Pest & Crop Newsletter

Sudden Death Syndrome– (Gregory Shaner)

Sudden death syndrome of soybean was widespread in Indiana during both the 1998 and 2000 growing seasons.  It appears that we may again have a problem with SDS in 2001.  In the July 6 issue of Pest & Crop (No. 16), I discussed symptoms and conditions that favor an outbreak of SDS.  Periods of heavy rainfall over much of the state as beans were entering their reproductive stage of development provided the conditions for development of the foliar symptoms of SDS.

I have received reports of SDS from several locations throughout the state.  It is likely that the disease will show up in more fields over the next 2 or 3 weeks.  Affected areas may consist of only a few small patches in a field, or larger areas.  Over time, affected areas may increase in size.

Growers should start scouting their fields to determine if and where the disease is present.  This information will be valuable for making future planting decisions.

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Grain Fill Stages in Corn– (Bob Nielsen)

The grain fill period begins with successful pollination and initiation of kernel development, and ends approximately 60 days later when the kernels are physiologically mature. During grain fill, the developing kernels will be the primary sink for concurrent photosynthate produced by the corn plant.

What this means is that the photosynthate demands of the developing kernels will take precedence over that of much of the rest of the plant. In essence, the plant will do all it can to ‘pump’ dry matter into the kernels, sometimes at the expense of the health and maintenance of other plant parts.

A stress-free grain fill period can maximize the yield potential of a crop, while severe stress during grain fill can cause kernel abortion and lightweight grain. Fortunately up to this point in the 2001 growing season, weather and moisture conditions have been reasonably favorable for grain filling.

Kernel development proceeds through several relatively distinct stages.

Silking Stage (Growth Stage R1). Some may argue whether silking should be labeled as a kernel growth stage, but nonetheless silk emergence is technically the first identifiable stage of the reproductive period. Silks remain receptive to pollen grain germination up to 10 days after silk emergence. After 10 days without being pollinated, silk receptivity decreases rapidly. Natural senescence of silk over time results in collapsed tissue that restricts continued growth of the pollen tube. Silk emergence usually occurs in close synchrony with pollen shed, so that duration of silk receptivity is normally not a concern. Failure of silks to emerge in the first place, however, does not bode well for successful pollination.

Kernel Blister Stage (Growth Stage R2).  About 10 to 14 days after silking, the developing kernels are whitish ‘blisters’ on the cob and contain abundant clear fluid.  The ear silks are mostly brown and drying rapidly.  Some starch is beginning to accumulate in the endosperm.  The radicle root, coleoptile, and first embryonic leaf have formed in the embryo by the blister stage. Severe stress can easily abort kernels at pre-blister and blister stages.  Kernel moisture content is approximately 85 percent.

Kernel Milk Stage (R3). About 18 to 22 days after silking, the kernels are mostly yellow and contain ‘milky’ white fluid.  The milk stage of development is the infamous ‘roasting ear’ stage, that stage where you will find die-hard corn specialists out standing in their field nibbling on these delectable morsels.  Starch continues to accumulate in the endosperm.  Endosperm cell division is nearly complete and continued growth is mostly due to cell expansion and starch accumulation. Severe stress can still abort kernels, although not as easily as at the blister stage.  Kernel moisture content is approximately 80 percent.

Kernel Dough Stage (R4). About 24 to 28 days after silking, the kernel’s milky inner fluid is changing to a ‘doughy’ consistency as starch accumulation continues in the endosperm.  The shelled cob is now light red or pink.  By dough stage, four embryonic leaves have formed and about 1/2 of the mature kernel dry weight is now in place. Kernel abortion is much less likely once kernels have reached early dough stage, but severe stress can continue to affect eventual yield by reducing kernel weight.  Kernel moisture content is approximately 70 percent.

Kernel Dent Stage (R5). About 35 to 42 days after silking, all or nearly all of the kernels are denting near their crowns.  The fifth (and last) embryonic leaf and lateral seminal roots form just prior to the dent stage. A distinct horizontal line appears near the dent end of the kernel and slowly progresses to the tip end of the kernel over the next 3 weeks or so.  This line is called the ‘milk line’ and marks the boundary between the liquid (milky) and solid (starchy) areas of the maturing kernels. Severe stress can continue to limit kernel dry weight accumulation.  Kernel moisture content at the beginning of the dent stage is approximately 55 percent.

Physiological Maturity (R6). About 55 to 65 days after silking, kernel dry weight usually reaches its maximum and kernels are said to be physiologically mature and safe from frost.  Physiological maturity occurs shortly after the kernel milk line disappears and just before the kernel black layer forms at the tip of the kernels. Severe stress after physiological maturity has little effect on grain yield, unless the integrity of the stalk or ear is compromised (e.g., ECB damage or stalk rots).  Kernel moisture content at physiological maturity averages 30 percent, but can vary from 25 to 40 percent grain moisture.

Harvest Maturity. While not strictly a stage of grain development, harvest maturity is often defined as that grain moisture content where harvest can occur with minimal kernel damage and mechanical harvest loss.  Harvest maturity is usually considered to be near 25 percent grain moisture.

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Yield Loss During Grain Fill– (Bob Nielsen)

Yield potential in corn is influenced at several stages of growth and development. Ear size potential (number of potential kernels) is determined quite early, from about knee-high to about shoulder-high, or from about leaf stage V6 to V15. Generally, conditions in 2001 during that time period were reasonably favorable for ear size determination.

The next influential period for the corn crop is pollination. Again, generally conditions in 2001 were not stressful for this critical yield-determining interval. Essentially no stress occurred from either excessive heat or moisture deficits.

The period following successful pollination and finishing at kernel black layer is defined as the grain filling period in corn and represents the final important yield determining time frame. Grain fill stages in corn are described in an accompanying article.  Perfect conditions for ear size determination and pollinations can still be negated if severe stress occurs during the grain fill period.  

Yield loss during grain fill can occur from 1) stand loss, 2) incomplete kernel set, 3) lightweight kernels, and 4) premature plant death.

Stand Loss During Grain Fill
Yield loss due to stand loss during grain fill is usually greater than that due to stand loss that occurs during the vegetative phase. When stand loss occurs prior to pollination, ear size (number of kernels) on surviving plants may compensate in response to the lesser competition of a thinner stand.  Additional compensation may occur during grain fill in terms of greater kernel weight. When stand loss occurs during grain fill, ear size has already been set.  Only kernel weight can compensate in response to the lesser competition of a thinner stand.

Incomplete Kernel Set in Corn
Kernel set refers to the degree to which kernels have developed up and down the cob.  Incomplete kernel set is not always apparent from ‘windshield’ surveys of a corn field.  Husks and cob will continue to lengthen even if kernel set is incomplete.  A wonderfully long, robust-looking, healthy green ear shoot can completely mask even a 100 percent failure of pollination or severe kernel abortion.

One of the causes of incomplete kernel set is unsuccessful pollination. Unsuccessful pollination results in ovules that are never fertilized and, subsequently, ears with varying degrees and patterns of incomplete kernel set.  Many factors can cause incomplete pollination and distinguishing between them can be very difficult.

Certain insects like corn rootworm beetles and Japanese beetles can interfere with pollination and fertilization by their silk clipping action.  These insects feed on pollen and subsequently clip silks as they feed on the pollen that has been captured by the silks.  Unusually early or late pollinating fields are often particularly attractive to these insects.

Drought stress may delay silk emergence until pollen shed is nearly or completely finished.  During periods of high temperatures, low relative humidities, and inadequate soil moisture levels, exposed silks may also dessicate and become non-receptive to pollen germination.

Unusually favorable conditions prior to pollination that favor ear size determination can result in ears with an unusually high number of potential kernels per row. Remember that silk elongation begins near the butt of the ear and progresses up toward the tip.  The tip silks are typically the last to emerge from the husk leaves.  If ears are unusually long (many kernels per row), the final silks from the tip of the ear may emerge after all the pollen has been shed.

Another cause of incomplete kernel set is abortion of fertilized ovules. Aborted kernels are distinguished from unfertilized ovules in that aborted kernels had actually begun development.  Aborted kernels will be shrunken, mostly white, often with the yellow embryo visible; compared to normal plump yellow kernels.

Kernels are most susceptible to abortion during the first 2 weeks following pollination, particularly kernels near the tip of the ear. Tip kernels are generally last to be fertilized, less vigorous than the rest, and are most susceptible to abortion. Once kernels have reached the dough stage of development, further yield losses will occur mainly from reductions in kernel dry weight accumulation.

Severe drought stress that continues into the early stages of kernel development (blister and milk stages) can easily abort developing kernels. Severe nutrient deficiencies (especially nitrogen) can also abort kernels if enough of the photosynthetic ‘factory’ is damaged. Extensive loss of green leaf tissue by certain leaf diseases, such as common rust or gray leaf spot, by the time pollination occurs may limit photosynthate production enough to cause kernel abortion. Consecutive days of heavily overcast, cloudy conditions may also reduce photosynthesis enough to cause abortion in recently fertilized ovules.

Decreased Kernel Weight
Severe stress during dough and dent stages of grain fill decreases grain yield primarily due to decreased kernel weights and is often caused by premature black layer formation in the kernels. Decreased kernel weight can result from severe drought and heat stress during grain fill; extensive European corn borer tunneling (especially in the ear shanks); loss of photosynthetic leaf area by hail, insects, or disease early in grain fill; and killing fall frosts prior to normal black layer development. 

Once grain has reached physiological maturity, stress will have no further physiological effect on final yield, because final yield is already achieved. Stalk and ear rots, however, can continue to develop after corn has reached physiological maturity and indirectly reduce grain yield.

Premature Plant Death
A killing fall frost prior to physiological maturity can cause premature leaf death or whole plant death.  Premature death of leaves results in yield losses because the photosynthetic ‘factory’ output is greatly reduced.  The plant may remobilize stored carbohydrates from the leaves or stalk tissue to the developing ears, but yield potential will still be lost.

Premature death of whole plants results in greater yield losses than if only leaves are killed.  Death of all plant tissue prevents any further remobilization of stored carbohydrates to the developing ear. Whole plant death that occurs before normal black layer formation will cause premature black layer development, resulting in incomplete grain fill and lightweight, chaffy grain.  Grain moisture will be greater than 35%, requiring substantial field drydown before harvest. 

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Short Corn at Tasseling– (Bob Nielsen)

This is a revised article from P&C#17 on July 13, 2001.

Early-planted corn in Indiana is well into, if not beyond, the pollination stage. Some folks have noticed that the height of plants in these fields is noticeably shorter than they normally expect to see. The causes of shorter than normal corn can be traced back to planting date and temperature during stalk elongation.

Remember that stalk elongation begins at about the V5 stage of development (five visible leaf collars). Prior to that stage, most of the plant’s energy is directed to root development and leaf initiation. After that stage, the plant enters its so-called grand growth phase wherein above- and below-ground growth accelerates to an exponential pace that peaks near tasseling.

Elongation of the stalk occurs primarily by cell expansion near the bases of the internodes at what are called the intercalary meristems. Stalk elongation is influenced by a number of factors, among which are light/shade relationships, daylength and temperatures. Shade tends to increase levels of the plant growth regulator auxin, which, in turn, encourages greater elongation of internodes. The ‘shading effect’ contributes to the greater plant heights of densely planted corn.  Intense solar radiation is thought to result in photodestruction of auxin, which leads to less internode elongation, which results in shorter plants.  Interestingly, though, longer daylengths tend to increase internode lengths and overall plant height. Cold temperatures are thought to increase the rigidity of basal internode cell walls, thus limiting cell expansion and internode elongation.

Given these physiological causes of short plants, one can think about this year’s corn crop and begin to understand why some of it is pretty darn short at tasseling. Indiana’s corn planting progress finished six days ahead of the previous record pace set in 1988. Early-planted corn normally reaches the V5 stage at dates earlier than does later-planted corn. Stalk elongation in early-planted corn, therefore, begins in a time period that is characterized by shorter daylengths and generally cooler temperatures than corn planted later in the season. As described above, both of these factors contribute to shorter internodes and plant heights.

Now consider the two- to three-week period beginning in mid-May when temperatures were significantly lower than normal throughout much of the state. Much of the early-planted corn was beginning or well within the stalk elongation period while most of the later-planted crop was younger than V5. This extended period of cool temperatures influenced the elongation of internodes in the lower third of the stalk and accentuated the expected typically shorter heights of early-planted corn.

Are there yield consequences of unusually shorter corn? There are probably no negative consequences, unless the short height is dramatic enough to significantly reduce crop canopy cover and harvest of sunlight. Conversely, shorter corn is usually a benefit from the standpoint that the risk of stalk lodging is decreased due to the lower center of gravity.

Don’t forget, this and other timely information about corn can be viewed at the Chat ‘n Chew Café on the World Wide Web at <http://www.kingcorn.org/cafe>.  For other information about corn, take a look at the Corn Growers’ Guidebook on the World Wide Web at <http://www.kingcorn.org/>.

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