Effect of leaf extracts on fruiting and sporulation of Cronartium pini and C. ribicola on susceptible alternate hosts

Resistance of plants to rust fungi may depend on the chemical quality of plants, suggesting that leaf extracts from resistant plant species could be useful in development of bio-based solutions against rust diseases. Pine trees ( Pinus spp.) are affected by Cronartium rusts ( C. pini , C. ribicola ). In their life cycle, these rusts infect alternate host plants, some of which are more rust resistant than others. We sprayed leaf extracts from resistant alternate host ( Melampyrum pratense , Impatiens glandulifera , Veronica chamaedrys , Ribes rubrum ) and tested—both in the laboratory and on live plants—whether those extracts reduce rust spore production on plants that are normally easy to infect ( Melampyrum sylvaticum , Impatiens balsamina , Paeonia lactiflora , Ribes nigrum ). Two commercial compounds, previously linked to rust resistance of alternate hosts to Cronartium rusts, a methanol control and an untreated control were also included to the experiment. Formation of rust uredinia and telia were estimated on the leaves after 6 weeks of incubation. In the laboratory, uredinia and telia of C. pini developed most abundantly on Paeonia lactiflora . Coverage of uredinia and telia did not differ between extracts on the leaves, but coverages of uredinia were significantly higher compared to dry control in the laboratory. In the greenhouse, infection percentage of leaves with uredinia and telia differed significantly among plant species but the treatments with extracts did not result in significant differences in infection percentage of the test plants. The results indicated that none of the tested extracts from rust-resistant alternate hosts and commercial compounds significantly inhibited fruiting and sporulation of Cronartium spp. on susceptible alternate hosts at medium (100 ppm) concentration.