Effects of temperature on rust development on mint infected with strains of Puccinia menthae

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Two principal groups of races of Puccinia menthae occur on mint. One infects native spearmint but not peppermint and is called native spearmint rust; the other infects peppermint but not native spearmint and is called peppermint rust. High summer temperatures are thought to be a limiting factor for rust development on peppermint. The effects of temperature on germination of urediniospores and length of germ tubes of native spearmint and peppermint rust isolates, and on length of latent period when host species were infected with respective isolates, were determined. Percent spore germination and length of germ tubes were significantly greater for the native spearmint isolate than the peppermint isolates; development of both types of rust isolates significantly decreased as temperature increased from 18° to 30 °C. Regression coefficients did not differ between the two types of rust for either percent germination or length of germ tubes. Length of latent period did not differ between isolates of native spearmint and peppermint rust at 18 °C and 23 °C, but latent period was significantly shorter for the native spearmint isolate than peppermint isolates at 28 °C. The increased germination, growth of germ tubes and shorter latent period for native spearmint isolates infecting native spearmint would favour the rust development in relatively warm environments. Conversely, peppermint rust would be disfavoured in increasingly warm environments, further explaining its limitation in high temperature environments.