Soybean disease pressure

If they haven’t already, soybeans will soon reach the growth stage that is “decision time” for a fungicide application. Growth stage R3 is known as beginning pod. At this stage, pods are three sixteenths of an inch long on one of the four uppermost nodes on the main stem with a fully developed leaf. It’s at this time that we start trying to answer the question: Do I need to spray my soybeans with a foliar fungicide? It’s a great question that will require some scouting.

Soybean diseases that might warrant fungicide control include frogeye leaf spot, brown spot, Cercospora leaf blight and pod and stem blight. Brown spot can be common in Kansas with Cercospora, and pod and stem blight has been an issue in excessively wet years.

Scouting for these diseases now is important for a couple of reasons. First, there are diseases that may look like brown spot, but are actually bacterial diseases. Fungicides won’t have any activity on them.

Second, work in Kansas would suggest that fungicides have not been economical unless significant levels of disease are present. Work by NE Area Agronomist Stu Duncan from 2013 to 2019 showed an average yield response across five dryland site years to be 2.8 bushels per acre. The yield response across nine irrigated site years was less than half a bushel. If disease is present, this could change significantly. Only scouting can help you determine that.

Third, fungicide resistance is increasingly becoming a concern. Blanket applications in the absence of disease with little potential for disease pressure may not seem to cause too much harm, but can increase resistance under the right circumstances.

Fungicide applications to combat soybean diseases will have the best opportunity for economic gain if frequent rainfall events occur through the R5 — seed is an eighth inch long in one of the four uppermost nodes on the main stem — growth stage and disease is present. Scout now to give fungicide applications the best chance to work for you.

For information on fungicide efficacy based on disease, check out this publication from Purdue online at https://www.plantpath.k-state.edu/extension/publications/BP-161-W-1.pdf.

A Crabgrass Invasion

If there was ever a summer to encourage crabgrass, this might be it. Cool snaps early slowed cool season grass growth and opened up the turf canopy. Warm temperatures mid-summer with good moisture have encouraged crabgrass ever since. Not a good combination.

Crabgrass control is best accomplished by preventing germination with a thick stand of desirable turf to keep seeds from germinating or light from getting to germinated plants. It’s difficult in our cool season grass systems, so we tend to apply crabgrass preventers to help out.

Most crabgrass preventers kill the seed as it germinates and do not have any effect on crabgrass already up. If we miss the preventer window, consider quinclorac. It kills crabgrass and foxtail as well as a few broadleaf weeds like black medic, bindweed and even clover.

Most crabgrass starts to go away in August and cool season grasses return, with crabgrass gone after the first frost. It will likely return next year, however, without efforts to combat it. Start planning now for thicker turfgrass next year – with a crabgrass preventer to help it along.

David Hallauer59 Posts

David Hallauer is the Meadowlark Extension District agent in the areas of horticulture and crops and soils.

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