Skip Navigation

8/2/2010 - 8/8/2010

Add Another Soybean Defoliator to the Mix

By Erin Hodgson, Mike McCarville and Adam Varenhorst, Department of Entomology

Imported longhorned weevils have been regularly collected in sweep net samples from soybean this year. A few reports of weevil damage in soybean are reported in Iowa annually since 1994. Damage is most likely to occur on border rows adjacent to grassy areas.

Imported longhorned weevil adults are about 1/4 inch long with a grey, pear-shaped body (Fig. 1). The forewings are covered with irregular tan markings and white scales. The antennae are long, prominent and elbowed. Adult weevils emerge from the soil in late June and become abundant during July and August. The adults are defoliators that create jagged leaf edges. Larvae are C-shaped and legless; they are root feeders with a wide host range. 
 
longhorn weevil

Fig. 1. These small weevils can be easily collected with a sweep net. Photo by A. Varenhorst.

Imported longhorned weevils have one generation per year. Interestingly, this species only has females and can reproduce without males, similar to aphids in the summer. Furthermore, adults have wings but are incapable of flight, which limit their dispersal. They are often restricted to border rows because the adults are not very mobile (Fig. 2).
 
weevil dam

Fig. 2. Widespread defoliation from imported longhorned weevil damage is unusual, but sometimes damage can be detected along border rows. Photo by M. E. Rice.

Because they are considered a minor soybean pest that colonizes in field margins, an economic threshold has not be developed. Marlin Rice conducted insecticide efficacy evaluations for imported longhorn weevil in 1994. He found it was difficult to kill the adults with the low rate of insecticides and recommended border treatments with full-rate products if defoliation becomes significant. We were not able to find insecticides with imported longhorned weevil on the label.

 

 

Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at ewh@iastate.edu or phone 515-294-2847. Mike McCarville and Adam Varenhorst are entomology graduate students working with Matt O'Neal, assistant professor of entomology.

Crop Minute Week of August 2

Daren Mueller, Iowa State University Extension plant pathologist, takes a look at the different crop diseases showing up or not showing up in Iowa this growing season in this week’s crop minute. Mueller mentions the lack of white mold, presence of stem canker in neighboring states and sudden death syndrome’s spread in Iowa.

At this point in the growing season, there are no treatments to save this year’s crop but scouting for crop diseases makes it possible for producers to set up management strategies for end of the season and for future years.

Common, Weird and Unusual Spots Showing up on Corn

Alison Robertson Department of Plant Pathology; Laura Jesse and Fanny Iriarte, Plant and Insect Diagnostic Clinic; and Erin Hodgson, Department of Entomology

 

There have been several recent reports of leaf spots showing up on corn in addition to the eyespot and Goss’s wilt reported earlier this growing season.  Some of these are expected – Northern corn leaf blight and gray leaf spot, but some such as southern rust are a little less common, and others are just weird.

Gray leaf spot
Identifying characteristics of gray leaf spot are rectangular lesions that start on the bottom leaves of the plant (Figure 1). Gray leaf spot can severely impact yield.  Susceptibility varies among hybrids. Infection is favored by warm, muggy conditions. 

Figure 1. Rectangular lesions of gray leaf spot

 

Northern corn leaf blight
Large, cigar-shaped lesions are characteristic of northern corn leaf blight (Figure 2).  Usually this disease also starts on the lower leaves of the plant, but in 2009, disease started high in the plant canopy.  Northern leaf blight is favored by wet weather; over 30 percent yield loss has been reported if leaves in the upper canopy are infected during silking. Susceptibility varies by hybrid.

Figure 2. Large cigar-shaped lesion symptomatic of northern corn leaf blight

 

Southern rust
Southern rust was reported by Pioneer Hi-Bred in southeast Iowa last week. Southern rust can develop rapidly, so it is always a good idea to keep a watchful eye on this disease. This rust is favored by high temperatures and humidity, unlike the more familiar common rust that prefers cooler conditions. Southern rust causes bright orange, small round pustules that generally only occur on the upper leaf surface.  Carl Bradley recently discussed southern rust in Illinois and has some good photos.

The above diseases can be managed with a foliar fungicide. Unfortunately we do not have defined thresholds for these diseases. We recommend scouting fields to see if there is disease pressure in the lower canopy and applying a fungicide if this is a susceptible hybrid and forecasted weather conditions are favorable for disease development. There are fields in Iowa where little to no disease has developed.  Previous work has shown that the chances of recovering the costs associated with a fungicide application in the absence of disease are low.

Corn blotch leafminer
Other spots seen in corn include pinhole punctures caused by the adult corn blotch leafminer (CBL) (Figure 3). These flies deposit eggs in the leaves and the larvae create transparent “window-pane” strips that can be confused with disease lesions. While it is not too unusual to see some of this every year, it seems that this year there is a feeding frenzy going on in northwest and central Iowa. Reports of CBL have also occurred in Wisconsin and Nebraska. Foliar insecticides are not recommended because they will not control protected larvae. 

Figure 3. Pinhole punctures caused by the adult corn blotch leafminer

 

Lesion mimic
A weird spot showing up in corn fields is lesion mimic (Figure 4). This is not a disease but is genetic. Although I heard of no reports in 2009, this spot was around in 2007 and 2008. It appears that some sort of stress may trigger the symptoms.  It usually occurs in patches and is more common in corn following corn fields.

Figure 4. Lesion mimic


 


Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases; contact at (515) 294-6708 or by email at alisonr@iastate.edu. Laura Jesse is an entomologist with the Iowa State University Extension Plant and Insect Diagnostic Clinic; contact at ljesse@iastate.edu or by phone 515-294-5374. Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at ewh@iastate.edu or phone 515-294-2847. Fanny Iriarte is a plant pathologist at the Plant Disease and Insect Diagonostic Clinic; contact at (515)294-5374 or by emailing firiarte@iastate.edu.

Be on the Lookout for Soybean Diseases

By Alison Robertson and Daren Mueller, Department of Plant Pathology

While this season is progressing much quicker than last year, one thing is remaining the same – lots of rain. With the rain, several diseases are starting to show up or are expected to show up during the next month.

Sudden death syndrome (SDS)
SDS has really started to show up across the state this past week. We received reports of it in most areas of the state. SDS can be identified by the characteristic yellowing between the veins, but be sure to split stems to distinguish it from brown stem rot.

sds

Sudden death syndrome, Iowa State University photo.

 

Phytophthora root rot
During the past week we have seen a few fields with plants starting to die from Phytophthora root rot. This disease often shows up a few weeks after excessive rains. Lower stems have characteristic dark purple-brown lesions that extend up from the roots of the plant.

White mold
There have been a few scattered reports of white mold, but nothing close to last year’s outbreak, which was the first major outbreak of white mold in almost a decade. While this year has been warmer, there is still a risk of white mold developing. Fields at a higher risk of getting white mold are those that had disease in previous years and are in high-yielding sites where the canopy closed early. Also, fields that have had plenty of soil moisture, high humidity and little airflow have increased chances of getting white mold.

Stem canker
I honestly have not seen stem canker in Iowa yet this year, but there have been reports of stem canker from neighboring states. Stem canker symptoms include slightly sunken brown lesions that occur near the base of the stem. It is common for stem canker to be misdiagnosed as Phytophthora root rot. Look for green stem tissue near the soil line to diagnose stem canker.

stem canker

Stem canker, Iowa State University photo.

 

So what?
What good would it be to identify these diseases this late in the season? Foliar fungicides are either not effective for sudden death syndrome or Phytophthora root rot, and are not effective after symptoms have developed for white mold and stem canker – thus fungicide applications are not recommended. There still is some value in scouting.

Locating “hot spots” of these diseases may trigger management strategies to reduce inoculum (i.e., the pathogen) in subsequent years. If you have white mold, you can reduce the number of sclerotia by apply a biological control to kill the sclerotia or perhaps tillage. Tillage and crop rotation are not effective management strategies for Phytophthora root rot and sudden death syndrome. Remember the pathogens for these diseases can survive over two years in the soil.

Also, knowing what fields are prone to getting these diseases may influence what cultivar you plant the next time that field is in soybeans. This is especially true for sudden death syndrome, Phytophthora root rot and white mold since cultivars with tolerance and/or resistance are available.

Finally, you can take extra precautions to keep your combine clean of soybean stems and residue after harvesting fields with white mold. This will help prevent spreading the fungus to new fields.

While there is little you can do for management of these diseases this year, information from scouting your fields may help with management in the future.

 


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. Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Robertson may be reached at (515) 294-6708 or by email at alisonr@iastate.edu.

Mid-Season Flooding: Impact on Corn Ear Fill

By Roger Elmore and Lori Abendroth, Department of Agronomy

Severe, localized flooding has occurred recently in eastern Iowa specifically Delaware, Jones, and Jackson counties. For some producers, their corn fields had flood waters up to the tassel. The impact on the corn crop is in question and possible outcomes are discussed here.

At the time of the flooding, most corn in these areas ranged in development from tasseling (VT) to blister stage (R2). One question is what impact the sudden, short-term flooding will have on corn grain yield? This type of flooding is extremely rare so the information available and our personal experiences with it are limited. A report published in 2004 by Dr. Peter Thomison, Extension Corn Agronomist at Ohio State University, provides some information on what may occur. Dr. Thomison discussed the impact following a flood that occurred at the dent stage (R5):

“…The impact of this flood damage on corn will be highly dependent on kernel stage of development, length of the flooding period, how much of the corn plant was immersed during flooding, and subsequent weather conditions…A major concern is the impact of flooding on grain and silage quality. In past reports, when corn in the dent stage was covered by flood water for six hours or more and nearly completely caked with mud for up to two weeks, damage from ear rots and premature kernel sprouting was extensive in those areas of fields where water had covered the ears the longest. Although such damage may be negligible in fields where water never covered the ears, prolonged flooding may cause significant injury to the roots, if not premature root death. Such plants will be more vulnerable to stalk rots thereby increasing the likelihood of stalk lodging…”


With this in mind, we can speculate on the situation in eastern Iowa:

Damage depends on the crops developmental stage
• The crop was at tasseling (VT) or silking (R1). Tasseling may proceed with little or no delay but silking will be delayed. This comes from the fact that silk emergence is affected by stress more than tasseling, which is related more to heat-unit accumulation. If the delay in silking is great, pollen may not be available when silks emerge. Kernel set could be limited and yield losses severe. On the other hand, if silks are not delayed and tassels unaffected, kernel set may occur normally.

• The crop was at blister stage (R2) or later. If flood waters penetrated the husk and warm temperatures follow the flood, as they have done, expect ear rots and lower yield potential. Dr. Thomison, 1995, states, “Research indicates that the oxygen concentration approaches zero after 24-hours in a flooded soil. Without oxygen, the plant cannot perform critical life sustaining functions, such as nutrient and water uptake is impaired, root growth is inhibited, etc. Even if flooding doesn't kill plants outright it may have a long term negative impact on crop performance.”

Length of flooding
The longer the period of submersion, the more chance of yield losses. Plants under water and roots in saturated soils are limited in their ability to function.

Water level
If the ears were not covered with flood water, we expect less yield loss. However, if the ears were covered the loss will be greater since we expect water to have penetrated the husks. Once corn reaches silking, shallow depths of flooding will not cause noticeable damage.

Type of materials deposited
If silt and sand are deposited onto the plants during the flood this will hamper recovery. If the flood waters contained less debris and/or significant rain falls on the crop soon after, this should help significantly as there will be less potential for disease infection. If the crop needs to be cut for silage, having the plants silt-free will be necessary.

Plant standability
Stalk and root lodging will likely increase in flooded fields; monitor these fields closely as harvest approaches.

 

Remember that at blister stage (R2) less than 10 percent of the grain dry matter is accumulated.  Stresses experienced recently by flooded corn can reduce yield significantly. When the waters recede and the crop has a chance to recover, examine plant and ear status as soon as possible. 

 

References
Thomison, Peter R. 1995. Effects of Flooding and Ponding on Corn AGF-118-95. Ohio State University Extension. 
http://ohioline.osu.edu//agf-fact/0118.html

Thomison, Peter R. 2004. Late Season Flood Damage to Corn: Management Considerations. C.O.R.N Newsletter 2004-33 September 27, 2004 - October 4, 2004. Ohio State University Extension.

 

Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. Lori Abendroth is an agronomy specialist with research and extension responsibilities in corn production. Elmore can be contacted by email at relmore@iastate.edu or (515) 294-6655; Abendroth can be contacted by email at labend@iastate.edu or (515) 294-5692.



This article was published originally on 8/9/2010 The information contained within the article may or may not be up to date depending on when you are accessing the information.


Links to this material are strongly encouraged. This article may be republished without further permission if it is published as written and includes credit to the author, Integrated Crop Management News and Iowa State University Extension. Prior permission from the author is required if this article is republished in any other manner.