Pathogenicity of Mycochaetophora gentianae, causal fungus of gentian brown leaf spot, as affected by host species, inoculum density, temperature, leaf wetness duration, and leaf position

When the influence of host species, inoculum density, temperature, leaf wetness duration, and leaf position on the incidence of gentian brown leaf spot caused by Mycochaetophora gentianae, was examined, the fungus severely infected all seven Gentiana triflora cultivars, but failed to infect two cultivars of G. scabra and an interspecific hybrid cultivar. Inoculum density correlated closely with disease incidence, and a minimum of 10(2) conidia/mL was enough to cause infection. In an analysis of variance, temperature and leaf wetness duration had a significant effect upon disease incidence, which increased with higher temperature (15-25A degrees C) and longer duration of leaf wetness (36-72 h). No disease developed at temperatures lower than 10A degrees C or when leaf wetness lasted < 24 h. At 48-h leaf wetness, disease incidence was 0, 28, 77, and 85% at 10, 15, 20, and 25A degrees C, respectively. Middle and lower leaves on the plant were more susceptible than upper leaves. In microscopic observations of inoculated leaves, > 50% of conidia germinated at temperatures > 15A degrees C after 24-h leaf wetness. More appressoria formed at higher temperatures (15-25A degrees C) with extended duration of leaf wetness (24-72 h). At 48-h leaf wetness, appressorium formation was 0, 8, 26, and 73% at 10, 15, 20, and 25A degrees C, respectively. These results suggest that temperature and leaf wetness duration were important factors for infection of gentian leaves.