Slow-Rusting Resistance in Native Spearmint to Puccinia menthae.

Rust, caused by Puccinia menthae, is often a serious yield-reducing problem on native and Scotch spearmint in south-central Washington State. Rust resistant mint cultivars would reduce a dependence on fungicides for disease management; however, conventional breeding practices are not possible because commercial mint plants are sterile. Mutant Scotch and native spearmint lines induced by irradiation were evaluated for partial resistance to rust. Latent period and number of uredinia per leaf from urediniospore inoculations in the greenhouse and aecial and uredinial development in the field were quantified. Length of latent period, number of uredinia per leaf, incidence of shoots with aecia, and area under the disease progress curve (AUDPC) for uredinia development all varied significantly among lines and standard cultivars. Length of latent period was significantly longer and number of uredinia per leaf was significantly lower for native spearmint line N87-1 than for most of the other mutant lines, native spearmint, and Scotch spearmint. Line N87-1 had the lowest AUDPC values of all lines and standard cultivars for 3 years in the field. However, line N87-1 had a relatively high incidence of shoots with aecia both of two years in the field. Length of latent period was significantly and negatively correlated with AUDPC 2 of 3 years. Mean incidence of shoots with aecia was not correlated with AUDPC or with length of latent period after urediniospore infection. Rust in native spearmint in south-central Washington State could be satisfactorily managed with slow-rusting resistance when coupled with good irrigation water management and sanitation tactics that limit aecia development on early spring mint foliage.