Identification and Management of Bacterial Wilt in Alfalfa
April 1, 2024
Bacterial wilt, one of the most destructive diseases of alfalfa is caused by Clavibacter michiganese subsp. insidisum. The pathogen favors wet cool growing conditions and is more prevalent in poorly drained soils. Aboveground symptomology usually does not present itself until the second or third cropping year. The bacterium can survive on living or dead alfalfa plants as well as in the soil and has been found to survive on seeds and dead plant tissues for over ten years.1 The bacterium can easily be spread by surface water, tillage, harvest equipment, hay, seed, and animals. Plants can become infected through openings in the crown and root. These injuries may be from harvesting equipment, animal grazing, and winter injuries.
Aboveground symptoms are more noticeable in regrowth after cutting. Symptoms include a slow and reduced regrowth of stems and leaves. Stems are spindly and short, giving a ‘dwarfed’ appearance. Leaves turn light green to yellow and tend to curve upward and appear wilted (Figure 1). Infected plants wilt and die more rapidly during warm and dry periods; however, some plants may survive into the next season. In most cases, if an alfalfa plant is infected, it does not recover and eventually succumbs to the infection.
Below ground symptomology - upon cutting open the taproot of infected plants and pulling back the bark, there is a brown discoloration of the outer vascular tissue. This discoloration is easily observed versus the normal cream-colored tissue of healthy roots. As the infection progresses, the visible appearance in the root changes. Infection starts as scattered yellowish-brown dots, and becomes more of a ring, and eventually turns the whole root brown. Symptoms and eventual death of infected alfalfa plants are due to the restriction of xylem “water- transport” vessels, which become blocked by the bacterium.
Proper management is key to suppressing outbreaks of this disease. One of the most beneficial tools we have available is the selection of highly resistant varieties. In areas where infection is present, it is recommended to amend field cutting schedule to harvest newer stands of alfalfa first, to avoid contamination from older stands. Although it may not be practical, washing equipment with steam after harvesting infected fields may help limit spreading the disease to other fields. Crop rotation for two to three years helps reduce disease incidence. Limiting or reducing injury to the crowns helps to reduce wounds and entry points for the bacteria to infect the plant. Managing your soil health and fertility to promote plant vigor and overall health can limit the incidence of weak plants. Lastly, avoid planting alfalfa in poorly drained soils. Implementing suggested recommendations provides the best defense for managing bacteria wilt.
Channel Agronomist
Ashley Madson
Sources
1RPD No. 300 – Bacterial wilt of alfalfa. 1988. Reports on Plant Diseases. Integrated Pest Management. University of Illinois Extension. https://ipm.illinois.edu/diseases/series300/rpd300/
Web site verified 2/16/24. 1110_360867
