Yellow alfalfa and Aphanomyces root rot
Figure 1. Nitrogen deficiency in alfalfa.
Alfalfa fields in Minnesota that have had high levels of rainfall this summer may have plants that are yellow and stunted or slow growing. The symptoms are similar to those of nitrogen deficiency (Figure 1).
The problem is more pronounced in heavier soils that are poorly drained. The problem is likely the result of lack of oxygen to plant roots due to the saturated soil and possibly due to infection by the water mold known as Aphanomyces euteiches, that causes Aphanomyces root rot of alfalfa. This pathogen has swimming spores that are active in wet soil. Once the pathogen infects the roots, it degrades the fine fibrous roots where root nodules are located and degrades the lateral roots (Figure 2).
Figure 2. Aphanomyces root rot in alfalfa.
The roots of plants infected with this disease look like the root of a carrot with mostly just a taproot (Figure 2). The lack of nodules and fibrous roots is the main cause of symptoms of nitrogen deficiency. Once soils dry out, plants may be able to recover if there is sufficient time before frost occurs. Stress from water logging and disease may increase winter injury of these stands leading to greater than normal stand losses next spring. Two races of Aphanomyces occur in Minnesota, so cultivars should be selected with this problem in mind. However, under continuous heavy rainfall with wet soil conditions, even resistant cultivars will show signs of Aphanomyces root rot.
Please feel free to connect to us directly if you observed yellow alfalfa. We would like to know your location, and when you observed the alfalfa.