Spring glyphosate recommendation for established alfalfa: prevent plant damage by bacterial stem blight

Authors: Samac, Deborah - Debby

Submitted to: Forage Focus
Publication Type: Trade Journal
Publication Acceptance Date: 2/28/2020
Publication Date: 3/15/2020
Citation: Samac, D.A. 2020. Spring glyphosate recommendation for established alfalfa: prevent plant damage by bacterial stem blight. Forage Focus. (March 2020):20-21.

Interpretive Summary:

Technical Abstract: Recent wet fall seasons have delayed or prevented fall weed control in established alfalfa stands in some locations. Growers anticipating doing glyphosate treatments in early spring to established alfalfa should be aware of recent changes in recommendations for spring applications. If applications of glyphosate are made to established stands of Roundup Ready alfalfa before the first cut, research has found that herbicide timing is critical to prevent damage from a bacterial disease. In established alfalfa, glyphosate applications in spring should be done when alfalfa plants are no more than 4 inches tall. Recommendations are unchanged for treating newly seeded alfalfa stands. The change for established stands is due to observations that herbicide application can promote frost damage and disease caused by a bacterial plant pathogen. Several years ago, producers in northern California reported increased frost damage to Roundup Ready alfalfa that was sprayed with glyphosate in early spring. Close inspection of plants showed that plants had symptoms of bacterial stem blight caused by Pseudomonas syringae and P. viridiflava. These bacterial populations are low when plants are less than 4 inches tall, which may be the reason early glyphosate application does not lead to disease. Also, the environment during early plant growth may not be conducive to disease. Genetic studies of P. syringae isolated from California, Utah, Minnesota and Ohio indicate that they are similar suggesting extensive movement of the bacteria across the US. A current hypothesis is that glyphosate aids in entry of P. syringae into alfalfa plants. In support of this idea was the observation that Roundup Ready alfalfa plants in Ohio treated with glyphosate in early spring 2019 showed disease symptoms but without frost damage. Resistance to bacterial stem blight was found in several older cultivars that have high winter survival traits. The genes conferring resistance are being mapped and resistance is being transferred into several genetic backgrounds by traditional plant breeding methods. Greenhouse tests found that resistance is not specific to single strains but rather is broad and confers resistance to diverse strains from distant locations. Because bacterial stem blight is linked to frost damage in alfalfa, developing resistance to the disease may have the added benefit of increasing frost tolerance in alfalfa.