Hessian fly infestation was low in wheat trials sampled in Indiana in 2001, however, precautions should be taken to minimize the establishment of the fly in plantings this fall. The Hessian fly is present in wheat-growing areas throughout Indiana and often survives, although in lower numbers, in wheat stubble or grasses during the summer. However, there is potential for rapid increase of fly populations as a result of weather conditions or cropping practices that favor survival of eggs and young larvae in the fall. This was demonstrated in 1999 and 2000 during seasons of higher rainfall in mid-Atlantic and southeastern states when severe Hessian fly injury occurred in areas where populations had been relatively low for 4-5 years. Insecticidal control of the fly once infestation has occurred is poor, and generally not economically feasible. Management practices that prevent or delay build-up of fly populations, such as planting of resistant varieties and seeding after the fly-free date, provide the most cost effective means of control for wheat growers. Information about both types of management is given below.
As reported in 2000, the soft red winter wheat variety INW9811 with resistance to Hessian fly biotype L is available to Indiana wheat growers. Although many wheat varieties grown in Indiana have the H5 or H6 genes for Hessian fly resistance, INW9811 is the only variety resistant to biotype L, which is predominant in fly populations throughout the state.
INW9811 has demonstrated excellent resistance to field populations of the Hessian fly from Illinois, Indiana, northern Alabama and Arkansas, southern Delaware and Maryland, and eastern North Carolina that have a high frequency of biotype L.
Much of the fall fly population can be avoided by planting after the fly-free date. This is key to avoiding subsequent infestation by the spring brood. Additionally, it has been shown that following the fly-free date will help reduce wheat disease problems and reduce winter kill from excessive growth. To determine the fly-free date for your area of the state, refer to the enclosed map. Crop rotation, where wheat following wheat is avoided, also is one of the key management strategies for reducing Hessian fly problems. The Hessian fly passes the summer in the stubble of the current wheat crop. Plowing the stubble results in the destruction of the pest. Volunteer wheat, the wheat seedlings sprouting in the fall from grain left in the field during threshing, germinates and begins growing just in time for the fall emergence of the Hessian fly. These plants are readily infested resulting in a rapid build-up of the population. The use of resistant varieties, in combination with the above pest management strategies, increases the chance for a fly-free crop.
Specific characteristics and yield potential of varieties presently grown in Indiana can be determined by consulting Purdue Station Bulletin “Performance of Public and Private Small Grains in Indiana - 2000”, web access: <http://shawdow.agry.purdue.edu/agronomy/ext/smgrain/variety/sm~var.htm> or talk to your seed dealer.
In contrast to last year, bean leaf beetle numbers continue to remain at low levels. The few that can be found are first generation adults. Late in the season the second generation beetles will emerge and feed on leaves and pods and then find overwintering sites in field edges. Therefore, it is likely that if beetle numbers remain low, late season feeding should be minimal. This is good news to soybean seed producers concerned about this pest’s feeding on pods and lowering seed quality.
This is a reminder to start trapping with the yellow sticky traps for western corn rootworm beetles in soybean ASAP. Beetles have been emerging for some time and movement into soybean fields has certainly begun.
The extension publication E-218, Monitoring and Decision Rules for Western Corn Rootworm Beetles in Soybean has trapping and threshold information. This fact sheet was stapled to Pest&Crop #15 for our hard-copy subscribers and is also available electronically at: <http://www.entm.purdue.edu/Entomology/ext/targets/e-series/fieldcro.htm>.
Like the 13 and 17-year cycle of the cicadas, Purdue Extension weed scientists are also on a cycle. Every four years we try to send out a survey asking about the weed problems that producers are experiencing. In 2004 we will start pestering the county educators and producers to fill out these surveys once again. What this does is help study the shift of weed species through out the state. These shifts may be caused by environment or changing weed management trends. We would encourage you to take part in these surveys, when they are being circulated, because the information is only as strong as the response we get.
Below is a summary of the survey that Dan Childs, Thomas Jordan, and Ron Blackwell did in 1996 (publication WS-10) and a summary of the 2000 survey (publication in the works).
As some of you may already know, last week was Case Medlin’s last week here at Purdue University. He will be starting a position at Oklahoma State University. We are going to miss him, but wish him success in his new position. The process has been started to fill the vacant position. Dr. Bauman and myself will continue to be available to provide Weed Science Extension Services and look forward to seeing you this summer and fall.
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.
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.
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
Incomplete Kernel Set in Corn
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
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
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.
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/>.