Potential causes of discolored wheat seedlings
Each fall, some fields of wheat lose their healthy green coloration. This can be true whether the wheat was planted in mid-September or late October. The green tissue color usually gives way to hues of yellow, purple or brown, or all three. The seedlings are sometimes stunted and often occur in localized areas in the field.
The extent of the symptoms may differ with very small changes in soil texture, elevation and water drainage. It’s not unusual to find that these areas appear normal as growth resumes in the spring.
In other cases, the spring may find the stand has experienced plant loss or that seedlings exhibit similar symptoms to that in the fall.
There are numerous potential causes for discoloration. Here are a few to consider:
- Cold temperatures and soil saturation: Sometimes wheat will discolor largely reflecting weather conditions such as a sharp decline in temperatures, or a prolonged period of flooding or soil saturation.
- Plant diseases: Both leaf and root diseases can lead to discoloration. Where there is abundant growth, heavy powdery mildew and Septoria can lead to color loss in susceptible varieties. Barley yellow dwarf virus infections can also occur in the Fall. Below ground, root and crown diseases may be the source of stress.
- Phosphorus deficiency: See Dennis Pennington’s article in this Wheat Wisdom below on “Early scouting.” The yellowing and purpling may not carry over to the new spring growth depending on the level of deficiency.
- Manganese (Mn) deficiency: This appears as a general yellowing and, on more developed leaves, as yellowing between the leaf veins. The affected areas are often localized in the field. Mn is much more likely to occur where the pH or organic matter is high. It is important to apply Mn fertilizer as soon as possible or at least soon after green-up. Two applications are sometime needed.
- Magnesium (Mg) deficiency: Unlike manganese (Mn), Mg is much more likely to occur where the soil pH is low. The deficiency symptoms are like that of Mn with yellowing between the leaf veins.
- Sulfur deficiency: The lack of sulfur appears as a distinct general yellowing of the leaves. The deficiency usually doesn’t show up until the spring. Generous amounts of sulphur fertilizer should be planned for the spring if not done last fall.
- Nitrogen deficiency: If there is a general soft yellowing of the field, it could be due to a lack of nitrogen. This sometimes can be seen in early-seeded wheat, especially where the nutrient has leached through coarse soils. Where the deficiency is modest, there may not be any yield reduction as long as there is plenty of nitrogen available as the wheat develops in the spring.
- Herbicide drift: Grass herbicides can easily drift from adjacent fields. Perhaps the most common occurrence is where glyphosate is being used in the neighboring field as a burndown. In this case, the leaf color will be yellow and purple, and eventually browning beginning at the leaf tips.
- Herbicide spray injury: For fields that received a fall herbicide, injury is more likely if it was over-applied or the temperatures turned very cool. Herbicides can also cause injury where there is tank contamination or where a burndown application included a product having some soil activity.
Growers or field personnel may want to consider mailing plant samples to a private lab for a tissue analysis where a nutrient deficiency is suspected.
Where possible, seedling samples with and without the symptoms in question should be taken within a few feet of each other. This will be helpful in diagnosing nutrient concerns as one is able to compare the analysis of the “good” and “poor” samples.