Frost and Wheat Curl Mite

MSU Extension Pathologist, Dr. Mary Burrows has had several calls regarding the Wheat Streak Mosaic Virus (WSMV). She wrote the following in one of her ag alerts.

Host: When we consider host, we usually think of resistant varieties to the pathogen or vector. Varieties with resistance genes for WSMV and/or Wheat Curl Might (WCM) are grown in more southern states, but none are available adapted to Montana (The plant breeders and Dr. Burrows are working on it!). That said, there is a range in tolerance that can be found in the Cereal virus Montguide (http://store.msuextension.org/publications/AgandNaturalResources/MT200911AG.pdf). There is no true resistance. Losses will depend on the time of infection, the variety of crop, the strain of the virus, and weather conditions. Later infections lead to less yield loss.

The second thing we consider for ‘host’ is the presence of host plant material. As you all know, the mite and virus are facilitated by the green bridge, or the presence of green plant material between the harvesting of one crop and planting of the next. Hosts vary in their ability to support mite and virus replication. The best host by far is spring wheat, followed by winter wheat and barley. A somewhat distant third is downy brome (cheatgrass) which due to its life cycle and high populations does serve as a significant alternate host in Montana. Then, there are numerous grassy weeds that I would say overall are ‘intermediate/poor’ hosts of the mite and virus, but could be sources in some years if conditions are favorable. They essentially maintain a background level of the virus and mite that we will never eliminate. A good summary of these alternate hosts and their capacity for hosting WCM and WSMV is in this guide from Nebraska: http://extensionpublications.unl.edu/assets/pdf/ec1871.pdf and the MontGuide mentioned above.

In talking to people, Burrows was pleased that in general, they were aware of the problem and had done everything possible to eliminate the green bridge. Once your own land is taken care of, you also have to look around at your neighbors and any adjoining CRP, pasture, etc. If there are green host plants, there is likely a source of inoculum. If there is unharvested spring wheat or barley, that is a potential source of inoculum. Low temperatures will lower the movement of mites off of standing spring wheat and barley.

Host removal and survival of the WCM: Burrows sent out an ag alert earlier about host removal methods, but an article by Jiang et al (2005) provides some evidence for how long wheat curl mites survive on a host after a termination method has been applied. The full article is available at http://apsjournals.apsnet.org/doi/pdfplus/10.1094/PD-89-0853. In this greenhouse study they either applied glyphosate or paraquat and measured retention of wheat curl mites on the host plant. The research resulted in showing that mites are not retained as long on paraquat-treated plant material as they are on glyphosate-treated plant material. This is likely due to the rapidity of plant kill with the two different chemistries. Time to ‘brown’ tissue will be dependent on a number of factors including soil moisture and temperature. They did a separate ‘stem cutting’ experiment to simulate swathing of plants as compared to drought, and mites were significantly reduced on plants within 3-5 days after cutting. In addition, plants laying flat on the ground would not serve as good ‘launching pads’ for mites into the wind.

Burrows did several years of experiments where she put out ‘trap’ plants, spring wheat, and collected them weekly. She can now look at what effect our experimental treatments and weather conditions had on migration of WCM to these ‘trap’ plants. What she found was that a prolonged cold period significantly reduced mite migration into our trap plants. A single frost event is UNLIKELY to significantly reduce mite migration into a newly planted crop. So, the next week or so of weather is going to be relatively warm. Two to three weeks from now we are back to long-term average weather trends of colder weather that will reduce mite movement. Bottom line: Dr. Burrows still recommend planting later than recommended for our since we are in a WSMV hot spot. The risk of WSMV must be balanced with other crop needs, but WSMV may be the primary risk in our area next year.