Stem‐Tip Inoculation of Alfalfa with Sclerotinia trifoliorum: I. Development of Necrosis on Resistant and Susceptible Selections

Abstract

A stem‐tip inoculation procedure has been successfully used in divergent selection for resistance to Sclerotinia trifoliorum (Eriks) on alfalfa (Medicago sativa L.). One unreported observation made in early studies was that lesion development was not continuous for some alfalfa plants over the 14‐d incubation. A more in‐depth study of disease development was needed to quantify the variability, and the stem‐tip inoculation procedure was modified for this purpose. Inoculated stems were placed in plastic sleeves and marks were made daily on the plastic to show the development of necrosis over time. Eight plants previously selected for their higher levels of resistance and eight plants selected for minimal resistance (susceptibility) developed necrosis after 14 d in the plastic sleeves, which was similar to that measured in earlier studies. The length of the lesion on susceptible plants was described by a linear equation with 0.9 to 1.3 cm of necrosis d−1. Six of the resistant plants stopped the lengthening of the necrotic stem lesion about 8 d after inoculation while two of the resistant selections continued linear development at rates much less than that measured on the susceptible selections. When the necrotic region stopped developing, a dark border usually developed between the green and necrotic tissues which was not seen when disease continued to develop. Statistical analysis suggested disease resistance might be effected by either of two qualitatively different systems.