Major concerns: Wheat head scab and alfalfa weevils

Frank Peairs photo: Alfalfa weevil larvae feed on alfalfa leaves.

Sow and Grow

Temperatures have heated up across the state this past week, stressing small grains and worsening drought conditions in many areas.

Winter wheat is heading or very near the heading and flowering stage in many areas. As flowering nears, the main pest concern is fusarium head blight (head scab), which is a fungal pathogen that produces a mycotoxin that can pose a significant livestock health risk and can cause wheat grain yield loss.

If you have winter wheat that hasn’t flowered yet, now is the time to scout fields and consider a preventative fungicide application for head scab at early flowering. The hot weather much of the state has experienced over the last few days may be accelerating the time between heading and flowering to just a couple days, so it’s vital to check fields daily. Timing for effective fungicide application is very important to achieve reliable crop protection.

Deciding whether to apply fungicides can be a difficult decision. To avoid scab, applying the right product at the ideal timing can protect your crop from significant yield loss; however, economics should also be taken into consideration. If conditions are wet at flowering and temperatures are high, you likely have a high risk of head blight infection. If conditions are dry and temperatures are moderate, there may be a lower risk, but inoculum in nearby fields or crop residues may still provide a source for infection.

Two tools are available to help assess head scab risk, the Fusarium Risk Tool provided by the national Fusarium Head Blight Prediction Center (wheatscab.psu.edu), and the NDAWN Small Grains Disease tool, hosted by the South Dakota Mesonet (climate.sdstate.edu/tools/smallgrains/). Both of these tools provide the likelihood rating for risk of scab based upon date, nearest weather station data, variety susceptibility ratings, flowering date and other factors. Consider using these tools to aid in deciding whether to apply a fungicide for scab prevention, and what fungicide may suit your situation the best.

Weevils

Alfalfa weevils have been spotted in fields the past couple weeks – fields with heavy defoliation often appear white or light-brown. Weevils can do a great amount of damage to an alfalfa crop in a hurry and many producers chose to cut hay slightly early, prior to their intended cutting window, in order to slow damage. This method is a viable management strategy to reduce feeding, but does not guarantee that weevils will not remain in the field or return. In fact, they often continue eating on windrows as they dry.

Previous research has shown significant stunting of alfalfa when weevils continue feeding on alfalfa stubble following cutting and harvest. Weevils feed mainly on the terminal buds of the plant, causing stunting. This is why scouting field is highly important, especially if weevils were present in your field prior to the first cutting.

Alfalfa weevil activity is based on growing degree day accumulation; however, this isn’t always 100% accurate and weevil development can deviate slightly from the expected calculations. To scout fields moving forward, the best method is to use a sweep net and 5-gallon pail. Once you have determined weevil presence, population density needs to be determined. To scout, walk in a ‘Z’ pattern across the field and randomly sample a total of 30 plants total (pull plants out gently or cut at soil surface). Tap each plant against the inside of the 5-gallon pail to dislodge larvae as you sample. Count the number of larvae in the bucket and determine the height of the alfalfa plant. Once 30 samples have been taken, calculate the average number of larvae per plant and average plant height of the sample set. Using this calculation, the economic threshold to spray insecticide for weevils can be determined using SDSU Recommendations that can be found by searching “Scout Alfalfa Weevils” at extension.sdstate.edu and using the appropriate table to determine your economic spray threshold.

Upcoming events and announcements

• South Dakota Grazing Exchange: Short on grazing forage or hay? Check out the South Dakota Grazing Exchange powered by the SD Soil Health Coalition. This free resource connects forage producers and livestock growers together across the state. www.sdgrazingexchange.com

• June 15 – Garden Hour: Every Tuesday evening through Sept. 28, join the SDSU Extension horticulture team to discuss garden and landscaping questions virtually. Register at extension.sdstate.edu/events. No registration fee.

• June 18 – Ag Economic Dialogues: To participate in this virtual program, register by June 17 at extension.sdstate.edu/events. No registration fee.

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