Evaluating Medicago spp. plant genetic resources for resistance to spring black stem and leaf spot pathogen

Abstract

Alfalfa (Medicago sativa L.), also known as lucerne, is the fourth most widely grown crop in the U.S. and plays key roles in animal nutrition and crop rotations. Several 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 a damaging fungal leaf spot disease of alfalfa and needed levels 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 spp. accessions were screened for disease reaction in replicated trials and 2832 alfalfa accessions were screened for disease reaction in non-replicated evaluations. An ideal concentration of 5 × 10⁴ spores mL⁻¹ was determined by measuring the reaction of standard susceptible (Lahontan) and moderately resistant (Ramsey) check cultivars. Several cultivars and related Medicago species appeared to be more resistant than the moderately resistant checks. Many of the alfalfa accessions originating from colder and wetter environments (e.g., northern latitudes) also showed improved resistance compared to reference cultivars. Resistant germplasm selections were made from these screening efforts for further development of improved alfalfa populations.

Plant phenotyping relevance

アルファルファ葉斑病抵抗性の評価に用いる接種プロトコルと、病徴表現型を識別する改良評価尺度を開発し、検証・大規模適用しているため、植物表現型取得法が中心である。

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