Common Leafspot of Alfalfa: Ascospore Discharge and Plant Infection in the Field

Publisher Website
Google Scholar
Abstract
Peak daily discharge of Pseudopeziza medicaginis ascospores in the field occurred after sunrise with attendant rises in temperature, wind, speed, and. dissipation of moisture. Protracted discharge over many days rose with initial development of robust, uncrowded apothecia and fell as these became spent and were joined by less productive, small, crowded ones. In the laboratory, detached field‐infected leaves discharged ascospores at 5°C to 32.5°C and at relative humidities as low as 90% when fresh, and at 94% or higher when wilted or dried. Pathogen‐free alfalfa plants placed overnight in a diseased affalfa plot were inoculated but uninfeeted more frequently in July than they were inoculated and infected. Artificially‐inoculated plants placed at the same time in a nearby sugar beet plot became infected every overnight. From early September through early October artificially inoculated plants placed in a plot of sugar beets or a plot of nearly disease‐free alfalfa for 24 h periods, became infected in 18 and 19 of 29 such periods. Light rain, fog and dew were frequent during these periods and early morning temperatures fell between ‐ 3 °C and 5 °C‐ in one‐half of the overnights. Dormant ascospores on foliage remained fully infective in the greenhouse for at least 10 days.