Comparison of host range and pathogenicity of isolates of Pythium myriotylum and Pythium zingiberis

Apart from morphology and genetic characteristics, species status of Pythium zingiberis and P. myriotylum may also be confirmed based on their pathogenicity and host range. An Australian putative P. zingiberis isolate and imported type isolates of P. myriotylum and P. zingiberis were subject to both in vitro and in vivo pathogenicity tests. In vitro tests were carried out on excised carrot, ginger, potato, radish, and sweet potato tuber/root sections, and on seeds and seedlings of cucumber, cauliflower, millet, rye, sweet corn, tomato, and wheat. In all assays conducted, the Australian isolate was found to be the most pathogenic, followed by type specimen of P. zingiberis (UOP 275), and then the type specimen P. myriotylum (CBS 254.70). An in vivo experiment on ginger plants at 35°C (with 10 h day light) in quarantine conditions showed that the ginger plants inoculated with the Australian isolate and also the type specimen of P. zingiberis died at 21 days after inoculation, whereas those inoculated with P. myriotylum CBS 254.70 were still green and healthy. Along with cardinal growth rate, the Australian isolate was confirmed to be closely related to P. zingiberis. This is also the first direct comparison in pathogenicity of P. zingiberis and P. myriotylum.

An assessment of Pythium spp. associated with soft rot disease of ginger (Zingiber officinale) in Queensland, Australia

In Australia, Pythium soft rot (PSR) outbreaks caused by P. myriotylum were reported in 2009 and since then this disease has remained as a major concern for the ginger industry. From 2012 to 2015, a number of Pythium spp. were isolated from ginger rhizomes and soil from farms affected by PSR disease and assessed for their pathogenicity on ginger. In this study, 11 distinct Pythium spp. were recovered from ginger farms in Queensland, Australia and species identification and confirmation were based on morphology, growth rate and ITS sequences. These Pythium spp. when tested showed different levels of aggressiveness on excised ginger rhizome. P. aphanidemartum, P. deliense, P. myriotylum, P. splendens, P. spinosum and P. ultimum were the most pathogenic when assessed in vitro on an array of plant species. However, P. myriotylum was the only pathogen, which was capable of inducing PSR symptoms on ginger at a temperature range from 20 to 35 °C. Whereas, P. aphanidermatum only attacked and induced PSR on ginger at 30 to 35 °C in pot trials. This is the first report of P. aphanidermatum inducing PSR of ginger in Australia at high temperatures. Only P. oligandrum and P. perplexum, which had been recovered only from soils and not plant tissue, appeared non-pathogenic in all assays.