Overwintering of Ampelomyces mycoparasites on apple trees and other plants infected with powdery mildews

摘要

Apple shoots and aerial parts of 13 other plant species infected with powdery mildews during the previous season were collected in late winter and early spring between 1998 and 2003 at a total of 34 sample sites in Hungary. Samples were examined for the presence of overwintering structures of Ampelomyces, common mycoparasites of powdery mildews. Pycnidia and resting hyphae resembling those of Ampelomyces were found on six plant species, including apple. Their viability and subsequent mycoparasitic activity of the hyphae emerging from the overwintered fungal structures were studied in vitro to determine whether they can serve as sources of primary inocula of Ampelomyces in the spring. Overwintered pycnidia of Ampelomyces collected in the spring, and produced in both the ascomata and the conidiophores of powdery mildews during the previous season, initiated the life cycle of these mycoparasites when placed close to fresh powdery mildew colonies in vitro. Similarly, thick-walled resting hyphae, found in the dried powdery mildew mycelia which covered the overwintered aerial parts of the host plants, also germinated and gave rise to new intracellular pycnidia of Ampelomyces when powdery mildew colonies were inoculated with them in vitro. On apple trees, Ampelomyces mycoparasites overwintered as resting hyphae in the dried powdery mildew mycelia covering the shoots and in the parasitized ascomata of Podosphaera leucotricha on the bark and the scales of the buds. Approximately 31% of the field samples collected from apple trees in spring between 1998 and 2003 contained overwintered structures of Ampelomyces. Artificial bursting of apple buds in the laboratory showed that both P. leucotricha and Ampelomyces start their life cycle during or soon after bud burst, but Ampelomyces can only slowly follow the spread of its mycohost on infected leaves. Most probably, the mycoparasites did not overwinter in the dormant hyphae of P. leucotricha in the buds, but only on the bark and the bud scales, as their hyphae were not found in the young hyphae of apple powdery mildew that appeared on the leaf tissues during bud burst. This study demonstrated that Ampelomyces mycoparasites can survive the winter in the field as pycnidia and as resting hyphae in the dried mycelia of their mycohosts.