Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 22, 2013
Publication Date: May 13, 2013
Repository URL: http://hdl.handle.net/10113/57972
Citation: Samac, D.A., Lamb, J.F., Kinkel, L.L., Hanson, L.K. 2013. Effect of wheel traffic and green manure treatments on forage yield and crown rot in alfalfa (Medicago sativa). Plant and Soil. DOI:10.1007/s11104-013-1746-5. Interpretive Summary: Harvesting alfalfa requires that tractors and other equipment travel over the field several times during each harvest. Over the life of an alfalfa stand, this can result in significant mechanical injury to plants, particularly the crown area from which shoots emerge, creating opportunities for crown rotting pathogens to enter the plant. However, the impact of wheel traffic on plant health has not been investigated. Developing resistance to crown rot is difficult due to the large number of pathogens that can cause crown rot. Incorporating plant materials called green manures into soil before planting crops has been shown to increase the populations of bacteria that inhibit growth of plant pathogens providing broad disease suppression. This study found that wheel traffic reduced forage yield 12-17%, reduced the number of surviving plants 13-16%, and increased the amount of crown rot. Cultivars were identified that had greater yields and less disease when subjected to wheel traffic indicating that improving alfalfa for traffic tolerance by selection and breeding is possible. Green manure treatments increased the proportion of bacteria with activity against pathogens when planted in late summer and incorporated in the spring. The green manure treatments increased yields under traffic but did not reduce the amount of crown rot. Thus, green manure treatments that can stabilize soil and reduce weeds also provide yield benefits in alfalfa production systems. Minimizing mechanical wheel traffic on alfalfa fields will improve forage yields and plant health.
Technical Abstract: Harvesting alfalfa (Medicago sativa) results in mechanical wheel traffic on plants, which may damage crowns and increase the opportunity for entry of pathogens causing crown rot. Developing resistance to crown rot is problematic due to the large number of pathogens involved. Incorporation of plant biomass (green manures) into soil has been shown to increase population density of streptomycetes with broad pathogen antagonist activity. This study aimed to measure the impact of wheel traffic on forage yield and plant health and determine the effect of green manures for reducing crown rot. Green manure crops, buckwheat and sorghum-sudangrass, were incorporated into soil 3 weeks before seeding alfalfa. Bacterial density, streptomycete density, and proportion of pathogen antagonists were measured from soil cores prior to planting green manures and alfalfa. Wheel traffic was applied 2 days after each forage harvest over every plant in the traffic treatment. Forage yield was measured over 2 years. Plant health was assessed at the end of the second year from plant counts and crown rot score. Wheel traffic reduced forage yield 12% to 17% depending on year and location, significantly reduced plant counts, and increased crown rot compared to the no traffic control. Cultivar had a significant effect on yield, plant counts, and crown rot. Streptomycete density and the proportion of pathogen antagonists increased when fall-sown green manure crops were incorporated in spring. Forage yields were significantly higher in plots with greater antagonist density when traffic was applied but fallow-control plots had highest yields in the no traffic control. Green manure treatments did not affect plant counts or crown rot. Mechanical wheel traffic reduces forage yield and increases disease. Selection of plants with increased tolerance to traffic may be possible. Green manure crops provide benefits in alfalfa production systems by increasing pathogen antagonists.