Development of Secondary Crowns Reduces Crown Rot Severity in Alfalfa Cultivars

Abstract

AbstractCrown rot occurs wherever alfalfa (Medicago sativa L.) is grown, but the disease reaction of cultivars has rarely been examined. Differences in crown rot reaction among cultivars were followed for up to 9 yr in two studies. In one study, crown rot severity in nine alfalfa cultivars was assessed at Saskatoon (1985–1993) and at Outlook (irrigated test; 1986–1991) in Saskatchewan, Canada. From 1986 to 1991, there were no consistent differences in severity among cultivars. Crown rot symptoms were severe in almost every plant of every cultivar by 1991, so the trial at Outlook was discontinued. When the trial at Saskatoon was sampled in 1993, cultivars with more of a subsp. falcata background had less crown rot than those with subsp. sativa parentage. The primary crowns of most plants had been consumed with rot, but secondary crowns had formed in many plants. As a result, crown rot severity (rated mainly on secondary crowns) had declined substantially. To determine if these observations were consistent between trials, a second trial at Saskatoon (seeded 1985) was sampled in 1994. The same pattern was observed; crown rot severity was substantially lower 9 yr after establishment than at 5 yr, and the reduction was associated with development of secondary crowns. Forage yields in this mature stand were higher than early in the life of the stand, indicating that moderate levels of crown rot do not affect yield. Differences in disease reaction among cultivars were small, but may affect the development of secondary crowns, which allows alfalfa plants to compensate for progressive injury to the primary crown.