Integrating Disease Severity and Growth Retention Reveals Distinct Resistance Responses to Spring Black Stem and Leaf Spot in Alfalfa

Alfalfa spring black stem and leaf spot (ASBS), caused by Ascochyta medicaginicola, threatens alfalfa yield, forage quality, and stand persistence. Conventional resistance assessments of alfalfa cultivars generally rely on disease incidence, disease severity, or the percentage of healthy plants. Although these measures describe symptom occurrence and development, they do not capture biomass retention or root growth after infection. In this study, 12 alfalfa cultivars were spray-inoculated with A. medicaginicola under greenhouse conditions and evaluated 14 days after inoculation. Disease incidence was positively correlated with the disease severity index and negatively correlated with relative fresh weight, relative dry weight, and relative root length. We integrated these five indicators using a membership-function-based standard-deviation coefficient weighting method to derive a comprehensive response score (D value). D values ranged from 0.0340 to 0.9611. Magnum 2 had the highest D value, followed by Gannong No. 3, Adrenalin, and Dryland, whereas Thunder, Zhongmu No. 1, and Aohan had the lowest values. Compared with the National Alfalfa and Forage Alliance classification based on the percentage of healthy plants, the D-value ranking provided additional discrimination among cultivars within the same resistance class under the present greenhouse conditions. The first two principal components explained 89.24% of the total variation and separated resistant, intermediate, and susceptible response types. Thus, combining disease suppression with growth retention yielded a complementary quantitative description of cultivar performance under the conditions tested.