Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Sweet growers are noting that much of the emerging corn is currently very yellow in color. This is due to cold soils during emergence (cool air temperatures, cloudy weather, and wet soils). In sweet corn seedlings that are more advanced, cold wet soils can reduce root growth and also lead to plants becoming lighter green or yellow. With warmer temperatures expected this weekend, seedlings should green up quickly.
We often seed purpling in some sweet corn varieties in poor growth conditions. This is because purple pigments (anthocyanins) form in response to stresses such as cold soils and wet conditions. Under these conditions, sugars produced are not fully utilized for growth and accumulate, triggering the pigment production (the pigment is a sugar-containing glucoside). Other stresses such as root feeding by insects, compaction, or herbicide injury can increase the purpling. This purpling is very variety dependent and usually shows up in sunny days with cool nights and the corn in the V3-V6 stage. Varieties with more purple pigment genes will show the most purpling. Phosphorus deficiency also can cause purpling in plants but is not common in most of our vegetable soils. In most cases, the purpling will disappear rapidly in warmer weather, unless there is an underlying problem such as insect injury or herbicide damage.
Some growers have commented that as sweet corn is uncovered in clear plastic production systems, plants have turned yellow rapidly. This too is because of colder conditions. We also see sweet corn turn pale at the V3-V5 stage when seed reserves are used up, especially if roots are not growing well. Root activity drives these color changes. As soils warm up and sidedress N is applied, this normally will self-correct.
Other color changes this time of year can be seen as yellowing between the veins of the corn plant, especially in younger leaves. This striping can be variety related and is also often related to poor growing conditions and stresses on the plant followed by rapid growth with better growing conditions. However, we do see occasional problems with yellow striping due to nutrient deficiencies; magnesium deficiency would appear as interveinal chlorosis of older leaves, manganese and zinc deficiencies as interveinal chlorosis of younger leaves.
Yellow or bleached bands across leaf blades can also be caused by low temperatures.
Sunscald can also occur on young sweet corn, especially right after plastic covers are removed in plastic production systems. This will be seen as bleached areas on leaves.