addition, we found that the leaf extract of the disease-resistant germplasm effectively inhibited the growth rate of A. alternate mycelium on the plate, and GC-MS components found that the leaves of resistant germplasm contained more volatile antifungal organic compounds, of which the abundant falcarinol and Germacrene D might play an important role in resistance to chrysanthemum black spot disease. In summary, epidermal trichome density, wax content and terpene substance content are three important reference indicators for disease resistance evaluation of related genera of chrysanthemum. The identified resistant germplasm can also be used as parents for future cross-breeding or as rootstocks.