By Jochum Wiersma
Reports of yellowing in small grains have started to reach us. There are several reasons why young wheat, barley, or oat plants have a pale green/yellow color. Some of the common reasons for early season yellowing are:
- Nitrogen deficiency
- Sulfur deficiency
- Early tan spot infection
- Herbicide injury
The nitrogen (N) deficiencies can readily be indentified as the symptoms are worst on the oldest leaves and start at the tip of the leaves, progressing towards the base as the deficiency gets worse. The causes of the N deficiencies are several, all which have common denominator, namely excess precipitation. Excessive rainfall causes:
- Leaching
- Denitrification
- Inability of the plants to take up available N
Leaching is a potential problem in coarser textured soils. Saturated soils/standing water will cause both denitrification and inability to take up available N. Denitrification is a microbial process and slows down considerably as soil temperature decrease. According to Univ. of Illinois data (Hoeft, 2004), denitrification losses are 1-2% if soil temperatures are less than 55oF, 2-3% when soil temperatures are between 55 and 65oF, and 4 -5% once soil temperatures exceed 65oF. As soils are saturated, the plant's roots also are unable to take up N - even if available. Often the crop recovers quickly if the growing conditions improve and the excess water has drained.
If the N deficiency is severe, a supplemental application of N as either urea (46-0-0) or urea ammonium nitrate solution (28-0-0) can be advantageous. Research by George Rehm and Russ Severson in 2005 showed that 40 lbs of supplemental N at the 4 leaf stage yielded 7 bu/A extra over the untreated check.
Sulfur (S) deficiencies are generally found on coarser textured soils and can readily be identified as symptoms are worst on newest leaves and less on older growth. This is opposite to N deficiencies as can be explained by the difference in mobility of the element in the plant; N can be more readily be recycled from older growth and redirected to the younger leaves compared to S. Cool and dry conditions tend to make S more pronounced as less S becomes available from the breakdown of organic matter.
Cool conditions make some of the micro nutrients also less available to the plant. These symptoms are often first noted on the coarser textured soils. Again, George Rehm has chased this problem in the past and found that no single culprit was to blame. As soon as growing conditions improved, the symptomology would disappear.
Early season tan spot infection can also cause the young wheat and barley crop to turn a bright yellow. Especially young seedlings up to the 3 to 4 leaf are very sensitive to a toxin that is produced by the fungus. This yellowing affects the whole seedling. If tan spot is identified as the cause of the yellowing, an early season fungicide treatment is warranted. Additional details on how to effectively control early season tan spot can be found in a follow-up article by Dr. Madeleine Smith.
Although few if any acreage has received an herbicide to date, yellowing of the crop can also be caused by herbicides. Cool growing conditions make a number of our common small grain herbicides more prone to cause temporary injury. Especially the ACCase class of grass herbicides is more active with cool(er) growing conditions. This temporary yellowing will dissipate in one to two weeks after application with no effect on grain yield.
Source : umn.edu