Isolates of Pythium attributable to thirteen named species and to species group F (sensu van der Plaats-Niterink) were isolated from roots of field-grown rice seedlings showing poor root development and lack of vigour. P. arrhenomanes and group F were the most commonly isolated species. The roots of 15-week-old rice plants yielded only P. flevoense, P. vanterpoolii, P. rostratum and Pythium group G of which the latter two were not obtained from seedling roots.
In pot tests with representative Pythium isolates, P. arrhenomanes was most pathogenic to rice seedlings, causing pre- and post-emergence death and average reductions in shoot and root growth in surviving seedlings of 48 and 70%, respectively. P. irregulare killed seedlings before but not after emergence, whilst the response of rice seedlings to P. myriotylum apparently depended on the length and timing of a cold shock treatment. P. pyrilobum did not reduce seedling number or shoot growth but reduced root growth. P. vanterpoolii and group F were not pathogenic in the tests. P. coloratum, P. echinulatum, P. flevoense, P. oedochilum, P. oligandrum, P. periilum, P. pyrilobum and P. rostratum were isolated infrequently and had no adverse effects on seedlings in pathogenicity tests, although P. myriotylum, P. oligandrum, and P. periilum were associated with significant increases in shoot growth. P. tracheiphilum was isolated from one site but its pathogenicity was not tested.
This is the first record of P. coloratum, P. echinulatum, P. flevoense, P. irregulare, P. oedochilum, P. oligandrum, P. periilum, P. pyrilobum and P. tracheiphilum from rice roots.