Field Performance of Alfalfa Cultivars Resistant and Susceptible to Phytophthora Root Rot1

Abstract

AbstractBecause phytophthora root rot (PRR) resistant cultivars of alfalfa (Medicago sativa L.) are new developments, there have not been opportunities to determine what effect PRR resistance may have on forage production in different alfalfa growing areas. This paper presents results from several experiments comparing the performance of PRR susceptible and resistant cultivars grown under a range of environments in Minnesota.Broadcast plots were established in April and May in three different years. Some plots received only natural rainfall; others received an additional period of supplemental sprinkler irrigation during mid‐June each year to simulate a 10 to 14‐day wet period.Natural levels of Phytophthora megasperma Drechs. inoculum were adequate in the soil to cause significant yield losses. PRR resistant cultivars usually out‐yielded susceptible cultivars in plots receiving supplemental irrigation or when natural rainfall was sufficient to induce PRR. The most severe losses occurred during the seedling year. PRR killed seedlings and damaged the normal tap root system of alfalfa. Injured plants usually produced adventitious lateral roots, resulting in a shallow root system. This appeared to make the plants more sensitive to stress, such as drought and frequent harvesting. Resistant cultivars appear to be a feasible method for reducing losses from PRR. A minimum of 60% resistant plants may be necessary in a cultivars to provide an adequate amount of protection under severe PRR conditions.