Leaf Surface and Endophytic Fungi Associated with Onion Leaves and Their Antagonistic Activity Against Alternaria Porri.

Sixty-eight fungal species belonging to 29 genera were isolated as leaf surface and endophytic fungi from healthy and purple blotch diseased onion leaves. The fungal populations associated with diseased onion leaves (1.360 × 10 CFU/g leaf in the phyllosphere, 2.614 CFU/leaf segment in the phylloplane and 1.324 CFU/leaf segment in the surface-sterilised diseased leaves) were higher than those in healthy samples (0.804 × 10 CFU in the phyllosphere, 1.184 CFU in the phylloplane, and 0.35 CFU as endophytes). Endophytic fungi of healthy leaves were represented by 12 genera and 15 species, while fungi of surface-sterilised diseased leaves included 17 species from 13 genera. The mycobiota associated with surface-sterilised diseased leaves were different from the endophytic fungi of healthy samples, whereas the disease may stimulate colonisation of opportunistic fungi causing secondary infections such as Botrytis cinerea, Penicillium aurantiogriseum, Alternaria alternata and Cladosporium spp. In contrast, healthy leaves were a source of antagonistic endophytic fungi such as Trichoderma harzianum and T. koningii. Testing the antagonistic effect of 91 fungal isolates against Alternaria porri showed that nine isolates of Trichoderma produced the highest suppressive potential (73.1%) depending on competition and mycoparasitism. Epicoccum nigrum and Penicillium oxalicum exhibited antibiosis against A. porri producing a 12 mm broad inhibition zone. In conclusion, the quantitative and qualitative compositions of fungi associated with onion leaves were distinctly influenced by A. porri infection. Mycobiota associated with asymptomatic onion leaves such as Epicoccum nigrum, Penicillium oxalicum and Trichoderma harzianum are a natural source of eco-friendly bioagents. They showed an effective antagonistic potential against A. porri, and may thus be applied as an alternative to fungicides.