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It is time for a ne-crop-sy

The proof is in the pudding and that is never truer than when harvesting. What looked like a really good HRSW crop is turning into some real disappointment from some.  Apart from problems with sprout damage and low Hagberg Falling Numbers (HFN), yields have been much lower than anticipated or hoped for.  The causes of these low yields vary. It is, however, important to not scapegoat. Especially if the goat, or Wheat Stem Sawfly,  didn't cause the yield losses.

While the nuisance of Wheat Stem Sawfly (WSS) spread farther across the heart of the Red River Valley, it is not likely the culprit of the low yields that have been reported.  There is little evidence in the literature that the feeding of WSS is a major yield robber. The inability to pick the crop up is by far the greatest cause of yield losses.  In this area, harvest losses due to WSS are minimal as evidenced by the close shave I see in many fields.

The fungal leaf diseases than most commonly rob yield were either very well controlled (tan spot and Septoria spp.) or absent (leaf rust) this year.  Bacterial Leaf Streak (BLS) could, however, readily be found across the region.  The yield losses attributable to BLS will vary greatly and depend in large part on when the BLS was found in the upper canopy.  Yield losses as much as 40% have been reported when infections start early in the season.  The disease thrives with warm and humid weather, something we didn't encounter until after heading/anthesis.  The later the infections start the lower the yield losses will be and once the crop reaches soft dough the yield losses are relatively small.

So what caused the (very) low yields in some fields?

First, the humid and warm night in the first half of July really did not do the crop any favors.  Warmer nights mean more respiration and having more of the sugars produced during the daytime with photosynthesis go to maintenance rather than yield.  This was further aggravated by drought stress in areas north of Crookston.  Drought stress causes the plants to close their stomata to reduce further water loss and the risk of irreversible damage. By closing the stomata the diffusion of oxygen out of the plant and carbon dioxide into the plant halts and photosynthesis becomes a zero-sum game.

That same warm, humid weather did result in plenty of early infections of Fusarium Head Blight (FHB).  Especially these early FHB infections cause disproportionate yield losses as the fungus has the opportunity to into the rachis.  Once in the rachis, the developing kernels above the initial point of infection are cut off from nutrients and grain fill is halted.  The resulting kernels are not the telltale thumbstone kernels we associate with FHB infections but rather kernels that are perfectly intact but very small.

We might have to dig a little deeper, literally.  The hot dry conditions favored the development of Fusarium Crown Rot (FCR).  As I have been digging up HRSW crowns to look for WSS larvae I have been finding a lot of FCR infections.  FCR thrives under warmer and, curiously enough, drier conditions. Something we weren't short of until about a month ago.

The telltale signs of FCR are dark tan to antique bronze colored first internodes sometimes accompanied by the presence of a carmine red color on the remnants of the leaf sheets or the stems themselves (Photo 1).  These infections got established early in the season and choked off the water when the crop needed it the most, i.e. during grain fill. Often affected stems die prematurely and there are fewer and smaller kernels in the heads.


Photo 1 - Single wheat stem with symptoms of Fusarium Crown Rot

The possibility that FCR contributed substantially to yield losses in the region is not a complete surprise. About 5 years ago, Dr. Ruth Dill-Macky surveyed wheat fields across the valley and found that FCR had replaced common root rot as the most prevalent root rot disease present. Seed treatments do not provide season-long control of this disease. Crop rotation and the use of resistant varieties are the most effective means of reducing yield losses.  Unfortunately, genetic resistance to FHB does not guarantee that the variety is also resistant to FCR.

Bottomline - start not just digging into your memory and through the grain samples you have kept from each field but also in some of your fields and perform a necropsy on the wheat crowns themselves to determine to what extent drought, WSS, BLS, FHB, and FCR all contributed to that disappointment.
 

Source: umn.edu