Over the past two weeks some fields in Iowa have shown silver areas on the corn leaves. Purple corn plants have also shown up this growing season. And where there's been a lot of rain, corn plants in areas of fields are showing signs of nitrogen deficiency.
A timely way to check the N status of the soil is to take 1-foot depth soil samples when corn is 6 to 12 inches tall, says Mark Johnson, Iowa State University Extension field agronomist in central Iowa. Take at least 16 soil cores (24 is better) for each sample, then take a subsample from this, about a cup of soil, and send that cup of soil to a testing lab.
"The cores of soil you pull from the field to gather your sample should be pulled in a systemic way going across corn rows," says Johnson.
For example, take the first core from within a row, walk a ways and take the next core one-eighth the distance between rows, walk a ways and take the next core two-eighths the distance between rows, the next three-eighths, the next four-eighths and so on for a set of eight cores. Two or three "sets of eight" comprise one sample for a test. For more information on the Late Spring Soil Nitrate Test, go to bit.ly/1K9JtkF. Click here for sending samples to ISU bit.do/5p3e. Cost for analysis is $8 per sample.
Silver leaf showing up on more cornfields this year
Over the last two weeks, Brian Lang has seen many cornfields in northeast Iowa with "silver leaf." The corn plants have irregularly-shaped silver areas on leaves.
"This condition occurs when there is bright sunshine while the early morning plants are cool and covered with dew," explains Lang, who is the ISU Extension field agronomist at Decorah. "This silver leaf symptom is considered to be a very minor event in terms of yield as yield loss does not occur."
Purple corn also common early this growing season
The most common reason for widespread purple corn is a combination of bright, sunny days and cool nights when corn ranges from V3 to V6 growth stage development, says Lang.
Corn hybrids with more anthocyanin-producing genes will show a greater amount of purple than those with fewer "purpling" genes, he notes. In most cases, the purpling will slowly disappear as temperatures warm and the plants transition into the rapid growth phase (post-V6 growth stage of corn). A hybrid's genetic makeup greatly determines whether corn plants are able to produce anthocyanin. A hybrid may have none, one, or many genes that can trigger production of anthocyanin.
Most corn researchers agree that the anthocyanin pigments develop in young plants in direct response to a number of stresses that limit the plants' ability to fully use the photosynthates produced during the day. These stresses include cool night temperatures, root restrictions, and water stress (both waterlogged and droughty conditions).
Several causes of corn plants having purple color
Lang sums up the purple corn situation as follows:
1) Cool conditions, slowing growth or cold events such as a light frost
2) Damage to the root system from insects, herbicide carryover or compaction
4) P deficiency, more likely with low soil P, high soil pH, poor nutrient uptake conditions
5) Fallow syndrome.
Effects and actions: what should you do, if anything?
1) If the weather is cold or if there is genetic tendency for the hybrid to produce purple symptoms, there should be little if any impact on yield.
2) Problems before the 4 to 5 leaf stage of the corn plant are easily overcome and have little lasting effect in many cases.
3) A non-uniform problem suggests problems with root systems. You should look for 2-dimensional roots, problems from soil compaction, insect damage, fertilizer burn from starter fertilizer or ammonia-N.
4) Sample the soil and have it tested if phosphorus deficiency is suspected.
Anthracnose leaf blight also showing up on some corn
The recent rainy weather, especially if you have high amounts of crop residue in corn-on-corn fields, has enhanced the chance for anthracnose leaf blight development on early season corn. Anthracnose leaf blight is caused by the fungus Colletotrichum graminicola. The fungus survives in crop residue and is splashed onto the leaves.
Anthracnose is definitely more severe where corn follows corn, says Lang. Symptoms are brown, oval, or elliptical spots (up to 1/2 inch in length) with a dark brown or purplish border, often surrounded by a yellowed zone. There may be black speckles within the dead tissue. This fungus is also often noted to infect leaves showing potassium (K) deficiency symptoms.
"Corn with anthracnose at this stage usually grows right out of it and is not likely to cause economic injury," says Lang, "but there have been exceptions, especially in fields where K deficiency was also noted. However, usually it just occurs on the lower leaves, does not move up the plant after those leaves are shaded out, and does not cause anthracnose stalk rot."
Farmers who are concerned about this disease could apply a foliar fungicide (you need to apply it at the V5 stage of corn) but university research does not show reliable economic returns for that practice, he says. Details of last year's research on this practice can be found here.
Early season K deficiency symptoms on corn plants
"Every spring we see some cornfields with potassium deficiency symptoms," says Lang. The K deficient plants are often stunted, with lower leaves yellowing and browning along the margins. This condition can be caused by low soil K levels, but it is often due to poor root function. Usually the fields look fine until the corn reaches the rapid-growth phase when K needs for the plant rapidly increase.
Along with the leaf symptoms, the plants are often stunted with internodes stacked closer together, but stage of development is usually similar to the rest of the field, says Lang. If root function improves, the yellowing will not progress up the plant, in which case they usually yield better than expected. There can be large differences among hybrids showing this K deficiency phenomenon. Anything that restricts root growth during the initiation of the nodal root system can lead to the problem.
Negative influences include shallow planting and/or soil settling or eroding after planting aggravates the problem, compaction, planter slots that open back up, erosion, seedling root rot, and/or very wet or droughty soil conditions for an extended period. It shows up in no-till and tilled fields. "There is no rescue treatment, however, if the nodal root system is at or very near the soil surface, a cultivation throwing soil up around the plants may help by stimulating root growth," he says. "Soil samples should be taken to make sure it is not a true K deficiency problem."