Issue No. 20, Article 9/August 10, 2007
Unexpected Problems of Corn Ear Development
Continuing dry conditions are causing concerns about the ability of the corn crop to complete the filling of grain in much of the southern half of Illinois. Whether the crop will recover to fill grain more completely following rainfall depends strongly on whether green tissue remains on the crop at the time that rain comes. Leaves that turn gray in the afternoon but still are green in the morning remain alive and capable of producing sugars if they can get water. Leaf tissue that is crispy and tan is no longer capable of coming back.
More surprising are recent reports, so far mostly from central Illinois, that some fields have small or even missing ears, even in areas that have not been deficient in water. Leaves, canopies, and stalks are in good to excellent shape in most of these fields, and there is no indication that a problem exists until you see ears that are not developing.
Most of the fields we are hearing about have symptoms much like those that we saw in a group of fields near Rock Falls in 2006. Plants appeared normal, but some ear shoots carry either no ear or only a short remnant of an ear. Cobs may extend past the kernels, and in most cases the end of the cob is the undeveloped remnant of the cob, with few or no silks present. Down the row, damaged ear shoots might alternate with full-sized ears, with no apparent cause. Reported damage in 2007 is extremely high in some fields, with virtually no healthy ears.
Undeveloped ear in a field near Rock Falls in August 2006.
We are still investigating the possible causes of this unusual injury, and it is not clear that there is a single cause across all fields. Many of the affected fields, including those described in 2006, were treated with a foliar fungicide (typically Headline), usually just before tassel emergence. While no direct connection has been made between such fungicides and such damage, it is likely that ears at this point in late vegetative stage are about the same size as those found with "arrested development" in these fields. We do not know of any herbicide that is known to persist from much earlier in the season to cause such damage, especially without affecting plant size at the same time. The problem also tends to be uniformly distributed across fields, regardless of percentage of plants affected. This suggests a uniform cause, such as a broadcast application.
It is important to go soon into fields to see whether this problem might be present. Be sure to check fields where fungicide was applied before tassel emergence, where application was by ground (higher volume and better coverage might mean faster and greater uptake), and where other additives, such as insecticides and fertilizer nitrogen, were applied along with fungicide.
The problem just described is similar to, but I believe slightly different from, the so-called "blunt ear" or "beer-can-ear" symptom that has been described by Dr. Bob Nielsen. This type of injury has also been reported in some fields this year, typically distributed unevenly across the field, with damage confined to or worse in the lower-lying parts of fields, including plants along waterways. We think that this is related to damage from the light frost that occurred on May 18 in parts of Illinois. Affected ears are often normal in row number and husk length, but rows might have only 5 to 15 kernels, and the cob may or may not extend past the kernels. In some cases, the cob has a sharp projection on the end that resembles the tip of a tassel branch.
Another phenomenon we are hearing about is the appearance of multiple ears from the same shank. In mild cases of this, the small, second ear never developed, and there will likely be no effect on yield (see my article in issue no. 18 of the Bulletin, July 26, 2007). There are definitely some hybrids that are highly likely to produce such ears. In some fields, as many as 5 or 6 "side" ears developed, forming what Bob Nielsen calls a bouquet. The side ears in these cases might be well developed, though many likely failed to form kernels due to late silking and lack of pollen. In general, the larger and more numerous the side ears, the more likely that the main ear was damaged in some way or has low kernel number. We think that the secondary ears are able to grow faster when the primary ear either shows less dominance or when it just uses less sugar, leaving more for the other ears. Causes of damage to the main ear might be different in different fields, but we can't rule out a cause such as that discussed above.
"Bouquet" ears from a single shank, with damage to the primary ear.
While some of the symptoms described here might differ among hybrids, it is not clear that there are groups of hybrids now on the market that are especially susceptible or immune to such problems. To the extent that stage of development affects the development of such symptoms, some hybrids that seem especially susceptible compared to others in the same field might only have been "in the wrong stage at the wrong time." At the same time, we cannot rule out that certain genetics common to a number of hybrids might bring along susceptibility. If the combination of factors that trigger such problems occurs very rarely, such hybrids might reach the market without the problem ever being observed.--Emerson Nafziger