By Emily Unglesbee
DTN Staff Reporter
ROCKVILLE, MD. (DTN) -- Adolescence is the worst. Just ask your corn.
Corn plants in the V5 to V7 growth stage may be showing some symptoms of a giant growth spurt -- twisted whorls, yellow leaves and weakened stems, agronomists told DTN.
These strange physical symptoms can occur when corn plants undergo unusually fast development early in their life. "It's been my observation that these symptoms occur more commonly when a period of relatively slow growth -- maybe because of cool temperatures -- is followed by rapid shift to warm conditions and rapid growth," said Purdue corn agronomist Bob Nielsen.
That's what many growers in the Corn Belt have faced this June, after a wet spring and delayed planting, Burrus sales agronomist Stephanie Porter noted. Scout for the issues, but don't be too alarmed if you spot them. The corn plant usually grows out of these symptoms given good growing conditions, both agronomists noted.
This symptom goes by many names -- "twisted whorl," "wrapped whorl," or most comprehensively, "rapid growth syndrome." It most often occurs between the V5 and V7 growth stage.
The exact cause isn't confirmed, but one hypothesis involves the "exudate," or liquid secreted by leaves, often during the evening hours, Nielsen said.
"The exudate can be a little sticky, so when this happens during a period of rapid whorl development, some think that the stickiness prevents the whorl leaves from unwrapping easily," he explained.
Nielsen likens what happens next to the old-fashioned Chinese finger trap toy, which wraps tighter and tighter the harder you pull. "Once those leaves are stuck and that wrapping begins, the plant is growing so fast, the leaves can't pull themselves out of it," he said.
The result is tightly twisted and even bent whorls, as the rapidly growing leaves try to force their way out. The whorl will usually unfurl itself after about a week and normal growth will resume, Nielsen said.
The field pattern of plants with this strange appearance varies, said Porter. She believes some hybrids may have a genetic predisposition to it, which means the symptoms could show up across a field if conditions are right.
Porter has also seen the phenomenon occur in small patches or along the field edge -- which likely means the root system is the culprit. "Something is holding that plant back," she explained. "Often it's the root -- either they're compacted or injured by insect or disease, so the plant is one growth stage behind everything else. Then -- due to good growing conditions or putting on new roots -- that plant jumps ahead and grows too fast."
Also known as "yellow flash" or "yellow top," this symptom is a direct result of twisted whorls.
After the whorl unfurls, the wrapped leaves will appear light yellow for a period of time, until they are able to absorb sunlight, Nielsen said. They can also appear wrinkled or kinked from the tight wrapping.
The yellow leaves are bright, easy to spot, and easily confused with other agronomic issues.
Because they often emerge shortly after postemergence corn herbicide applications, many farmers fear herbicide injury, Porter said.
ALS herbicides, in particular, can cause similar yellowing in corn plants, Nielsen said. It can be tricky to tell the symptoms apart, but yellowing from rapid growth will be limited to the top leaves emerging from the whorl. Yellowing from ALS injury will occur where the herbicide made contact, so depending on the growth stage at application, it can be anywhere on the plant, Nielsen noted.
Parts of the central Midwest have received heavy rainfalls, and some corn fields may have standing water, which can also cause leaf yellowing, Nielsen added. In those cases, the yellowing will occur at the base of the plant, as it tries to remobilize nitrogen and other nutrients to the top of the canopy, Nielsen said.
See more details on twisted whorls and yellow leaves from Nielsen here: https://www.agry.purdue.edu/… Porter here: http://burrusseed.com/…
Stems can also suffer from bursts of rapid plant growth.
As stalks mature, a compound called lignin is deposited on the outer surface, Nielsen explained. It acts like cement within cell walls to make the stalk rigid and strong.
When corn plants are young, they naturally have less lignin in their stalks, making them more fragile and easily bent.
During a period of hyper-fast plant development, often before pollination, sometimes the lignin deposition can't keep up with the stem growth, Nielsen explained. "Sometimes the stalks are elongating so fast that they haven't yet formed this lignified outer stalk circumference, so they're more susceptible to strong winds," he said.
In severe cases, these plants will be susceptible to green snap or lodging until the lignin content of the stalk increases.
See more details on this problem from Nielsen here: https://www.agry.purdue.edu/…
Emily Unglesbee can be reached at Emily.firstname.lastname@example.org
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