Spearmint Rust Resistance and Immunity in the Genus Mentha 1

W HILE Verticillium wilt is by far the most .serious and damaging disease of mint, rust caused by Puccinia menthae Pets. would rank second in importance. The rust disease may be severe in some areas and practically nonexistent in others. In a given area, rust may be important one season and unimportant the next season. A light rust infection is taken for granted as a necessary evil, but even a light infection must have a debilitating effect on the plant sufficient to result in some yield reduction. Under epiphytotic conditions, the fields are infected early, and there is a rapid buildup of the spores by secondary spore production. This results in the plants losing almost all, or at least a sizeable percentage, of their leaves prior to a normal harvest date. The farmer may save yield by premature cutting and distillation of the hay, but this practice often results in the production of oil of a substandard or inferior quality. There are two principal strains of rust recognized by agricultural workers. One affects Mentha piper#a L. and is called peppermint rust. The other (spearmint rust) affects the common or American variety of spearmint, M. spicata L. The peppermint rust strain will not infect the spearmint plant, nor will the spearmint rest infect the peppermint plant. However, the newer Scotch variety of spearmint M. cardiaca Baker is subject to both types of rust as indicated by Baxter and Cummins (1) and Hornet (3). Within the past 12 years, peppermint rust has resulted in severe yield losses in the Willamette valley of Oregon and in the lower Columbia River basin. Remedial measures adopted by farmers include the spraying of fence rows, plowing under of all stubble, and burning back the initial spring plant growth with a dinitro weed killer spray. These practices suggested by Horner (3) to reduce the overwintering spore population seem to be most useful in reducing the severity of the disease. Spearmint rust is not important in the Oregon area since little spearmint is grown there. Neither peppermint nor spearmint rust has been important in the irrigated Yakima valley of central Washington, although peppermint rust was found on plants in a fence row near Toppenish in the late fall of 1954. The high summer temperatures and low humidity found in this arid region presumably are not conductive to rust development since stolons brought from Oregon or the midwest must certainly have introduced both spearmint and peppermint rust. In contrast, the midwest area (southern Michigan, northern Indiana, and southern Wisconsin) is not troubled with peppermint rust but invariably has spearmint rust. The infection varies greatly from season to season, but seldom reaches epiphytotic proportions commensurate with that of the peppermint rust in the Willamette valley. No special remedial measures have been employed in the Midwest. This foreword serves to delimit the problem and to indicate the desirability of genetic plant improvement. Our present data are confined to spearmint rust, since these basic genetic data on Mentha species were obtained from plantings in southwestern Michigan where peppermint rust is seldom found.