White Heads

By Jochum Wiersma

It’s not the name of a band storming the Billboard charts but a simple description of partially or completely bleached wheat heads that stick out like a sore thumb in both spring and winter wheat this time of the year.  The following key will help you decipher the most likely cause of these white heads.

• Is the whole plant white or light green, or is just the head/upper portion of the stem white or lighter green?  If you answer is just the upper portion of the stem, please go to Step 2, otherwise go to Step 8.
• Pull on the white head and see whether the head and upper most internode, also known as the peduncle, pulls away from the lower portion of the plant.  If you can pull the head and upper most internode out, go to Step 3, otherwise go to Step 6.
• Inspect the bottom portion of the peduncle that you pulled out.  Does the bottom look like it was cut-off with a sharp instrument, leaving a clean, precise cut?  If yes, go to Step 4, otherwise go to Step 5.
• The likely culprit is wheat stem sawfly.  Wheat stem sawfly is rare in Minnesota and rarely, if ever, an economic pest.
• Does the bottom of the peduncle look chewed on and ragged?  If yes, the culprit is likely wheat stem maggot.  Wheat stem maggot is common throughout Minnesota.  As other grasses can also harbor wheat stem maggot, incidence of wheat stem maggot can be higher near the field edges.  Nevertheless, incidence rarely, if ever, exceeds 0.5% across a whole field.  There are no effective control options of either the adult fly or the larvae that cause the feeding damage.
• Inspect the spikes.  Do they look off-white as opposed to bleached white? Look for the lowest spikelet that is off-white white and peel away the outer most glume of this spikelet and the spikelet above it on the opposing side.  Inspect the developing seed.  If the seed is chalky white, then the likely culprit is likely Fusarium Head Blight (FHB).  If not or the seed is completely absent, go to Step 8.
• Inspect whether there is a pattern and/or gradient across the field.  Does the pattern/gradient across the field coincide with difference in elevation or soil type.  If so, go to step 8.  If not, the culprit may be glyphosate drift.  Glyphosate drift at very low concentration (less than 2 fl. oz. /acre) can cause sterility if the drift occurred right around heading.  
• The likely culprit is excess water if the worst symptoms coincide with low lying areas or drought stress if the worst symptoms coincide with higher lying areas or lighter soils.  Drought in combination with temperatures in excess of 92F in the later part of the boot stage or heading stage can cause partial development of florets and/or sterility, ultimately resulting in poor seed set.  Partially or completely sterile heads tend to ripen faster.  
• Did the whole plant including the crown pull up?  If no, go to Step 10, otherwise go to Step 14.
• Inspect the roots and cut the crown longitudinally.  If the crown itself is brown  or the stem base has a honey -colored appearance, and the inside is also brown, then go to Step 11, otherwise go to Step 11.
• The likely culprit of the white heads is latent common root rot.  The initial infection occurred earlier in the season but was not severe enough to kill the seedling.  Instead the plant was able to develop normally until the demand for water increased to the point where the vascular was no longer able to support the plant, causing premature death.
• If the node immediately above the crown is tan/light brown, than the likely culprit is Fusarium Crown Rot (FCR).  Sometimes there a pink color can be found on the leaf sheath or stem base surrounding the lowest node. This pink color will often disappear once the plant is pulled from the ground and stored for a few hours. If the roots and crown look black, go to step 13.
• If the roots, crown and or stem base have a very black appearance, then the likely culprit of the white heads is take-all.  This disease is less common in Minnesota and tends to be found more in areas where wheat is under irrigation.
• If the crown did not pull up with the plant, but instead only the stem down to the second internode looks like it collapsed or has an exit hole on one side, the likely culprit is Hessian fly.  Hessian fly is a relatively rare pest in Minnesota.  The highest risk is in winter wheat that is seeded prior to September 15.

Source:umn.edu