Conquering the devastating diseases of alfalfa

Authors: Samac, Deborah

Submitted to: Forage Focus
Publication Type: Trade Journal
Publication Acceptance Date: 4/28/2025
Publication Date: 5/15/2025
Citation: Samac, D.A. 2025. Conquering the devastating diseases of alfalfa. Forage Focus. May 2025:16-17.

Interpretive Summary:

Technical Abstract: In 1972, Dr. Fred Frosheiser, a Plant Pathologist with USDA, wrote an article for Crops and Soils magazine titled “The Worst Diseases of Crops: Alfalfa.” In that article he listed the diseases that caused the most severe yield losses: bacterial wilt, Phytophthora root rot, anthracnose, and Fusarium wilt. However, today we do not consider these diseases to be major problems. What happened? Since the 1970s, alfalfa cultivars were developed that have high levels of resistance to the worst alfalfa diseases. However, spring black stem and leaf spot (SBSLS) remains problematic. The disease occurs in both spring and fall, although symptoms and yield losses are most severe in spring. Recently, USDA scientists in Prosser, WA revised the standard test for SBSLS and evaluated 79 standard check alfalfa cultivars and 2,834 alfalfa accessions. Many of the accessions originating from northern latitudes appeared to be more resistant than reference cultivars. Plants with the lowest disease symptoms were retained from these screening efforts for further development of improved alfalfa populations. Interestingly, plants in related species show immunity to SBSLS that is not found in alfalfa. In the annual species Medicago truncatula, also known as barrel medic, some accessions show no disease symptoms after inoculation. USDA scientists in St. Paul, MN investigated resistance in these accessions in detail and found that resistance is associated with loss of certain genes that make plants susceptible to infection by the fungus. Also, over-expression of a gene called MtKCS12 involved in long-chain fatty acid production increased disease resistance in barrel medic. Research is underway investigating the role of the susceptibility genes in SBSLS resistance in alfalfa and to produce alfalfa plants overexpressing MtKCS12. This multipronged approach, recurrent selection for increased resistance, deletion of susceptibility genes, and increasing expression of genes enhancing resistance will improve understanding of disease resistance and offer ways to transfer resistance to commercial cultivars and conquer this destructive disease of alfalfa.