Black light trap catches, and splattered windshields have confirmed that certain areas of Indiana are experiencing a significant European corn borer moth flight. One must now evaluate high-risk fields for egg masses and borers to determine if economic levels exist. Typically, egg laying of second generation corn borer is highest in late-planted fields, especially those actively pollinating. Fields that were wet and delayed in development this spring or drowned out and replanted may be acting as a “trap crop” for egg laying. Consider that infestations after the blister stage do not have as much of a physiological effect on yields as they do at earlier stages.
It is very time consuming and frustrating to scout for second generation egg masses and larvae. The adult moths usually lay their eggs on the underside of corn leaves in the ear zone area (90% of the time). The tiny borers (1/16-1/8" long) hatch and crawl to leaf axils, behind leaf sheaths, in ears, or ear shanks to seek protection from the environment and feed. When one inspects these areas, it is not easy to see the larvae. Also, areas with pollen, anthers (pollen sacs), mold, etc., make it even more difficult. Once the larvae are large enough to easily be seen, they probably have already entered the stalk, ear shank, and/or the ear itself.
Fields in areas where the moth flight has been extremely high and where income will be high enough to warrant corrective measures if an economic population of borers develop, should be scouted for egg masses and borers. If you find that 1/3 of the borers have already bored into the stalks, etc., a control should not be applied. Remember, to be effective, 2 applications of an insecticide will normally be needed to control the second generation. This can get expensive!
Something to consider and observe while inspecting is the presence of beneficial organisms. Some fields have high numbers of lady beetle adults and larvae, they will devour corn borer egg masses and small borers. There are many other predators and parasites present in fields that can have a significant impact of corn borer populations.
As the rains continue to hit n’ miss in Indiana, the areas with little to no rain are observing some soybean fields with yellowing, and in some cases stunting. Although the damage caused by twospotted spider mites comes to mind, a number of factors can cause leaf yellowing. These include soybean cyst nematode, nutrient deficiencies, thrips, early senescence of leaves due to lack of moisture, diseases, compaction, etc. This is certainly not to say that spider mites are not causing some of the problems. However, it points out the fact that one should not jump to conclusions concerning the cause of a particular plant problem without making a thorough field evaluation. If the problem is due to spider mites, a good understanding of the pest’s biology, level of infestation, potential for damage, and management alternatives is needed to properly deal with the infestation.
Spider mite damage is often first noted on field borders and seemingly spreads to areas of the field (clay/sand nobs, compacted areas, poor fertility, etc.) where moisture stress has the greatest impact. Where yellowing is observed, twospotted spider mites could be the culprit. However, before considering control, it is very important that spider mites are identified as the source of the problem. Shake some discolored soybean leaves over a white piece of paper. Watch for small dark specks moving about on the paper. Also look for minute webbing on the undersides of the discolored leaves. Once spider mites have been positively identified in the damaged areas of the field, it is essential that the whole field be scouted to determine the range of infestation. Sample in at least five different areas of the field and determine whether the spider mites are present or not by using the “shake” method.
Reduction of crop yield is directly related to duration and intensity of the mite attack. The most severe damage occurs when the infestation starts in the early stages of plant growth and builds throughout the season. However, a heavy infestation at seed set can still cause economic damage. With the above in mind, it is extremely important that producers closely monitor their fields to determine if they have a mite problem. However, before applying controls there are certain factors that should be taken into consideration. These include:
When the discoloration from twospotted spider mite feeding is first noticed along field borders, or in spots within fields, and scouting information from the remainder of the field reveals no movement, then spot treating may suffice. Success of spot treating depends on spraying beyond the infested area, not just the damaged area. Spray a buffer zone 100 to 200 feet beyond spider mite colonized plants. If scouting results indicate that movement has occurred within several areas of a field or throughout a field, then treating the whole field should be considered. Although spot treating was of limited value in 1988 due to the earliness of the infestation, spot treating is a viable option at this time since we are in the advanced stages of plant development.If a control is warranted, two pesticides are recommended for use. These include dimethoate (Dimethoate 400 and 4 EC) and chlorpyrifos (Lorsban 4E). Proper placement of these pesticides is the key to successful control results. Nozzle pressures of 40 psi and 30-40 gallons of water per acre for ground application helps distribute the pesticide throughout the foliage. If using aerial application, the control material should be applied in 3-5 gallons of finished spray per acre. Normally, aerial applications are not as efficacious as ground applications due to limited surface-area coverage. So where possible, use ground application. Also, research has shown that mite controls work best in the early morning or evening hours. This is primarily due to more stable weather conditions, less convection currents and evaporation, resulting in better targeting of the pesticide.
Overnight (Aug. 9) corn earworm pheromone trap catches boomed in numbers! This and the black light trap catches indicate that this pest is on the move. Late market sweet corn and late pollinating seed corn should be closely monitored with pheromone/black light traps or for eggs on silks.
Diplodia ear rot is confirmed to be present in at least some west central Indiana cornfields. It is likely present in fields in other areas of the state as well. At its most extreme, husks and shanks of infected ears are completely brown. When the husks are peeled back, white mycelial growth of the fungus can be seen on the surface of the kernels. The ear leaf sheath and blade may also be dead. Less severe infections may appear as small (1/4 inch) to fairly large (2 inches) dead areas on the surface of the husk. On these ears, mycelium of the fungus may not yet be evident on kernels, but if the dead husk tissue extends into the ear, the kernels are probably already infected. These infections may be more recent than those in which the shank and husks are completely dead. With time, these milder infections may become more severe.
Anthracnose leaf blight is appearing on upper leaves of corn. Lesions are quite variable in size, ranging from small oval areas to large, irregular areas of dead tissue. Under humid conditions, it is possible to see dark structures on the surface of the lesion. These are somewhat smaller than a pinhead, and if they are examined with a hand lens at an oblique angle, it may be possible to see tiny, black spines projecting from the surface of the leaf.
Definite symptoms of sudden death syndrome have been seen in west central Indiana soybean fields. Dan Egel, plant pathologist at the Purdue Southwest Agricultural Center near Vincennes, reports seeing the disease in southern Indiana. Upper leaves show interveinal chlorosis and necrosis and, when stems are split, the cortex is brown and the pith is white. The disease will probably appear first as patches within a field, particularly in areas where soils hold water longer.
In a number of soybean fields, areas of pale green or yellow plants are appearing. This could be from a number of causes in addition to sudden death syndrome – soybean cyst nematode, brown stem rot, Phytophthora rot, ponding, and other causes. It is important to investigate these areas to determine the cause of the problem. Knowing the reason for poor growth or premature plant death will permit appropriate management decisions for future crops.
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. Thus far in 2000, weather and moisture conditions have been very favorable for grain filling.
Kernel development proceeds through several relatively distinct stages.
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 2000 during that time period were quite favorable for ear size determination.
The next influential period for the corn crop is pollination. Again, generally conditions in 2000 were excellent 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.
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.
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/.