Wheat and barley can handle some flooding but it’s not without cost. When the water recedes and soils drain, you’ll likely notice that the wheat crop (and barley for that matter) has turned pale green or even yellow.
Common reasons for early-season yellowing include:
- Temporary nitrogen deficiency.
- Temporary herbicide injury.
- Early tan spot infection.
Other possible causes include Barley Yellow Dwarf virus or temporary micronutrient deficiencies.
Nitrogen (N) deficiencies with readily identifiable symptoms are most noticeable on the oldest leaves. Symptoms start at the tip of the leaves, progressing toward the base as the deficiency gets worse.
There are several causes of N deficiencies, all of which stem from excess precipitation. Excessive rainfall causes:
- Inability of plants to take up available N.
Leaching is a potential problem in coarser textured soils. Saturated soils and standing water will cause both denitrification and inability to take up available N. Denitrification is a microbial process and considerably speeds up as soil temperatures increase.
According to University of Illinois data, denitrification losses are:
- 1 to 2 percent if soil temperatures are less than 55 degrees Fahrenheit.
- 2 to 3 percent when soil temperatures are between 55 and 65.
- 4 to 5 percent once soil temperatures exceed 65.
Because soils are saturated, the plant's roots also can’t take up N – even if it’s available. The crop often recovers quickly if growing conditions improve and the excess water has drained.
If the N deficiency is severe, it can be advantageous to apply supplemental N as either urea (46-0-0) or urea ammonium nitrate solution (28-0-0).
Research in 2002 (PDF) showed that 40 pounds of supplemental N at the 4-leaf stage yielded 7 bushels per acre extra over the untreated check. The small, replicated trial looked at the impact of timing of supplemental nitrogen during a spring when excess rain saturated soils and yellowed the wheat crop.
Prior to the supplemental N, the field – where wheat had followed corn – received 80 pounds of N per acre as 82-0-0 and 50 pounds of N per acre as 18-46-0. Supplemental N was applied at either Zadoks growth stage 14 or Zadoks growth stage 60.
The early application of fertilizer N increased yield by 7 bushels per acre compared to the wheat that didn’t receive supplemental N. The later application didn’t increase grain yield compared to the untreated check.
Cool and wet conditions make some of the micronutrients less available to the plant. These symptoms are often first noticed in crops grown on coarse-textured soils.
Researchers have chased this problem in the past and found no single culprit to blame. As soon as growing conditions improved, the symptoms also disappeared.
Cool growing conditions make many common small grain herbicides more likely to cause temporary injury. The ACCase class of grass herbicides is especially more active with cooler growing conditions.
This temporary yellowing will dissipate in one to two weeks after application with no effect on grain yield.
Tan spot infection
Early season tan spot infection can also cause the young wheat crop to turn a bright yellow. In particular, young seedlings up to the 3- to 4-leaf stage are very sensitive to a toxin that’s produced by the fungus. This yellowing affects the whole seedling.
If you identify a tan spot as the cause of the yellowing, an early season fungicide treatment is warranted. Use half the labeled rate of any of the labeled fungicides – such as Tilt, Quilt, Stratego and Headline – to halt the development of the tan spot and allow the crop to recover.
Reviewed in 2018