PATHOGENICITY OF PYTHIUM-IRREGULARE, PYTHIUM-SYLVATICUM AND PYTHIUM-ULTIMUM VAR ULTIMUM TO LUCERNE (MEDICAGO-SATIVA)

The ability of Pythium irregulare, P, sylvaticum and P. ultimum var. u ltimum to induce damping-off of lucerne (Medicago sativa) seedlings cv . SA Standard was tested in artificially infested planting medium. Thr ee levels of sand-bran inoculum were used: 5, 25 and 50 g inoculum per kg planting medium. All three Pythium species caused more than 80% da mping-off at each inoculum level and are therefore pathogenic to lucer ne. There were differences in virulence among the species and isolates tested. Pythium sylvaticum was the most virulent while P. irregulare was the least virulent species tested. The highest survival rates in i noculated soil were recorded for seedlings growing in soil containing the lowest levels of inoculum for each species tested. At the high ino culum levers (25 and 50 g/kg) in some cases (P. sylvaticum and P. ulti mum var. ultimum) survival was so low that intra-species differences i n virulence could no longer be detected. Since there were differences in virulence among the species and isolates tested, it is important to test a number of isolates of a species before drawing conclusions abo ut the pathogenicity of the species as a whole to a particular host, a nd to select appropriate levels of inoculum to elucidate these differe nces. As far as could be determined this is the first valid report of P. sylvaticum as a pathogen of lucerne seedlings.