Alfalfa Stem Nematode and its Link to Climate Change in Yolo County
Alfalfa stem nematode outbreak in Yolo County
Alfalfa is a major crop in Yolo County, accounting for 17% of all irrigated acres in 2008. While nematodes and other pests have always been of concern for alfalfa farmers, a large outbreak of alfalfa stem nematode in 2009 and 2010 resulted in significant losses in yield countywide. In response, local growers, extension staff, and scientists are working to understand what factors may have contributed to the outbreak and what solutions may help control the pest.
What is alfalfa stem nematode?
The alfalfa stem nematode is a parasitic roundworm in the phylum Nematoda. It's life cycle begins with an egg. After hatching, the nematode molts two times to become an infective juvenile. The juveniles can live in the soil and withstand extreme temperature and moisture fluctuations. During favorable conditions, the juveniles travel up the stem of alfalfa seedlings and enter the plant through the stomata. Once within the plant, the juveniles undergo another molt to enter the adult stage. Plant hormones are released by the nematodes that stunt growth and swell plant tissue (Figure 1). Adult females can lay 200-500 eggs within the alfalfa plant. The average generation time is 19-23 days at 41-86 deg F .
What are the symptoms?
- leaves small and "mouse-eared" in appearance
- shortened internodes and swollen nodes
- plants appear yellow or white in color ("white flagging")
- brittle stems that are easy to break off
Figure 1. Normal internodes with no swelling of nodes on healthy stem (left) compared with shortened internodes and swollen nodes on the infected stem .
What are the possible links to climate change?
While the exact cause of the recent stem nematode outbreak is unclear, an increase in winter temperatures is likely to be an important contributing factor. Stem nematodes do not actively reproduce below 41 deg F . In Yolo County, average minimum winter temperatures have increased increased 3 deg F since 1983 and are currently approaching the lower reproductive threshold (Figure 2). Higher temperatures allow the nematode to compete a larger number of breeding cycles during the winter and thus impact the severity of the infestation. If climate change causes winter temperatures to rise further, outbreaks of alfalfa stem nematode may become more frequent in the region. Additionally, the use of organophosphates and carbamate in alfalfa crops has decreased 50% since 2005 (source). These pesticides are known to suppress stem nematode populations, but are being replaced by pyrethroids, which do not affect the stem nematodes. Consequently, the decreased use of these pesticides may have also played a role in the recent outbreak.
Figure 2. Average minimum monthly ambient temperatures in Davis, CA November to February 1983-2010 . The lower reproductive limit of stem nematode females is 41deg F .
How does alfalfa stem nematode spread?
Alfalfa stem nematode can be transmitted through plant debris, irrigation water, soil and farm equipment. Once a nematode outbreak occurs, effective treatment options are limited. However, certain precautionary measures may help prevent the introduction of ASN and/or limit its spread to other fields.
- Farm equipment should be washed before and after use.
- Irrigation runoff from infected fields should be directed away from other fields.
- Crop rotation with non-host plants can reduce nematode populations
- Livestock manure can contain stem nematodes, and livestock grazing can aid in dispersal
How can alfalfa stem nematode be controlled?
In addition to the methods mentioned above, farmers can also use varieties of alfalfa that are resistant to stem nematode. As of 2011, 53 varieties of certified-eligible alfalfa seed with high stem nematode resistance (>50% individuals resistant) are available to farmers . However, plants with high resistance can still become affected by stem nematode. Varieties with higher levels of resistance are under development and should be available in a few years.