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8/10/2009 - 8/16/2009

More On Soybean White Mold This Year

By XB Yang and S.S. Navi, Department of Plant Pathology

Soybean white mold has been a production problem for soybean producers since early 1990s. White mold outbreaks often occur in even years due to crop rotations, with rare severe occurrences in odd years. Iowa’s cool, wet summer has increased the white mold risk for some growers in eastern Iowa – even though it is an odd year.  Extension field agronomist Virgil Schmitt first reported the occurrence of white mold in east central Iowa and Jim Fawcett, northeast Iowa field agronomist, has received a report of severe white mold in a 60-acre field.

Although many Iowa soybean producers are experienced with this disease, there are questions when it shows up in mid-August. Some are asking if applying chemicals is a solution. Others want to know if an immediate application of a fungicide can stop the infestation of this disease.

The short answer is that it is too late to use any fungicides for white mold control.  It wastes resources if fungicides are applied now.  White mold fungus attacks soybean plants during flowering stage and treatments to protect soybean have to be made before or during the flowering period, depending on chemicals used. The dead plants visible now are the results of fungus infestation that happened before mid-July.

Management
If you have severe white mold in your soybean fields, consider no-tillage if corn is a rotation crop. Another consideration is to use Contan after soybean harvest. Contan is a biological control agent proven to be effective in white mold control in many crops.  A tolerant variety of soybean should be planted in this location during the next rotation or a consideration given to using Cobra, a herbicide that can reduce white mold infection if applied correctly. There are other chemicals in the same family that are effective in reducing the risk of this disease.

 

white mold wilting

Soybean field showing wilt symptoms due to white mold.

 

white mold symtoms

Mycelia (a) and sclerotia (b) formation on white mold infested soybean plant.

 

XB Yang is a professor of plant pathology with research and extension responsibilities in soybean diseases. Yang can be reached at (515) 294-8826 or by emailing xbyang@iastate.edu. SS Navi is an assistant scientist working on soybean diseases.

 

Aphids Are on the Move, Remember to Scout in August!

Erin Hodgson and Matt O'Neal, Department of Entomology

As soybean fields enter seed set throughout the state, we continue to get questions about how long to scout for soybean aphid. This year, aphid populations have been extremely variable with a limited number of fields exceeding the economic threshold (250 aphids per plant with increasing populations on 80 percent of the plants) in late July. In August, however, there have been more reports of increased aphid activity throughout the state (i.e., some fields were clean until last week). Many of the extension field agronomists also indicate winged aphids are showing up in fields. This correlates well with current aphid captures in the Regional Aphid Trapping Network (Figure 1).


 
aphid trapping

Figure 1. Summary of winged aphids collected at the four Iowa suction traps in 2009. In addition to soybean aphid, there have been several other common species identified.

 

How do the winged soybean aphid numbers compare to other years? So far the actual number of soybean aphid trapped in Iowa is quite low and is similar to 2006 (Figure 2). However, other states are reporting high soybean aphid captures. Does this mean aphids recently found in Iowa fields came from other places? Winged aphids are capable of long distance migration via jet streams, but we aren't going to blame other states. 
 

summary of winged aphids

Figure 2. Summary of winged soybean aphid trapped from all four Iowa locations.

 

General guidelines recommend scouting for soybean aphid every 7-10 days through seed set (R5-R6), regardless of insecticide treatments applied during the year. Do not assume a seed treatment or preventative application will control aphid populations. In rare cases, fields treated at the economic threshold may need to be treated twice if aphids flare again during seed set.

We encourage growers and consultants to continue scouting because of all the winged aphids moving right now. Late-season infestations are possible and aphids can cause economic loss during seed development. As plants mature, they become less attractive to aphids. However, soybean aphids strongly prefer to feed on soybean and aphids will continue to feed as plants enter R7 (beginning maturity). In 2008, some fields had aphid populations increasing into September. What are the treatment recommendations for late-season infestations? Refer to ICM News article on this topic.

 

Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at ewh@iastate.edu or phone (515) 294-2847. Matt O’Neal is an assistant professor of entomology with research and teaching responsibilities. He can be contacted by email at oneal@iastate.edu or phone (515) 294-8622.

Are Late-Season Soybean Aphid Treatments Worth It?

Erin Hodgson and Matt O'Neal, Department of Entomology

As soybean fields enter seed set, growers are getting concerned about late-season soybean aphid infestations. And with good reason - winged aphids are everywhere and some areas are experiencing increasing aphid densities. There is much evidence to show treating aphids when they exceed the economic threshold (250 aphids per plant with increasing populations on 80 percent of the plants) up to R5.5 will protect yield. But what happens if aphid populations are still increasing past seed set? This was the trend in 2008 when aphid populations were still increasing into September. So do late season insecticides financially make sense for managing soybean aphid? This is a difficult topic for us to discuss because of the lack of replicated data throughout the North Central Region.

Research conducted at ISU research farms is trying to answer that question. In 2008, replicated plots at the Northeast Research Farm near Nashua were naturally infested in August (Figure 1), reaching 250 aphids per plant when the plant was at the R6 stage. This late season infestation was used to determine if such a population should be treated. To see all the results from this experiment, which was part of our soybean aphid efficacy trials from 2005-2008, go to www.soybeanaphid.info. A condensed summary of the efficacy trial is summarized here with only four treatments shown:

 1) Untreated control that never received an insecticide
 2) Seed Treatment (mefenoxam +fludioxonil) only
 3) λ-cyhalothrin applied at threshold (plants were at R6)
 4) Aphid-free (plots treated with λ-cyhalothrin + chlorpyrifos on July 22, Aug. 1, and Aug. 22)
 

small plot researchFigure 1. Small plot research near Nashua, Iowa in 2008. The total number of aphids for the entire year is estimated in "cumulative aphid days" to reflect seasonal pressure. Statistical differences are represented by unique letters for cumulative aphid days (upper case) and yield (lower case).

 

Summary Research Points
• There was a significant difference in the 'aphid-free' cumulative aphid day treatment compared to the other treatments.
• There was no significant difference in yield between any of the treatments, although there was slightly lower yields in the 'untreated control.'

These data show a R6 treatment may not be worth it. Plants at R6 and beyond may be able to tolerate more aphids without experiencing yield loss. Unfortunately, we do not have a robust data set to help us make recommendations at R6 like we do for aphid outbreaks that occur during R1-R5. So going into August, consider the following factors before treating for soybean aphid. Careful monitoring of fields throughout the summer should give you an indication of the aphid trajectory - are numbers going up and how fast? If aphids are exceeding the threshold, then you must consider the overall treatment costs for an application in late August. Depending on the delivery method (ground or aerial) and product choice, control costs can exceed $15/acre. Keep in mind treating with ground equipment after the canopy closes can reduce yield by 1-2 bushels/acre.

If you decide to treat for aphids, remember to use sufficient volume and pressure to make contact with aphids on the undersides of leaves and in the lower canopy. Whenever possible, leave a check strip so that you can make evaluations of the value and performance of your insecticide application at harvest.

 

Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at ewh@iastate.edu or phone (515) 294-2847. Matt O’Neal is an assistant professor of entomology with research and teaching responsibilities. He can be contacted by email at oneal@iastate.edu or phone (515) 294-8622.

Managing White Mold at This Stage of Development

Virgil Schmitt, Extension Field Agronomist and Daren Mueller, Department of Plant Pathology

White mold has become evident in soybeans during the last two weeks, especially in eastern Iowa. Although infection occurred shortly after the beginning of flowering in late June and early July, the characteristic white myecial growth on infected plants has only become apparent the past two weeks. Really the only good news about this disease is that it does not have too much of a secondary disease cycle. In other words the disease itself is no longer spreading or is spreading one plant at a time.

The availability of fungicides for soybeans has raised many questions about their efficacy against white mold, particularly at this stage in the development of the disease. Despite some of the fungicides being classified as "curative", there most likely will be little positive effect from any fungicide applied at this time because of the stage of the disease. Remember that "curative" fungicides do NOT cure the plant of disease.

The most important thing for growers to do at this time is to note the presence of white mold in the field and then select for varieties with lower susceptibility or higher tolerance for white mold the next time soybeans are grown in the field. Wider rows may help with white mold, but wide rows have other drawbacks. If the conditions are good for white mold infection (cold and wet) at the beginning of flowering, the application of an appropriate fungicide at that time may help. An application of Cobra at or just before the first bloom has also been shown to lessen the impact of white mold.

 

Virgil Schmitt is an extension field agronomist serving eastern Iowa. Schmitt can be reached at (563) 263-5701 or by email at vschmitt@iastate.edu. Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative. Mueller can be reached at (515) 460-8000 or by email at dsmuelle@iastate.edu.

Crop and Weather Update - August 10

This week Doug Cooper interviews Iowa State University Extension plant pathologist Daren Mueller, integrated pest management specialist Rich Pope, and soybean agronomist Palle Pedersen.

Mueller says wind- and hail-damaged crops may develop disease problems this year, however much of the damage may be too severe for fungicide application.

Pope relates Extension field agronomists reports of crop damage done across the center of the state in Sunday hail and wind storms.

Pedersen encourages growers to continue monitoring insects and managing when they reach threshold numbers.

Making Fungicide Decisions on Hail Damaged Crops

By Daren Mueller, Department Plant Pathology

During the past weekend hail storms once again wreaked havoc on corn and soybean fields across Iowa. Much like hail damage last month and last week, growers have an option to spray fungicides to protect remaining leaf tissue. We wrote an article last month walking through a fungicide decision on hail-damaged crops at that point in the growing season. Much of the information in that article remains relevant.

Points to once again consider:

1- There has only been one replicated study completed with the proper checks looking at hail damage on corn. The main conclusions from that two-year study completed in Illinois was that foliar fungicides did not significantly improve yield in either the damaged or non-damaged plots compared to the non-treated controls. Results from this research indicated that foliar fungicides provided very little benefit to corn injured by simulated hail.

2- This late in the season, there is not much time left for additional foliar disease to develop and contribute to yield loss, even with tattered leaves. Many of the diseases that are managed with fungicides do not require the wounds created by hail.

3- Depending on the percent of damage, yield potential is less in hail-damaged crops. Remember that fungicides protect yield, not create more yield. So, if yield potential is lowered by hail, this reduces the chances of recouping your costs of applying a fungicide.

eyespot and hail 2

Hail damage to corn with eyespot, five days after hail storm. Lang, 2009.

 

Additional information related to hail damage to crops and making related decisions is available on the Hail Damage Recovery Web page.

 

 

Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative. Mueller at (515) 460-8000 or by emailing dsmuelle@iastate.edu.

Degree Days - Average IS a Good Thing

By Rich Pope, Department of Plant Pathology

A nearly average week of heat and some rain to boot! August could not have started better for Iowans, except for the band of hail and wind damage along the U.S. 20 / Iowa 175 corridor.  For the week, we only lost around 10 to 15 base-50F degree days to average across Iowa. Like they say, just what the doctor ordered (except for the hail!).

Statewide, corn development ranges from brown silk (R2) to dough stage (R3), and soybeans are busily filling pods (R4--R5).

Accumulated degree days from May 1 through August 9, 2009

Soybean aphid numbers are increasing with some fields reaching the economic threshold of 250 per plant and increasing; most notably in the northeast, but in other scattered areas as well.

Corn leaf aphids have been bothersome in northwest and north central Iowa as well. There are not good thresholds available for treatment  for aphids on corn, but extension entomologist Erin Hodgson posted a recent ICM News article that highlights the corn aphid. Corn leaf aphid numbers typically decline after tasselling, so the problem may soon start to ebb.

Also of interest is the Web page assembled by ISU Extension addressing late-season hail issues.

 

Rich Pope is a program specialist with responsibilities with Integrated Pest Management. Pope can be contacted at ropope@iastate.edu or by calling (515) 294-5899.

Corn Hail Loss Chart and Things to Consider Following Hail

By Roger Elmore, Department of Agronomy

Hail storms again devastated portions of Iowa’s corn crop on Aug. 9. Storms cut a 1 to 8 mile swath across north central Iowa. Producers in its path are asking, “How does hail after tasseling affect yield?” 

Most corn in the affected area was in the blister stage (R2).  According to hail industry tables, complete leaf loss at R2 results in 73 percent yield loss (see table).  Amount of leaf loss as well as development stage greatly impacts losses.  For example, if half of the leaves are lost, yields are reduced by 22 percent.  Damage at tasseling, VT, affects yield more severely while losses both earlier and later in development are less severe. 

Table.  Estimated percent yield reduction caused by hail damage.
 yield reduction due to hail

Consider a few other things relative to hail damage:
• Stand losses also occur with hail storms.  Yield losses are directly related the number of plants broken below the ear at these later development stages. For example a 10 percent stand loss – plants broken below the ear will result in a 10 percent yield loss in addition to any losses from defoliation.
• Bruises on stalks and ear husks may allow pathogen entrance, thus further reducing yields and increasing issues with stalk and grain quality.
• No research data supports fungicide applications as a method to improve crop recovery.
• If a portion of the crop is worth harvesting, adjust combines well.  Volunteer corn will likely be a problem in 2010 whether the crop is harvested or not.

These losses are devastating.  Prior to the storm, the crop looked better than ever.  Recovery of the crop, and growers, will be difficult.

For more information see the article on assessing hail damage from the National Corn Handbook.  In addition, Iowa State University Extension developed and posted resources for producers affected by hail on a special Web page.

 

 

Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. Elmore can be contacted by email at relmore@iastate.edu or (515) 294-6655.

A Summer 2009 Update on Diseases in Iowa Soybean

By XB Yang and S.S. Navi, Department of Plant Pathology

The association of plant disease with extreme weather is well documented, so it is no surprise that this year’s extreme weather with record cool temperatures and plenty of rain has created favorable conditions for disease development in soybean. Disease that we are seeing includes sudden death syndrome (SDS) and several soybean foliar diseases, the most prevalent are Cercospora leaf blight, soybean brown spot, bacterial blight, and downy mildew. As August temperatures begin to heat up diseases such as Cercospora and frogeye leaf spot can increase. This will create a mixed bag of cool and warm diseases that can cause premature defoliation.  Because of the potential disease mix, assessing the risk to crops is difficult.

Cercospora leaf blight was wide spread in July, but was low in intensity because low temperatures aren’t favorable to the development of this disease. With increased August temperatures, reports show that this disease has picked up steam and built up quickly. ISU field agronomists Mark Carlton and Virgil Schmidt in southern Iowa are seeing fields with high levels of infestation of Cercospora leaf blight. Similar high level of the disease was reported by Brian Lang in northeast Iowa. 

Soybean leaves infected by Cercospora have a leathery appearance and turn a mottled purple-to-orange color on the upper surface. Expression of these symptoms varies with varieties. Sometimes, this disease can be confused as “sunburn,” with symptoms appearing on the lower leaf surface. If a summer is cool, the chance of sunburn is small. This July has been one of the coolest summers in Iowa, which is not favorable for sunburn development. This also happened three years ago in a cool summer. Identifying this disease requires experience in taking free-hand sections for observation of the fungus under a microscope.  All samples from Iowa that we examined had Cercospora kikuchii, the causal fungus of this disease. 

cercospora leaf blight

 

Brown spot, caused by the fungus Septoria glycines, is another foliar disease commonly seen in July. Symptoms include many irregular, dark brown spots on both upper and lower leaf surfaces. Adjacent lesions frequently merge to form irregularly shaped blotches. Like bacterial blight, this disease occurs every year. Disease symptoms occur on the lower leaves of soybean plants and progress upward as the plant develops.  Brown spot usually does not cause early defoliation unless the disease progresses due to frequent rains later in the season.

Downy mildew has been wide spread this summer because of cool weather. The last time the disease was wide spread was in 2004, also a cool, wet season.  High August temperatures should stop the development of this disease until late in the month. Fields with downy mildew may see more disease at that time if there are dews and mists. Except for affecting seed quality, little data is available on the reduction of soybean yield by this disease in Iowa.

Use of fungicide can prevent early defoliation when foliar disease risk is high. One question I receive most often from producers is whether to spray or not. This is a site specific question. For fungicide applications, please see our other articles on fungicide applications.

downy mildew

 

Sudden death syndrome is showing up in east central Iowa this year.  In the areas we have visited, the severity was the highest we have experienced.  Although the disease has also been observed in other regions of Iowa, the level and extent of this disease is yet to be determined; that will be clearer later in August.  In 2006 the disease showed up very early, with many reports before the first week of July.  Like 2006, cool temperatures this summer may have contributed to its occurrence. Plants with SDS have leaves exhibiting chlorotic spots and necrosis between green veins. The roots of these plants have deteriorated. The simplest method of identifying SDS is to look for bluish fungal colonies on the taproot in severely affected plants.

Once patches of SDS are found, it would be wise to avoid planting susceptible varieties the following season. Resistant or tolerant varieties should be planted in fields that have history of SDS. Good varieties have been developed and are available from many seed companies in Iowa.

sudden death syndrome

sudden death syndrome

 

White mold infection was also encouraged by the cool, wet July conditions and the disease risk can be high when fields met these two conditions:
1) had the disease before and did not use Contans
2) was planted early and had dense closed canopy in early July

Fields that has been applied with Contans should have low risk and growers have observed less disease after use Contans in soybean fields previously had severe white mold.

white mold  

 

 

XB Yang is a professor of plant pathology with research and extension responsibilities in soybean diseases. Yang can be reached at (515) 294-8826 or by emailing xbyang@iastate.edu. SS Navi is an assistant scientist working on soybean diseases.



This article was published originally on 8/17/2009 The information contained within the article may or may not be up to date depending on when you are accessing the information.


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