Improved assessment of mycelial growth stimulation by low doses of mefenoxam in plant pathogenic Globisporangium species

Globisporangium Uzuhashi, Tojo & Kakish. (syn. Pythium Pringsheim) species cause many plant diseases, including Pythium damping-off, leaf and fruit blights, and root rots. Fungicide resistant isolates are selected by repeated use of a single active ingredient on infected crops without rotation. Previous studies demonstrated increased pathogenicity and radial growth in a mefenoxam resistant isolate of Pythium aphanidermatum when exposed to sub-lethal doses of fungicides and ethanol. In those studies, reproducibility of in vitro assays was difficult to achieve due to large variations among trials. This study aimed to examine two protocols for improved reproducibility during the assessment of biphasic dose-responses in mefenoxam-resistant isolates of Globisporangium ultimum and G. irregulare. Two different growth related endpoints, total growth area and total dry mass weight, were assessed. Assays were conducted using ten concentrations of mefenoxam ranging from 0.01 to 1,000 mu g/ml. Statistically-significant stimulatory effects were observed in the two Globisporangium species using the two growth related endpoints. Because of its better reproducibility, mycelial growth area is recommended as an endpoint for future studies of chemical hormesis on growth of Globisporangium spp.