Aphanomyces is often an "unseen" crop pest that can significantly limit yield gains and profitability for alfalfa growers. Aphanomyces root rot is caused by a soil fungal pathogen that causes plant disease under the right soil moisture conditions. Dr. Craig Grau, UW-Madison retired, conducted Aphanomyces research and provides much of our understanding of this disease described here.
Although Aphanomyces can occur in a wide range of field conditions including sloping contour strips it is most severe in flooded soil conditions.
Resistance to Aphanomyces was discovered in alfalfa and almost all new varieties since the early 1990's have had good to high levels of resistance to Aphanomyces root rot. These first varieties are resistant to what became designated as the Race 1 pathogen.
However, around 1990, another strain of Aphanomyces was found that was capable of infecting the first generation of resistant varieties. This strain was coined as "Race 2" isolates representing a new form of the pathogen.
What to look for
Symptoms of Aphanomyces root rot are not easy to distinguish and are not characterized by unique alterations of the roots. Infection of young seedlings with Aphanomyces can result in the death of seedlings, but more often results in stunted, chlorotic (yellow) plants. Yellowing is predominant on the first two cotyledon leaves that emerge on seedlings.
Aphanomyces can also infect mature stands causing large yellowing or light green areas in the field with stunted plants. These plants have a much smaller root system with very little of the extensive fine root hairs found on uninfected plants. Nitrogen fixing nodules are frequently absent or are in some stage of decay. The yellowing foliage of the infected plants resembles symptoms of nitrogen deficiency. While Aphanomyces can stunt plants and reduce forage yield it does not kill plants as compared to Phytopthora.
Yellowing areas in alfalfa fields are often associated with fertilizer deficiencies or shallow top soil. Observing these areas a little more closely with an examination of plants and their root systems can help clarify the cause of poor performance. The yellowing areas often have an irregular shape relating to soil/subsoil conditions if Aphanomyces is the cause.
Aphanomyces is often confused with sulfur deficiency. To further complicate the symptoms, root rot caused by Aphanomyces can restrict nutrient uptake causing sulfur deficiency. To help separate the two a tissue test of the top 6 inches of the alfalfa near harvest and a corresponding soil test with a mineral analysis can help verify sulfur deficiency.
Fungicides commonly used to coat alfalfa seed are not active against Aphanomyces and therefore, avoiding poorly drained soils and using resistant varieties are the main methods useful for control. The number of alfalfa varieties that are resistant to Race 2 Aphanomyces has increased significantly in the last three years.
The laboratory work required to test for Race 1 and Race 2 Aphanomyces takes six to eight weeks at a cost of $150 per sample. For this reason, one management strategy to follow when Aphanomyces is suspected is to assume it is the Race 2 pathogen and plant Race 2 resistant varieties. Choosing highly resistant (HR) varieties provides a greater measure of resistance to Aphanomyces.
Differences have been observed among Race 2 resistant varieties for performance in soils testing positive for race 2. Sometimes all race 2 resistant varieties perform equally, but in some cases some are better than others. If possible, growers might consider planting several Race 2 resistant varieties in problematic fields. The differences in performance are not due to false claims in resistance to Race 2 but rather the existence of a third race or possibly another pathogen that requires wet soil conditions to become damaging.
Bay is the Extension crops and farm management agent for Grant and Lafayette counties.