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April 19, 2002
Diseases Insects Equipment
Weed control
Alfalfa and Hay Management Updates
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Neighboring states are seeing similar increases -- most states in the North Central Region list it as the most common viral disease of soybean. Like Nebraska, other states reported lower disease occurrence last year as well as lower bean leaf beetle populations. Bean pod mottle virus is a viral disease that is vectored or moved very efficiently by the bean leaf beetle.
Factors to consider
The first point to consider is that yield reductions as high as 52% have been documented. Does this mean all fields will be reduced this much? Absolutely not.
The key to yield reduction is when the virus is introduced to the plant. Viruses infect plants systemically. Once a plant is infected, it can never get rid of the virus. Greater yield loss occurs when plants are infected early in development. Plants with BPMV also can have higher levels of Phomopsis seed infection and produce discolored seed, which can have a significant impact on seed quality. It should be noted that seed coat mottling is not an indicator of seed infection by bean pod mottle virus. Sometimes bean pod mottle virus is confused with a soybean mosaic virus (SMV) infection, which generally are not common in Nebraska.
Symptoms of bean pod mottle virus are green to yellow mottling (blotchiness) of younger leaves in the upper canopy. In severe cases leaves may show puckering and distortion and plants can be stunted. The only way to confirm the presence of this virus is with serological testing (Elisa). Bean pod mottle resistance has not been identified, however, varieties will respond differently to the virus.
Virus vector -- bean leaf beetle
The bean leaf beetle is a vector of the bean pod mottle virus. The beetles first appear in soybean fields in the spring during seedling stages. These are the beetles that have successfully overwintered, and appear to be the primary vectors of the virus. Growers also see bean leaf beetles during the later vegetative and reproductive stages of soybean production, but the role of these mid to late season beetles in vectoring the virus is not clear and probably not as important as the spring beetles.
The overwintering beetles emerge from leaf litter in early spring and move to other legumes, such as alfalfa, before soybean fields are typically planted. When soybean plants begin to emerge, the beetles move to the seedling soybean. The earliest planted fields are the first to be colonized and usually end up with the largest beetle populations. It appears that as the beetles move around and feed on seedling soybean, they transfer the virus from plant to plant. Much about the relationship between the beetle, virus, and soybean remains unknown.
Will controlling the spring, overwintered beetles help control bean pod mottle virus? Preliminary studies conducted in Iowa indicate that this may be an effective strategy, but many questions need to be answered, such as "When is the best time to treat the beetles -- at plant emergence, the unifoliolate stage, or in a herbicide/insecticide tank-mix?". We don't yet know the answers to these questions, but studies are underway in Nebraska and other states to address these issues. What about planting date? Planting date studies in 2001 showed that by planting soybean later in the year, less virus occurred. This is similar to what is observed for bean leaf beetle populations. This was based on mid-April and mid-May planting dates at sites near Mead and Fremont.
2002 NU research plans
for bean pod mottle virus We will continue monitoring the amount of bean pod mottle virus throughout the state. One point, which will be addressed in 2002, is how much yield loss current soybean varieties suffer from BPMV infection. We will plant the top three entries from several companies to assess this. Currently varieties are not rated for susceptibility to BPMV. Management studies focusing on early-season insecticide applications also are planned for this production season. Management studies are being funded in part from a grant by the North Central Soybean Research Program. Other aspects, including variety evaluation and virus monitoring, are being funded by the Nebraska Soybean Board. Hopefully, through these efforts we will also get a better idea of the yield loss associated with bean pod mottle virus in Nebraska and how to manage it.
Loren Giesler
Extension Plant Pathologist
Thomas Hunt
Extension Entomologist,
Haskell Ag Lab, NEREC
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Early spring irrigation tends to help warm the soil since irrigation water usually is about 55oF. This will help speed early alfalfa growth.
Your biggest advantage from reserve water will come after each summer cutting. Alfalfa roots need oxygen in the soil if plants are to regrow rapidly. Watering right after cutting suffocates roots, slowing regrowth.
Immediate watering also stimulates shallow rooted or sprouting weeds, especially at a time when alfalfa plants are not very competitive. Both problems are solved when water is available for deep alfalfa roots while the top several inches of soil remain dry.
So, improve your alfalfa irrigation by watering early, with a goal of six to eight feet of soil at field capacity at first cutting.
Thomas Hunt
They reported that wheat condition is behind last year and average, with 6% rated very poor;
19% rated poor; 43% rated fair and 32% rated good. Last year at this time wheat rated 54% good
and excellent with the five-year average at 62% food and excellent. The crop was jointed on 1%
of the acareage, comapred with 2% last year and 3% average.
Oat planting was ahead of verage at 60% complete and compares with 19% last year and 47% for
the five-year average.
Sugarbeet planting is well ahead of last year with 28% complete; 3% had been planted by this
time last year. Corn planting was just beginning.
Extension Entomologist, Haskell Ag Lab, NEREC
Wheat condition below normal;
Nebraska jumped into spring with a little bit of summer last week. Overall, temperatures
averaged from five to eleven degrees above normals aross the state, according to the Nebraska
Agricultural Statistics Service.
crop planting forges ahead across state
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Bean leaf beetles vary in color, but are usually reddish to yellowish-tan. They are about ¼ inch long and commonly have two black spots and a black border on the outside of each wing cover. These spots may be missing, but in all cases there is a small black triangle at the base of the wings near the thorax.
Because they move to soybean fields so soon after seedling emergence, early-planted fields will usually have more beetles and suffer the most injury. This has become more of a problem in recent years because planting dates seem to be getting earlier each year. Although the defoliation the beetles cause can appear quite severe, research in Nebraska and elsewhere has shown that it usually does not result in economic damage. Soybean plants can compensate for a large amount of early tissue loss, so it takes a considerable amount of beetle feeding to impact yield. Generally, unless insect populations are large enough to cause more than 50% to 60% defoliation of seedling soybeans, it is unlikely that treatment would be economically justified. Tables 1 and 2 show economic thresholds for bean leaf beetle on seedling soybean. Be aware that these thresholds are for defoliation of beans at VC - V1. If beetles enter the field right at or during seedling emergence, the thresholds will likely be lower because the beetles do not have leaf tissue to eat and will feed on the growing point, stem, and cotyledons. We do not have a good research base for bean leaf beetle injury to newly emerging soybean, but the thresholds are probably about 1.5 beetles lower than the VC thresholds.
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Another reason some producers treat bean leaf beetle on seedling soybeans is to reduce the pod-damaging F2 generation that emerges in August; however, UNL Extension does not recommend this practice. There are many environmental factors that can impact beetle populations throughout the growing season, making it impractical to use spring beetle numbers to accurately predict if beetle populations will reach economically damaging levels in August. Regular scouting and the use of the appropriate economic thresholds are the best way to manage late season bean leaf beetle in soybean. Economic thresholds will be included in CropWatch later this summer.
Beyond the beetle Of course, there are other insects that may be found in soybean fields. Last year we had numerous reports of several species of caterpillars infesting Nebraska soybeans, including wooly bear, thistle, and red admiral caterpillars. These are sporadic pests of soybeans, so we will report on these insects as necessary throughout the season.
Finally, a new soybean pest, the soybean aphid, has been receiving a lot of attention the last couple years. First found in Wisconsin in 2000, the aphid has since spread to 13 states. Although we have yet to find this insect in Nebraska, it was found just across our borders in South Dakota and Iowa last summer, so we expect to see it eventually.
The soybean aphid, Aphis glycines, originated in Asia. A small yellow aphid with black cornicles ("tail-pipes"), it can be found on soybean from seedling through reproductive stages. It typically feeds on new growth or the undersides of mature leaves. Heavy feeding may produce wilting, leaf distortion and yellowing. Yield losses exceeding 25% have been observed in Minnesota and Iowa.
Because of the potential impact this insect may have for Nebraska soybean farmers, the Nebraska Soybean Board has funded a "Soybean Aphid Survey and Early Alert Program". If you find colonies of yellowish aphids with black cornicles in Nebraska soybeans this summer, please contact Tom Hunt at (402) 584-2863 or email him at thunt2@unl.edu. More information concerning the aphid will be presented in subsequent issues of CropWatch.
Thomas Hunt
Extension Plant Pathologist, Haskell Ag Lab
Keith Jarvi, Extension Assistant
Integrated Pest Management
Both at the NEREC
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How to check for soybean cyst nematode
There are two ways to check for soybean cyst nematode. 1) examining the roots or 2) submitting a soil sample to a laboratory. On the roots, cysts will appear as cream to brown-colored bodies, which are pinhead in size. They should be present on the roots anytime after the Fourth of July. If you decide to take a soil sample for soybean cyst nematode, this can be done at any time of the year, but populations will be highest at harvest. Often people take soil samples after harvest when things slow down.
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In fields found positive for soybean cyst nematode, plan to monitor the population every few years. Remember that pathogen populations can and do change. By monitoring your situation you can hopefully say in pace with the change.
Loren J. Giesler
Extension Plant Pathologist
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Two nozzles, the Turbo FloodJet and the Air Induction nozzle from TeeJet, reduced drift compared to the standard XR TeeJet @ 40 psi. The Turbo Drop and Turbo TeeJet also reduced primary drift (the distance to last dead plant), but were not statistically better than the standard treatment. When secondary drift (the distance to the last injured plant) reduction is compared, all four drift-reducing nozzles outperform the standard flat fan.
The second year of the study included three new nozzles from Lechler Inc., providing nine treatments. The Turbo FloodJet, both AI nozzles and the Turbo Drop outperformed the standard flat fan in controlling primary drift. When comparing the performance of reducing secondary drift, only the Turbo FloodJet was statistically better than the control.
Drift-reducing additives can be best evaluated by comparing the nozzle itself against the nozzle plus an additive. Data from 2000 and 2001 was combined to evaluate all the additives. None of the additives significantly reduced primary drift. Although Border reduced drift compared to Placement, none of the treatments were statistically different than the AI nozzle alone. Border applied with the AI TeeJet nozzle did reduce the amount of drift compared to the AI nozzle used alone.
When comparing the efficacy of the original six nozzles, data was pooled over both years. This research was completed in planted crops, all of which were approximately the same size. It is possible that the larger spray particle size may not be as critical as if smaller plants were present, such as in a field with different weed sizes. The standard treatment in this experiment was the XR TeeJet nozzle @ 40 psi. In general Roundup Ultra had higher injury ratings than Liberty. Since there was no herbicide/ nozzle interaction, data was pooled over herbicides.
The Turbo FloodJet, Turbo TeeJet and Turbo Drop nozzles provided significantly greater efficacy than the two XR flat fan treatments. Another drift-reducing nozzle, the AI TeeJet had significantly greater efficacy than the XR TeeJet @ 20 psi but was similar to the other nozzles. The top four nozzles are designed to produce larger droplets. These experiments were conducted on days with little wind.
Nine treatments were compared in 2001. Again, herbicide/ nozzle interaction was not significant, so data was pooled over herbicides. The data are similar to previous results. All the additives tested were sprayed with a conventional nozzle (XR TeeJet) and a venturi style nozzle (Air Induction (AI) TeeJet). When the data is averaged over the three additives, the nozzles were not statistically different. The venturi nozzle performed as well as the standard flat fan.
When the data is averaged over nozzle type, Array performed statistically poorer than Border and Placement. Array had an 80.1% injury rating while Placement had a 83.9% and Border had an 84.2% rating. While the additive's main purpose is to reduce drift, it should not reduce weed control efficacy compared to the nozzle treatment alone. Figure 1 indicates that none of the additives had a negative impact on weed control compared to treating the crop with the XR nozzle and no additive.
Adam K. Johnson
Preemergence herbicides need to be incorporated into the soil in order for them to have the
desired activity of preventing weeds from surviving. Many times herbicides are applied to soil
with residue on the surface, blocking the spray from soil contact. This herbicide will be
ineffective until it is moved to the soil via either rainfall or mechanical incorporation. Most
preemergence herbicides require 0.5 inch of rainfall or greater for adequate incorporation. With
mechanical incorporation, soil moisture must still be adequate for herbicide uptake and activity.
How long the herbicide can remain on the soil surface before incorporation and still remain
adequately active depends on a number of issues, primarily the type of herbicide and the weather
conditions. For example, Prowl is moderately volatile and should be incorporated within seven
days before activity is lost. Hot temperatures and high winds may reduce this time. Many corn
preemergence herbicides such as Dual, Bicep, Harness, Surpass, Balance, Frontier, and Atrazine
are much less volatile and can remain on the soil surface longer without loss of activity. Under
cool weather conditions, these herbicides can remain on the soil surface for two or more weeks
before incorporation without significant loss. If rainfall has not occurred by the third week, other
incorporation measures may be needed. Again, temperatures in the 80s with high winds will
reduce this time.
If dry weather follows the application of a surface applied herbicide, weeds may emerge even
though the herbicide is on the surface. If the weeds get more than 1 inch tall before rainfall
occurs, most surface applied herbicide will not control them.
For the most part, producers do not need to worry about incorporation of preemergence
herbicides immediately after application. Producers may want to keep this information in mind
when buying herbicides, as some are much better than others with respect to vapor loss or
degradation. Choose a herbicide that can remain unincorporated for two or more weeks and hope
for rain.
Brady Kappler
Former NU Graduate Student in Weed Science
Robert N. Klein
Extension Cropping Systems Specialist
Without rain, what happens to the preemergent herbicides you applied?
While some areas of the state have been fortunate enough to receive rainfall in the past week,
other areas have not received any significant rainfall for at least 10 days. Producers in these areas
are now asking questions about preemergence herbicides. At the forefront of these questions is
how much rain is needed to incorporate preemergence herbicides and how long can they remain
on the soil surface without rainfall.
Extension Educator-Weed Science
Alex Martin
Extension Weed Specialist
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Under non-irrigated no-till conditions, yields were increased in two cases with early planting but decreased in three cases. The average yield was 38.6 bu/A for early planting and 42.4 for the normal planting time. The two cases with a yield advantage with early planting occurred in 2000.
Two trials were conducted under irrigated, ridge-till conditions two were under pivot irrigation. Mean soybean yield was six bushels per acre more with early planting.
Water deficits were severe in July to early August 2001. Adequate soil moisture during this period is probably more important for the early planted soybeans while normal planted soybeans may benefit more from rain in mid to late August.
Jerry Mulliken, a crop consultant and Nickerson farmer participating in the NSFGP project, used the DSSAT crop growth model for soybean, SOYGRO, to simulate the effects of different planting dates on yield of a Group 3 variety. He used historical weather data for 1983 to 1998 for Mead. The outcome of these simulations sheds light on the inconsistent results with early planting (see Table 2).
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Risk of frost damage is a concern but minimized by planting seed two inches deep into cool soil. Risk is further reduced with no-till as the crop residue cover prevents soil temperature from responding quickly to increases in air temperature. Emergence is delayed until air and soil temperatures are warmer.
Seed-applied fungicide should be used to protect the seed from soil borne diseases due to the long wait in the soil and slow early growth.
An important consideration is pest management. The bean leaf beetle, a foliar feeder, has been observed to most affect early planted beans. The bean leaf beetle transmits the bean pod mottle virus which also was observed to be worse with early planting. Careful scouting and properly timed insecticide application may be needed to control the beetle on early planted beans.
Early planting of soybeans should be considered when planting normally continues well into June, especially if the land is irrigated. The chances of good results are less under rainfed conditions if the soil does not have good water holding capacity.
Charles Wortmann
Extension Nutrient Management Specialist
When pastures start out relatively dry like they may this spring, this loss in carrying capacity often becomes quite serious. Not only do you lose potential growth for the grass this year, you also lose the carryover feed value from last year's residue. It may not be very high quality feed, but when pastures get short during a drought, almost anything is better than nothing. Green-up and rate of growth right after the fire also is much slower when soils are dry at time of burning. This increases the chance of wind and water erosion, and delays when these pastures will be ready for grazing.
Burning still might be appropriate on CRP lands, though. The loss of production isn't nearly as important as improving stands. Even so, when it's dry you must be extremely careful. Fire is valuable tool. But like any other tool, in the wrong hands or at the wrong time it can be costly or dangerous.
Bruce Anderson
Extension Forage Specialist
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