Notes from the Pod Doctor - July 27, 2009

Weather and Crop Development

    Last month I told you that the effect of the cool weather on crop development during the first two weeks of June would likely be offset by weather conditions throughout the remainder of the growing season.  Gambler’s odds notwithstanding, I may have spoken prematurely.  Little did any of us know that cool summer weather would continue in July in the northern half of the cornbelt.  Growing degree day (GDD) accumulation (base 50) since May 1 is about 160 – 260 heat units (or about 8 – 10 days) behind normal for much of northern Iowa and southern Minnesota, as of July 27.  Both corn and soybean development would appear to be about that far behind normal also at this date.  Some areas in southern Iowa and east of the Mississippi River are even further behind because late planting has made the growing season even shorter.

    Will GDD accumulation catch up to normal by late September?  Only Mother Nature knows the answer to that question.  However we do know that soybean, and to some degree corn, both have the ability to sense the shortening days in the fall, and in response, begin to hasten their advance toward maturity.  This phenomenon is called photoperiod (or daylength) sensitivity.  That is why even when a soybean variety is planted very late, eg. – mid-June, the crop will mature only 7 – 10 days later than the same variety planted in early May.  The late-planted soybean may not yield as much as the earlier planting, but at least it will mature and be harvestable.

Soybean Grain Filling

    I promised last month to talk more about growth stage R5 in soybean.  Stage R5 is defined as the stage at which developing seeds within the pods at the fourth leaf node from the top of the plant are 1/8 inch long.  Stage R5 represents the onset of rapid seed dry matter accumulation.  It is the most energy-expensive and nutrient expensive stage in the life of the soybean plant.  There is huge competition for energy and other resources between the developing seeds, the nitrogen-fixing root nodules, and the photosynthetic machinery in the leaves.  These three sinks are competing for energy from carbohydrates produced by photosynthesis, and nutrients and water taken up by the roots.  Every tillage, fertility, weed control, and pest control practice done before or after planting should be done primarily in order to make the soybean crop as healthy and vigorous as possible going into stage R5, so that the plant can provide as much energy, nitrogen, and other nutrients as possible to these competing uses within the plant.

Soybean Nodulation

    Soybean plants obtain the majority of their nitrogen through the symbiotic relationship with a nitrogen-fixing bacterium, Bradyrhyzobia japonicum, found in most Midwest soils with a history of soybean production.  This bacterium will invade soybean root hairs, and form small brown or gray nodules which can grow as large as 1/16 to ¼ inch in diameter, within which the bacteria reside.  This symbiotic relationship is mutually beneficial for both the soybean and bacteria.  The bacteria take nitrogen from the air in the form of N2 and convert it into a ureide form, and eventually into an ammonium form, NH4+, or nitrate form, NO3-, for later use by the soybean.  In return the soybean provides carbohydrate energy to the bacteria.  Nitrogen is an essential key component of all proteins, which of course are a major component of soybean, comprising about 36 – 40% of the soybean seed.  Nodulation begins within a few days following soybean emergence.  Small amounts of N2 fixation have been documented as early as growth stage V2 – V3, and will continue throughout the life of the soybean plant, in proportion to the amount of energy the soybean plant can provide the bacteria.

    Healthy nodules, when split open, will have a reddish tint on the inside.  This is a good indication that the nodules are functioning properly.  Stress to the soybean plant, due to drought, flooding, weed competition, aphids, or any other factor that competes for, or reduces the amount of available water, nutrients, and energy, can reduce optimum function of the nodules.

    Fixation of N2 requires substantial plant energy.  An agronomist once asked me whether it was possible to genetically modify corn to allow it to symbiotically work with a nitrogen-fixing bacteria in the same way as soybean, so that expensive nitrogen fertilizer would not be required for corn production.  My answer was, “Yes, with much work, it would likely be possible.  However, the energy cost to the corn plant would likely result in corn yields of 50 – 60 bushels per acre, instead of the 180 – 200 bushel per acre yields he was accustomed to.”  Of course, my answer was not acceptable to him, and the conversation switch to something else!  The point here is that N2 fixation, though essential to normal growth of the soybean plant, is an expensive process.  The obvious benefit of N2 fixation is that the nitrogen conversion takes place using photosynthetic energy, as compared to using industrial energy to make nitrogen fertilizer.

Soybean Cyst Nematode

    July is when visible symptoms of Soybean Cyst Nematode (SCN) begin showing up in some soybean fields.  SCN is the most destructive pest of soybean worldwide.  However, above-ground symptoms can be, and often are, confused with some other types of soybean damage such as iron deficiency chlorosis, soil compaction, drought stress, flooding, or herbicide injury.  As a result, SCN infestation can often go undetected for several years.  In fact soybean yield losses can occur from SCN infestation even when no visible above-ground symptoms are obvious.  Damage from SCN occurs when the hatching SCN juvenile penetrates into young soybean roots, robbing the plant of food, interfering with the flow of water and nutrients through the vascular system, and reducing the ability of the root to take up water and nutrients.

Most fields in the Midwest have at least a modest SCN infestation.  If you are unsure whether SCN is present in your fields, I recommend having your fields tested.  Many private soil testing labs in the Midwest, as well as public universities, offer SCN soil analysis.  Accurate results of SCN analysis are highly dependant upon thorough soil sampling.  Soil sampling protocols are described in several publications such as “Soybean Cyst Nematode Management – Field Guide” published by Iowa State University Extension, 2008.  As a general rule of thumb, submitted samples should be the thorough mixture of the composite of one subsample for each acre of ground in the field.  The composite sample should not represent more than about 20 acres of ground.

    A simple, qualitative way to determine if SCN is present in your soybean plants is to dig up the young, healthy roots of several plants, and simply look for presence of adult SCN females.  DO NOT PULL plants out of the ground by hand, as this will strip the SCN females from the roots.  Dig up the roots and carefully remove soil from the roots by shaking or crumbling.  The adult creamy white SCN females are about 1/32 inch in diameter, or about twice the size of the period at the end of this sentence.  SCN eggs will develop inside the female.  The enlarged female eventually dies and hardens into a tan, orange, or brown cyst containing 100 - 200 eggs.  Eggs, thus protected, can survive in the soil for up to 10 years.  The adult females and cysts are easily distinguished from nitrogen-fixing nodules in the root by their much smaller size and lighter color.  In ideal temperatures, the complete life cycle of SCN can occur in about 4 – 6 weeks.  Thus several life cycles can and do occur in one soybean growing season.

    If present, SCN cannot be totally eliminated from a field, if soybeans are ever grown in the field.  Rather, SCN management must be used to keep existing SCN populations at sufficiently low levels in order to avoid damage.  It is easier to maintain low SCN population levels than it is to reduce high SCN population levels.

    The most common management tools used to suppress SCN levels are crop rotation with a non-host crop like corn or alfalfa, and growing SCN-resistant varieties.  Use of these tools in combination has been shown to have an even bigger impact on SCN suppression.  There are several conventional (non-GMO) genetic sources of resistance to SCN used commercially today.  Galena Genetics currently uses several of these sources in our breeding program in order to diversify the resistance mechanisms at work in our germplasm.  This will reduce the possibility of SCN overcoming any single source of genetic resistance.

Insect and Disease Update

    Latest reports indicate soybean aphid levels vary considerably from county to county in Minnesota and Iowa as of July 27.  At this writing, I am aware of a few reports, but not many yet, where aphid populations may be reaching economic threshold levels of 250 aphids per plant.  My recommendation to all growers at this date is to continue scouting all your soybean fields at least once a week for aphids.  Continue scouting through growth stage R5.  If aphid populations reach levels of 250 live aphids per plant and are increasing before R5, then treat with an approved insecticide.  If aphid populations have NOT reached 250 live aphids per plant by stage R5, there is no need to continue aphid scouting, nor is there a need to treat your fields, since the potential losses are lower after stage R5, and will not justify treatment costs at that time.

    With the cool, wet conditions prevalent this summer in many growing areas, especially in Iowa and Illinois, some soybean foliar diseases are being reported.  There are a few reports in Iowa of the foliar fungal diseases frogeye leaf spot, brown spot, and Cercospora leaf spot.  These fungal diseases can be treated with a fungicide, if severe.  However the spray treatment should be done at growth stage R3 in order to be most effective.  Many soybean fields are nearing or are at growth stage R3 as of July 27.  Therefore if a fungicide treatment is to be used, it should be applied very soon.  The cost effectiveness of a fungicide treatment is difficult to calculate without additional field information and history.  The best advice provided by XB Yang, professor of pathology at Iowa State University, is that if the soybean field has a history of responding favorably to a fungicide treatment, it probably will respond favorably again.  In addition, if the soybean field has had high precipitation and is not well-drained, it probably will respond favorably to a fungicide treatment.  Higher, well-drained ground may be less likely to respond.

    In visiting with a grower near Greenville, IA today, I learned that he is going to spray for aphids and fungal diseases at the same time, since he is having a problem with both of them at the moment.  Thus, he is able to save on application costs and potentially “kill two birds with one stone”.

    Those of you concerned about soybean white mold infestations, be aware that the damage from white mold has already gone into effect, since white mold inoculation of the soybean takes place through the flowers.  Fungicide treatments will have little or no effect on white mold after most flowering has already occurred.

    Please note that bacterial diseases, such as bacterial blight, are NOT treatable with a fungicide.

    The cool, humid weather has also promoted the development of several foliar diseases in corn.  There are many reports of eyespot in southern Minnesota and northern Iowa in the last week of July.  Most of these reported eyespot cases are probably not at economic threshold levels, but should be watched on a field-by-field basis. 

There are also a number of reports in Iowa of gray leaf spot in corn, especially where high humidity levels and/or fog have occurred frequently this summer.  In southwest Iowa, some corn fields have reached economic threshold levels for treatment of gray leaf spot.

Grant Metz, PhD
Research Director
Galena Genetics, LLC
(grant_metz@rabbeusa.com)