Jumbled Kernel Syndrome Resulting From Late Glyphosate Application.
Insect silk clipping and feeding of pollen at VT/R1 can result in kernel jumbling (ear on left). Source: P. Thomison, OSU
Symptoms:
Jumbled kernel symptoms result when kernels fail to develop throughout the ear for some percentage of the ovules (Nielsen, 2014). Cob length is usually normal and surviving kernels expand to fill the empty spaces on the cob left by the absent kernels. The resulting kernel arrangement on the ear appears to be jumbled.
Causes:
Excessive heat and drought stress near the time of pollination or early kernel set.
Extensive damage to exposed silks during pollination due to insect feeding ( e.g. western corn rootworm beetles and Japanese beetles) can also cause incomplete kernels that leads to kernel jumbling.
Applying glyphosate herbicide in glyphosate-resistant corn later than recommended according to product label guidelines can result in ears with jumbled kernels (See Bubble Kernel or Translucent Kernel Syndrome). The problem often occurs when glyphosate is applied off-label to glyphosate-resistant corn that is shoulder high or closer to pollination (Nielsen, 2010). There is little to no metabolism of glyphosate molecules in the plant, meaning that the herbicide remains in its active form and can damage developing tissues that have inadequate expression of the resistant form of the target site (DuPont Pioneer, 2014).
Many RR hybrids are not homozygous for the glyphosate resistant gene (Nielsen, 2010). Rather, they are heterozygous for the trait; one resistant gene contributed from one inbred parent and one susceptible gene from the other. The resistant gene is dominant and thus the hybrid plant itself is resistant to glyphosate. Following pollination among the heterozygous “parents” in the hybrid field, the “next generation”, represented by the kernels (fertilized ovules) of the ears, segregates genetically such that approximately 1/4 of the developing kernels are homozygous for the susceptible genes, 1/2 of the kernels remain heterozygous (and resistant), and 1/4 contain two copies of the resistant gene (and resistant). Glyphosate applied late can be lethal to the 25% of the developing kernels that are homozygous susceptible if they come in contact with the glyphosate during their development. Thus, up to 25% of the potential kernels on an ear may be damaged by late (off-label) applications of glyphosate and, thus, up to 25% yield loss may occur from this damage (Nielsen, 2010).
Management:
Follow recommended guidelines for minimizing crop stress, including maintaining appropriate soil fertility, selecting adapted hybrids and seeding rates consistent for soil yield potential and date of planting, and adjusting planting depth with varying soil conditions. Avoid planting too early in wet soils and minimize weed competition with effective herbicide application and/or timely cultivation.
Follow label guidelines concerning Roundup application, e.g. Roundup WeatherMax® can be applied over the top to corn with Roundup Ready® 2 Technology up to the V8 stage or until the corn reaches 30 inches tall, whichever comes first. For corn 30 to 48 inches tall, treatments can only be made using a ground applicator equipped with drop nozzles.
References:
DuPont Pioneer. 2014. Corn Ear Injury Risk with Off-Label Glyphosate Applications. Crop Focus. DuPont Pioneer Agronomy Sciences. https://www.pioneer.com/CMRoot/Pioneer/US/Non_Searchable/agronomy/cropfocus_pdf/off_label_glyphosate_app_corn.pdf (URL verified 8/26/2019)
Nielsen, R.L. 2010. Jumbled Kernel Symptom in Corn & Late Glyphosate Application. Available at https://ag.purdue.edu/btny/ppdl/Pages/POTW_old/12-13-10.html [URL verified 3/25/2019].