Reports of yellowing in small grains have started to reach me. There are several reasons why young wheat, barley, or oat plants have a pale green/yellow color. Some of the more common causes of early season yellowing are:
§ Nitrogen deficiency
§ Sulfur deficiency
§ Early tan spot infections
§ Herbicide injury
§ Barley yellow dwarf virus (BYDV) infections
Nitrogen (N) deficiencies can readily be identified 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, and an 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 55F, 2-3% when soil temperatures are between 55 and 65F, and 4 -5% once soil temperatures exceed 65F. 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.
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. As soon as growing conditions improve, the symptomology should 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. For more details go to http://blog-crop-news.extension.umn.edu/2014/05/early-season-scouting-in-small-grains.html
The first aphids in small grains were reported already last week by Bruce Potter, Madeleine Smith and Ian MacRae (see here: http://blog-crop-news.extension.umn.edu/2016/04/cereal-aphids-in-small-grains.html ) Some of these aphids may have carried barley yellow dwarf virus (BYDV) which will cause yellowing and ,because both the winter cereals as well as the spring cereals are still small, possibly severe stunting.
Although few if any acreage has received an herbicide to date, yellowing of the crop can also be caused by herbicide carryover from pre-plant, pre-emergence or post-emergence herbicides used the previous year. As the use of other herbicide modes of action are increasing to combat glyphosate resistant weeds in soybeans and corn the incidence of carryover issues has increased. Some of the active ingredients are rather persistent in the soil, especially under drier conditions and/or fields with low organic matter. Carryover issues will often be more evident on headlands as overlaps will create small areas with twice the amount of active ingredient. For more details go to http://www.extension.umn.edu/agriculture/weeds/resistance/ and reference "Corn and Soybean Herbicide Diversification Strategies - 2015".