Yellow Wheat Part II - Causes
and Corrections
A number of growers are reporting patches of yellow in their winter wheat
fields. There are many potential causes for this discolouration - wet
soils, low pH, nutrient deficiencies, disease - any of which may be exacerbated
by backwards spring weather and slow root growth. The first step will
be to diagnose the cause of the yellowing so an effective treatment can
be applied. A quick guide is found in the following table.
| Suspected Nutrient Deficiency |
Plant Symptoms |
Soil Conditions |
Corroborating Tests |
| Nitrogen |
Stunting and yellowing of entire plant, starting at
lower leaves |
Often associated with wet soils, where denitrification
has caused N loss |
Plant tissue tests (compare good with poor areas) will
confirm if the yellowing is due to low N or low oxygen to roots |
| Magnesium |
Yellowing from the base of the plant, with
distinct interveinal chlorosis |
Generally associated with sandy, low pH
soils with low Mg. High K levels will make deficiency worse. |
Soil test to confirm low Mg levels; Squirt
test can give quick diagnostic |
| Manganese |
Yellowing from the top of the plant, sometimes with
faint striping of the leaves |
Patterns in field often more diagnostic than leaf symptoms;
appears on high pH knolls, and in hollows with muck soils |
Plant tissue test (compare good with poor areas); Squirt
test can give quick diagnostic |
| Iron |
Rare in Ontario; similar to Mn, with yellowing on younger
leaves first, but with more distinct interveinal striping |
Saturated, high pH soils |
Plant tissue test (compare good with poor areas); Squirt
test can give quick diagnostic |
| Low pH |
Stunting and poor vigour, may look like nutrient deficiency |
Most often associated with sandier soils and lower
slope positions (exception: Niagara, where the clays are naturally
acidic) |
Soil test for pH |
Once the deficiency has been identified, steps can be taken to correct
it. Magnesium deficiency will require soil applications of Epsom Salts
or Sulphate of Potash-Magnesia (K-Mag™), or dolomitic limestone if
it is associated with low soil pH. Micronutrient deficiencies can be corrected
with foliar applications. The product of choice for correcting manganese
deficiency is manganese sulphate, applied to provide 2 lb/ac of actual
Mn (8 lb/ac of product). This should be applied in high rates of water
(40 gal/ac), with a spreader-sticker to help carry the manganese through
the cuticle of the leaf. Chelated manganese is much easier to use, and
is more compatible with herbicides if they are mixed, but much more expensive.
Scientific opinions on the effectiveness of chelates range from about
2X as effective as the sulfate (U. of G.) to equally effective (Michigan
State), but the cost is roughly 10X higher for the chelate per pound of
manganese. Very low rates of chelated product are not effective.
Squirt Test Protocol
The "squirt and see" approach can often give results quicker
than the turn-around from a lab. Mix ½ teaspoon of nutrient with
water in a one liter hand spray bottle, adding two drops of a spreader
sticker (dish soap will work in a pinch). Lightly wet foliage with the
mixture in one metre strips in the affected area(s), marking the sprayed
area with flags. If the crop responds to the application within 48 hours,
you have identified the problem.
| Suspected Deficiency |
Nutrient Source |
| Magnesium |
Epsom Salts (MgSO4) |
| Manganese |
Manganese Sulphate |
| Iron* |
Ferrous Sulphate |
*Note: Iron is known to give plants a darker green colour, so interpret
these results with caution.
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