Growth and sporulation of Phytophthora ramorum in vitro in response to temperature and light

Phytophthora ramorum, recently found in the US, is causing concern for hardwood forests and the nursery industry. In an effort to identify some of the environmental limitations to growth and sporulation we undertook a laboratory study of four US and three European (EU) isolates. On V8 media, isolates grew when incubated at 2-28 C and produced chlamydospores at 8-28 C. Sporangia were produced at all temperatures tested: 10-30 C for US isolates and 6-26 C for EU isolates. Optimal temperatures were 16-26 C for growth, 14-26 C for chlamydospore production and 16-22 C for sporangia production. US isolates grew less and produced fewer spores when exposed to increasing doses of near-UV radiation (50-300 microW/cm(2)) and visible radiation (250-1500 microW/cm(2)). EU isolates were exposed to 300 microW/cm(2) near-UV only, which significantly reduced growth of one of three isolates and had no significant effect on spore production. In our studies P. ramorum tolerated a broad range of temperature and light conditions, which suggests that it is capable of establishment in a wide geographic area.