Evaluating Medicago spp. plant genetic resources for sources of spring blackstem and leaf spot resistance

Authors: Irish, Brian; Samac, Deborah; Porter, Lyndon; Heineck, Garett

Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/19/2025
Publication Date: 11/20/2025
Citation: Irish, B.M., Samac, D.A., Porter, L.D., Heineck, G.C. 2025. Evaluating Medicago spp. plant genetic resources for sources of spring blackstem and leaf spot resistance. Crop Protection. 201. https://doi.org/10.1016/j.cropro.2025.107485.
DOI: https://doi.org/10.1016/j.cropro.2025.107485

Interpretive Summary: Alfalfa is the world’s most important forage legume crop with significant production and economic impact in the United States. Alfalfa plant diseases reduce yield and quality and are often difficult to manage in a sustainable fashion. Alfalfa spring black stem and leaf spot is an important foliar fungal disease for which no good disease resistance is found in commercially available cultivars. To identify potential sources of resistance to this disease, large alfalfa collections held by the US National Plant Germplasm System were systematically evaluated. Previously described disease screening procedures were optimized and modifications suggested to improve the procedures. From the ensuing evaluations a subset of resistant plants has been identified and chosen to be incorporated into the program’s breeding objectives. The protocols and the improved populations generated will become publicly available and can be used to further develop cultivars to be used in producing alfalfa sustainably.

Technical Abstract: Alfalfa (Medicago sativa L.), the fourth most widely grown crop in the U.S., plays key roles in animal nutrition and crop rotations. However, foliar diseases reduce yield and quality of herbage, impacting both plant and animal production and farm economy. Spring black stem and leaf spot, caused by Ascochyta medicaginicola (formerly Phoma medicaginis), is the most damaging fungal leaf spot disease of alfalfa and resistance is lacking in commercial cultivars. To identify potential sources of resistance in alfalfa germplasm, an optimized greenhouse seedling inoculation protocol was developed. Several A. medicaginicola isolates were tested for pathogenicity and spore concentrations were optimized for ideal disease pressure. A rating scale, modified from an established protocol, with improved resolution was used to distinguish differences observed in disease phenotypes. After protocol improvements, 78 standard check alfalfa cultivars and 188 Medicago accessions were screened for disease reaction in replicated trials and 2,832 alfalfa accessions were screened for disease reaction in non-replicated evaluations. An ideal concentration of 5x104 spores mL-1 was determined by measuring the reaction