Molecular characterization and pathogenicity of Pythium species associated with damping-off in greenhouse cucumber (Cucumis sativus) in Oman

A study was undertaken in 2004 and 2005 to characterize pathogens associated with damping-off of greenhouse-grown cucumber seedlings in 13 districts in Oman. Identification of Pythium to the species level was based on sequences of the internal transcribed spacer (ITS) of the ribosomal DNA. Of the 98 Pythium isolates collected during the survey, Pythium aphanidermatum, P. spinosum, P. splendens and P. oligandrum accounted for 76%, 22%, 1% and 1%, respectively. Pythium aphanidermatum was isolated from all of the districts, while P. spinosum was isolated from seven districts. Pathogenicity tests showed inter- and intraspecific variation in aggressiveness between Pythium species. Pythium aphanidermatum, P. spinosum and P. splendens were found to be highly aggressive at 25°C. However, the aggressiveness of P. spinosum decreased when the temperature was raised to 30°C, which was found to correspond to the lower frequency of isolation of P. spinosum in the warmer seasons, compared to the cooler time of the year. Pythium aphanidermatum exhibited limited intraspecific variation in the sequences of the ITS region of the rDNA and showed 100% similarity to the corresponding P. aphanidermatum sequences from GenBank. The ITS sequence data, as well as morphological characteristics of P. spinosum isolates, showed a high level of similarity within and between P. spinosum and P. kunmingense, and suggested that the two species were synonymous. This study represents the first report of P. spinosum, P. splendens and P. oligandrum in Oman.

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.