Effect of leaf chemicals from susceptible and resistant alternate hosts of Cronartium on in vitro germination of aeciospores of Cronartium pini and C. ribicola

The chemical compounds of plant leaves may be used to defend against rust diseases. The effect of methanolic leaf extracts from four resistant and four susceptible alternate hosts of Cronartium pini and C. ribicola, and two pure phenolic chemicals, such as chlorogenic acid and quercitrin, were tested on the germination of C. pini and C. ribicola spores. Aeciospores of these rusts were dusted on water agar amended with the leaf extracts and chemicals in three concentrations and a methanol control, and germination of the aeciospores was counted after 24 h incubation. Germination of the rusts was compared between extracts and concentrations. All plant extracts tended to stimulate the germination of both rusts at 50 and 100 ppm, but the highest concentration, 500 ppm, inhibited the germination of both rusts regardless of the extract. The germination of C. pini spores was inhibited strongly by extracts from Veronica chamaedrys, V. longifolia, Ribes rubrum and Impatiens glandulifera, chlorogenic acid, quercitrin and methanol, and weakly by extracts of Ribes nigrum, Melampyrum pratense, M. sylvaticum and Impatiens balsamina at 500 ppm. The growth of C. ribicola was strongly inhibited by all extracts at 500 ppm. Since the results indicate stronger inhibition by the extracts from resistant host species, more research is required to identify the active compounds for bio-based control of Cronartium rusts. Keywords: aeciospores; alternate hosts; rust resistance; Scots pine blister rust; white-pine blister rust