Yellow Flash in Soybeans

Pale green, yellow leaves may be caused by a number of factors. Pale green soybean leaves at this time of year are generally caused by a lack of nitrogen. As bean foliage grows rapidly the roots are not able to supply all the nitrogen required. A dark green colour will return as the nodules catch up and supply the N required by the leaves. If proper nodulation and sufficient nutrients are present, soybeans will come out of the yellow phase quickly and there will be no yield loss.

If the yellow beans are concentrated on headlands or where there has been obvious glyphosate spray overlap the culprit may be yellow flash. Yellow flash sometimes occurs with the application of a high rate of glyphosate and when conditions are dry. It's thought that the glyphosate temporarily ties up Mn in the plant. These symptoms usually last for about a week and no yield declines have been reported. Research has found that adding chelated Mn to glyphosate to reduce yellow flash does not increase yield if the fields have adequate Mn to begin with.

Yellow leaves with dark veins are classic symptoms of true Mn deficiency and will cause yield losses, however. Symptoms of Mn deficiency are interveinal chlorosis (yellowing) on the newest trifoliates, although older trifoliates often show the symptoms as well.(see Figure 1).

Figure 1 - Manganese deficiency

If it is unclear whether Mn deficiency is the problem, take a hand spray bottle (i.e.: Windex type) of Mn in solution, and hand-spray 1 metre of row. Flag this section. If Mn is the problem, this section of row will turn dark green in 2-3 days. If the area does not turn dark green, further analysis is necessary to determine the culprit.

Factors Influencing Mn Deficiency

1. One of the most significant factors affecting the availability of Mn is the soil pH. As soil pH increases, less Mn is available to the plant. That's why Mn deficiency is not generally found in soils with a pH below 6.2. Deficiencies can also appear on eroded knolls where the pH is higher than the rest of the field.
   
   
2. The deficiency is most common on poorly-drained soils, especially clays and silt loams.
   
   
3. High organic matter also ties up Mn. Symptoms may occur in hollows where organic matter is higher.
   
   
4. Manganese is less soluble in well-aerated soils, which is why in compacted areas such as wheel tracks a dark green colour may remain while the rest of the field becomes symptomatic (Figure 2).
   
   
   
   
   Figure 2 - Compacted areas often show less symptoms of Mn deficiency.
   
   
   
   
5. An over-riding factor is that roots must reach manganese to absorb it. Wet soils, extremely dry soils, cool weather, soil compaction, root diseases, nitrogen deficiency and herbicide damage can limit root growth causing Mn symptoms.
   

The general recommendation is to spray with a foliar application of Mn as soon as the problem is identified. Several products are now available. Check the recommendations for the specific product.

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