August is a month when lots of important things happen physiologically in both corn and soybean plants. In soybean, growth stage R5 is the beginning of rapid seed fill. The soybean plant is under a great deal of stress at this time due to demand for starch-based energy produced by photosynthesis. The demand for energy comes from the developing seeds and pods, the nitrogen-fixing nodules on the roots, the moisture-seeking roots themselves, and the photosynthetic and transport machinery within the plant. These various energy sinks are in direct competition with each other for the limited supply of energy available. If soybean plants are already under stress due to diseases, insects, drought, low fertility, soil compaction, etc., there is even less photosynthetic energy available for the plant to distribute equitably.
Wise farmers have noted from experience that soybean grain yield is made in August, coincident with growth stage R5. If the soybean is growing in the upper Midwest USA, this is generally true. Everything done prior to growth stage R5, i.e. – fertilizer, seed bed preparation, weed control, insect control, etc., is done in order for the soybean to be as vigorous and as healthy as possible at R5, in order to enable the plant to adequately meet its energy demands and fill seed as efficiently as possible.
The physiology of a corn plant is considerably different than that of soybean. Consequently, although grain fill period in corn is very important to an acceptable corn crop, other growth stages like stage R1, pollen and silk emergence, are also very important. Since corn produces only one or two ears per plant, it is crucial that tassel pollen shed and ear silk emergence are synchronous with each other. Under severe drought stress, silk emergence can be delayed relative to pollen shed. If this occurs, there may be no more pollen dehiscing from the tassel when the silks are receptive. In this extreme case, no fertilization can take place and the corn plants will be barren. Fortunately, such extreme drought is infrequent; and furthermore, considerable breeding work has and is being done to select corn hybrids which will silk simultaneous with pollen shed, even under severe drought.
That being said, corn plants are subject to considerable energy demand during rapid grain fill just like soybean. A well-pollinated ear requires a huge amount of energy during grain fill. The stalk and roots also require considerable energy to remain healthy and functioning. Any stress to corn plants during rapid grain fill can limit the amount of energy that the plant can provide to the competing energy sinks. When corn plants are under stress during grain fill, several types of stalk and root rotting organisms can take advantage of the weakened corn plant. If this happens, the vascular transport system, which carries the starch energy and other nutrients and water up and down the plant, can be weakened or shut down completely. This may cause the plant to die prematurely in the fall or the stalks and roots to lodge. In both cases, grain yield can be lost.
I guess my take-home point in the above discussion is this. As farmers and growers, you know there are many cultural practices that can be done to allow both corn and soybean a good start in the spring and summer, such as a good seed bed, fertility, weed control, adequate and uniform stand (especially in corn). To be of any value, these practices must have an impact on how efficiently the plants can assimilate dry matter into harvestable yield. That harvestable yield is being produced in August and September during rapid seed fill in soybean or grain fill in corn.
Foliar fungicide in soybean.
I have discussed foliar fungicides in soybean previously. Daren Mueller and Alison Robertson from Iowa State University Department of Pathology have provided an updated summary of foliar fungicide work done in soybean and corn at Iowa State University from 2006 to 2009. The results are interesting.
·In soybean, across all experiments and fungicide treatments (831 observations) the overall yield response to foliar fungicide was 2.18 bushels per acre.
·Applications at stage R2 and R3 resulted in higher yield response (2.32 bu/A and 2.42 bu/A respectively) than applications at R1 (1.07 bu/A) or at R4 or later (less than 1 bu/A).
·Yield response was greater when disease severity in the field was >5.0% (1.79 bu/A) than when disease severity was <5.0% (0.68 bu/A).
·The predominant diseases noted in the experiments were brown spot, Cercospora leaf blight, frogeye leaf spot, and downy mildew.
I am not going to make an economic analysis of the various spray costs and yield responses. Each grower’s situation is different. I am told the cost, after rebates and price reductions, of various fungicides, such as Headline or Quadris, is about $12.00 per acre. Application costs depend upon whether application is done by ground or air, or whether it is custom-applied or grower-applied, or whether the fungicide is being applied in conjunction with an insecticide and/or other chemicals. If the soybean crop is going to be sprayed for aphids anyway, some growers will assume the fungicide application has no additional application cost. My recommendation for soybean is this. If you know a field has or likely will have a serious fungal disease problem that can be treated at stage R2 or R3, a foliar fungicide treatment will be helpful.
Foliar fungicide in corn.
·In corn, across all experiments and fungicide treatments (574 observations), the overall yield response to fungicide was 4.04 bu/A.
·The yield response was higher in corn following corn (4.54 bu/A) than in corn following soybean (3.96 bu/A).
·Mean yield response was greater when disease severity at corn growth stage R5 was greater than 5% (9.46 bu/A) than when disease severity at stage R5 was less than 5% (4.83 bu/A).
·Applications made at tassel emergence, stage R1, and stage R2 showed slightly higher yield responses than applications made at later developmental stages.
Again, I am not going to present an economic analysis for fungicide application in corn. Fungicide cost is about $12.00 per acre. Application costs are, on average, a little higher for corn than for soybean. However, everyone’s cost basis is unique. For both soybean and corn, growers can do their own math to see how it pencils out. My recommendation for corn is similar to that for soybean. If you know a field has or likely will have a serious fungal disease problem that can be treated at stage R2 or R3, a foliar fungicide treatment may be helpful.
Soybean aphids.
It is time to start scouting for soybean aphids once again. I have seen small numbers of aphids in fields since about July 21. However, aphid numbers as of July 27 have not yet been anywhere at all close to economic threshold levels of 250 LIVE aphids per plant. I have presented aphid scouting methods previously (Pod Doctor – June 25, 2009) which can easily be viewed from the NCS web site (NorthCountrySeed.com). I highly recommend scouting for aphids to know whether a problem actually exists rather than prophylactically spraying all your soybean acres for aphid. If aphid levels are not high, unnecessary insecticide expense is being incurred. In addition, the insecticide will kill beneficial insects that may in fact be natural predators of aphids. This in turn could open the door for later aphid infestation in a particular field. Lastly, if insecticide is being applied with a ground rig rather than aerially, a certain amount of crop damage is likely, due to wheel traffic and mechanical damage.
Ideally, genetic resistance to soybean aphid will go far in reducing the economic and environmental cost of applying insecticide. Galena Genetics will be introducing a new variety with aphid tolerance. This 2.6 RM experimental variety is undergoing foundation seed increase in 2010, and should be available for planting in 2011. Additional aphid resistance breeding work and germplasm are currently in the Galena Genetics breeding and development pipeline, in order to develop additional aphid resistant varieties.
Disease and insect notes.
White mold (Sclerotinia sclerotiorum) apothecia (mushrooms) have been observed in parts of northern Iowa and southern Minnesota in soybean fields with very good growing conditions and fields with previous history of white mold. For the most part, the summer of 2010 has been a little warmer than white mold prefers for optimum proliferation. However, wet, humid conditions have been favorable for white mold development. The mushrooms produce spores which infect soybean flowers and eventually proliferate through the plant. Certain fungicides, such as Domark, Endura, and Topsin-M, can help control white mold. However, since all but very late planted soybean fields are at late flowering stages as of the end of July, it is probably too late this year for effective fungicide treatment of white mold. The good news, at least in Iowa and Minnesota, is that there is not a high incidence level for white mold so far in 2010.
Soybean cyst nematodes (SCN) are widespread throughout cultivated Midwest soils. Some of the yellowish areas in fields in late July are evidence of SCN feeding on the roots of soybean plants. However, yellowing could be caused by other factors as well. The only way to quickly determine in the field if a soybean field has SCN infestation is to dig roots out of the ground and gently remove soil from the roots. Look for the round, white or creamy-white SCN females on the roots. They will be about the size of a period at the end of this sentence. This is a fairly easy way to determine SCN presence in a field and to evaluate the degree of SCN resistance in a resistant soybean variety. If there are less than about 20 cysts present on the root of a resistant plant, the resistance is holding fairly well. For more accurate determination of SCN levels in the soil, a representative soil sample must be sent to a laboratory equipped to determine SCN egg counts and infestation levels.
Green clover worm seems to be making a rare appearance in soybean fields this year. There are some reports of green clover worm larvae in central Iowa. I have personally seen larvae in fields in southern Minnesota during the last week of July. The last major outbreak of green clover worm was back in the mid-1970s. This insect does not overwinter in Minnesota or Iowa. Rather it migrates from southern states each year. Damage to soybean is caused by the larvae which feed on the leaves, causing defoliation. Larvae are pale-green, with one or two white strips along the sides. Each larva has three pairs of thoracic prolegs just behind the head, and three pairs of abdominal prolegs further back toward the rear of the larva. I have not seen any infestations close to economic levels at this time. In severe cases, insecticide treatments can effectively control green clover worm larvae.
Soybean stem canker (Diaporthe phaseolorum) has recently been observed in a field in southern Minnesota. This disease is caused by a fungus which resides in soybean residue or on soybean seed. Spores can be transported to healthy plants early in the growing season by splashing water or rain. Symptoms of stem canker are the formation of small reddish-brown superficial lesions at the base of branches or leaf petioles. The lesions can enlarge and encircle the stem, causing the stem to be girdled. The infected plant can die. Severity of damage caused by stem canker is highly influenced by the timing in which spores infest plants. Plants infested at stage V3 have a much higher likelihood of serious damage than plants infected at stage V10. Hail prior to infection has also been shown to increase the rate of stem canker infection when spores are present. Fungicides such as Headline or Quadris may help reduce the spread of this disease if applied early. Spores can reside on soybean residue for several years. However, the disease can be managed by deep tillage to bury spore-infested residue and by planting seed that is free of spores, i.e. – does not come from a field where stem canker was present.
Galena Genetics show plots.
We have a number of show plots planted in southern Minnesota and northern Iowa in 2010. These fields are signed, and showcase our newest genetics. Please stop by and take a look at these plots in your area whenever you have a chance. And as usual, I or the marketing staff will gladly take your calls and emails to answer any questions you may have regarding agronomy, Galena Genetics products, or current prospects for the Minnesota Twins to make the playoffs.
