What's wrong with my corn leaves? 12 common causes

Differential-diagnosis table

Nutrient deficiencies (causes 1–5)

1. Nitrogen. The plant turns pale yellowish green and the lowest leaves yellow in a V from the tip down the midrib, then die back as N moves to new growth. It shows first in wet low spots, where denitrification has turned nitrate to gas, and on sandy knolls where it has leached. Confirm with a tissue test and a soil nitrate test.

2. Sulfur. Sulfur is not easily moved within the plant, so yellowing is worst on the upper, younger leaves, sometimes as interveinal stripes on the newest leaf. It is most common on acid sandy, eroded or low-organic-matter soils, in cold dry springs, and Purdue has seen it at V4 after a rye cover crop. Plants often recover as soils warm; a tissue test settles whether to apply.

3. Potassium. Yellowing and then necrosis start on the margins of the lower leaves and move up the plant, while the upper new leaves may remain green even in severe cases. A pattern that follows headlands or dry ridges points to a soil condition limiting uptake; compare soil and tissue tests from good and bad areas.

4. Magnesium. Lower leaves develop yellow to white striping between the veins because magnesium is mobile; severe cases show beaded round dead spots that may turn reddish purple, with dead tips and margins. It is most likely on acidic coarse soils or soils high in calcium or potassium. Tissue test and check pH.

5. Zinc. Look for a whitish band from the base of the newer leaves toward the tip, with margins, midrib and tip still green, shortened internodes and a stunted plant; the severe form is called white bud. It is tied to calcareous soils, eroded knolls, cool wet weather and high phosphorus rates. Most plants outgrow it; fertilize next year from a soil test.

Colour changes from weather (causes 6 and 7)

6. Purple seedlings. Cool temperatures or restricted roots slow growth while photosynthesis continues, sugars accumulate and anthocyanin forms; the colour usually fades with warm weather with minimal to no yield loss. True phosphorus deficiency is a dark green plant with reddish-purple tips and margins on the older leaves and no colour on the newest leaves. Check soil test P before applying fertilizer.

7. Cold and flooding. Yellow-green seedlings after a cool cloudy spell are short of photosynthate and nodal root growth, and they recover with sun and heat. Flooding is more serious before V6 because the growing point is at or below the soil surface: oxygen in a flooded soil is gone within about 48 hours, and submerged plants may not survive one to three days above 77 °F (25 °C). Three days after the water drains, split stalks and look for a white or cream growing point.

Herbicide injury (causes 8–10)

8. Bleached white leaves. Pigment-inhibitor herbicides (Group 27 HPPD inhibitors and Group 13) bleach existing leaves white; glyphosate (Group 9) and ACCase (Group 1) drift produce a "white flash" in corn. Frost, root burn from spring anhydrous and cool cloudy weather can also whiten leaves. Match the pattern to the boom, field edge and spray record; most plants outgrow it.

9. Twisted whorls and buggy whipping. Leaves wrap around each other and fail to unfurl, usually around late V5 to early V6 after a shift from cool cloudy weather to warm and sunny; the trapped leaves emerge yellow and crinkled and green up within a day or two. Pre-plant or pre-emergence Group 15 (e.g., acetochlor, S-metolachlor) and Group 4 (e.g., dicamba, 2,4-D) herbicides cause the same symptom. Purdue puts the yield effect of weather-related twisting at essentially nil.

10. Yellow flash, stunting and odd roots. ALS-inhibitor (Group 2) injury or carryover shows as a yellow flash, shortened internodes and stubby bottle-brush roots. Photosystem II inhibitors (Group 5, e.g., atrazine) taken up from the soil yellow the margins of the oldest leaves while new leaves stay clean. Fomesafen (Group 14) carryover from last year's soybeans whitens the veins rather than the tissue between them, with a pinkish-red midrib and floppy leaves.

Diseases and insects (causes 11 and 12)

11. Leaf diseases. Discrete lesions with a definite edge are a pathogen, not a nutrient. Long narrow tan rectangles bounded by veins are gray leaf spot, cigar-shaped gray-green blotches are northern corn leaf blight, orange pustules that rub off are rust, and raised black spots that do not rub off are tar spot. Base the fungicide decision on which leaves carry lesions at VT.

12. Insect feeding. A row of straight holes across leaves emerging from the whorl is shot-hole injury from a caterpillar that fed on the rolled leaf, most often European corn borer in conventional corn. Check 20 plants in each of five areas and dissect whorls for larvae; an insecticide only works while they are still in the whorl.