• Following reports of high damage in Iowa, lab studies revealed resistance to Bt hybrids expressing Cry3Bb1 toxin (found in Monsanto hybrids targeting rootworms).
• Field locations where resistance was documented were characterized by high rootworm pressure, with a history of continuous Bt corn planting.
• This highlights the importance of refuge planting, and Indiana producers should remain vigilant.
Regular readers of Pest&Crop may recall that we have mentioned a few times that corn rootworm Bt toxins are not high dose toxins – meaning that many larvae survive exposure and reach adulthood on each acre of these hybrids. This is one of the reasons that the refuge is so critical in stewardship of this valuable IPM tool. Those points were underscored by the publication last week of the research findings of Iowa State entomologist Aaron Gassman and co-authors. After receiving persistent reports of high damage to Bt corn in northeastern Iowa, the group collected adults and eggs from the area. Rearing the larvae in the laboratory on Bt hybrids revealed that the larvae were able to survive on Bt corn hybrids expressing the Cry3Bb1 toxin at levels similar to survival on non-Bt corn. Hybrids expressing this toxin include those formerly labeled as Yieldgard RW and VT3 hybrids. This toxin is also one of the proteins found in SmartStax hybrids. The good news is that the study tested the other major toxins deployed in North America against this pest, Cry34/35 (found in Herculex hybrids targeting rootworms and also in SmartStax hybrids), and no enhanced survival was found. Although Cry3bb1 and Cry34/35 toxins are different, they are similar enough that cross-resistance (where surviving exposure to one toxin confers some level of survival to another), was a possibility worth investigating. No evidence of cross-resistance was found in these rootworm populations. The next questions to tackle involve untangling the mechanisms behind how these insects are able to survive toxin exposure – what combination of physiological and behavioral traits are at work here? Understanding these mechanisms will undoubtedly help find solutions and plan future control technologies.
Cry3Bb1 traited (left) versus untreated (right) roots from Purdue efficacy trial a few years ago
Given that other researchers have reported that Bt resistance is fairly easy to select for in the lab, we strongly suspected it was just a matter of time before we would see it in the field. The majority of corn planted in the US is Bt corn, and the Cry3bb1 toxin is the major one deployed against rootworms. So what does this mean for other corn-producing areas, including Indiana? It certainly is not good news, but it is not a total disaster either. First off, it demonstrates again what a remarkably adaptable pest the western corn rootworm is. There is no "putting the genie back in the bottle," and resistance in these areas is a problem that won't go away. This is an alert to keep our eyes open for similar occurrences elsewhere – i.e. Indiana. However, the vast majority of Bt corn continues to perform well and be a boon for producers. At the same time, remember that rootworms are the key pest of corn here in Indiana as well, and there is no reason why we could not see resistance occur at some point, either to this or other toxins. The fields to watch are those where the selection pressure upon the pest is highest: namely continuous Bt corn in areas of high rootworm pressure. Some northwestern and north-central Indiana fields may fit this description. In general, however, we have a more diverse cropping system than Iowa with a large proportion of fields rotating corn with soybeans. That helps delay resistance. Planting the recommended refuge certainly helps, and although compliance with the refuge requirements has been falling in recent years, this serves as a stark reminder of how important it is.
Finally, this research also underscores the fact that growers and consultants are not only the stewards of this technology, but they serve as the eyes and ears to report problem fields and initiate further study. None of this research would have been done without reports to Iowa State University extension staff, so please keep your eyes open and don't hesitate to report potential problems.
The full paper can be downloaded and read here:
• Rainfall has very little direct effect on spider mite populations, although rain and high humidity encourage beneficial fungal pathogens
• Depending on future weather, rain may offer a brief respite to spider mite damage and spread.
• Dimethoate is best to control mites, chlorpyrifos is best option when soybean aphid are also present
Scattered thunderstorms have brought moisture relief to some parched areas of Indiana. Now the question concerning spider mites is whether the rain will "control" them. Before attempting to answer the question, let's review the factors that have combined to create a spider mite problem in soybean fields.
Extended hot and dry conditions will:
1) encourage the movement of spider mites from drying field margins and roadside weeds onto soybean
2) favor rapid (sometimes explosive) reproduction of spider mites
3( cause spider mites to increase their feeding rates, while also allowing greater nutrient uptake from stressed plants. This creates a feedback loop that makes plants more stressed, leading to more mites, etc.
4) dramatically reduce fungal pathogens that normally keep spider mites in check
In contrast, a significant rainfall (1 inch or more) followed by high humidity will:
1) hydrate plants, removing stress and reducing the concentration of nutrients available to mites in the tissues
2) encourage the growth, development, and spread of beneficial fungal pathogens
3) physically kill spider mites by dislodging them from the plant (this is relatively minor)
The bottom line here is that rainfall does not make the spider mites go away, it merely helps plants recover and slows down the reproductive rate of mite populations. Mites are always in each and every field at some level. Unless fungal pathogens wipe them out (given continual high humidity), they will lurk and may possibly return to an outbreak situation should the weather return to hot and dry. Spider mite damage is irreversible. That is, leaves that are severely bronzed or brown will not "green back up" and contribute to the yield of the field.
Two pesticides are recommended to control spider mites. These include dimethoate and chlorpyrifos. Dimethoate is the most efficacious of these compounds for mite control. If soybean aphid is also present in the field, then chlorpyrifos would be a good choice to suppress both pests. Neither of these products will control spider mite eggs, however, and each will provide about 7 to 10 days at most of residual activity. Proper placement of these pesticides is the key to successful control. Nozzle pressures of 40 psi with fine to medium droplet size and 30-40 gallons of water/acre for ground application helps distribute the pesticide throughout the foliage.
Stippling, beginning of spider mite leaf damage
Spider mite bronzed plants near the field edge.
Indiana's "second" corn crop that was planted in late May or early June is at or approaching the tasseling and silking growth stage. In many areas, this late-planted corn is short, uneven in growth stage, and is experiencing nitrogen or other nutrient deficiencies. These problems are compounded by the fact that the tasseling and soon to tassel corn is going through pollination under high temperatures and low soil moisture (Figure 1). Extension corn Agronomists across the Midwest have been discussing the impact that the heat and lack of moisture can have on corn yields: http://graincrops.blogspot.com/2011/07/high-temeperatures-could-hurt-corn.html. The bottom line is that yields may be impacted not only by the late planting date, but also by the current weather conditions. Growers interested in maximizing remaining yield potential may be considering spraying this corn with a fungicide in the hopes that these applications will improve plant water use efficiency, and/or alleviate plant stress through grain fill.
Applying fungicides for purposes other than disease control has not proven to be consistently profitable in replicated field research trials across the Midwest. As we have reported in previous articles, the most likely chance to recoup the investment from a fungicide application is when fungicides are used in response to disease presence or disease risk factors. Across Indiana, development of foliar fungal diseases such as gray leaf spot and northern corn leaf blight has overall been low. Disease development is slowed under high temperatures (above 85-90°F) and dry conditions, and in many fields, lesions of gray leaf spot are well below the ear leaf, or hard to detect (Figure 2). Our scouting thresholds (http://extension.entm.purdue.edu/pestcrop/2011/issue15/index.html) would indicate that in these situations there is not enough disease pressure to warrant a fungicide application.
Additionally, there are very few replicated field research trials that examine the true effects of fungicides on plant water use efficiency and ability to alleviate drought stress. If fungicides will be applied to corn for these purposes, it would be beneficial to work with the applicator and leave several untreated strips within the field that can be harvested and yields averaged to determine if there was an actual yield benefit due to the fungicide application. This does require additional planning and effort, but it can mean the difference between knowing if the fungicides were worth the investment and whether or not they work under your specific production conditions.
Figure 1. Corn showing symptoms of moisture and heat stress
Figure 2. Lower leaves of corn plant with little or no gray leaf spot lesions present (Picture taken in Northeast IN on 8/1/11).
April-planted corn is in milk or early dough stages, and weather conditions have been favorable for ear rot disease development, especially in southern IN. Three potential ear rot problems for 2011 are discussed in this article.
Aspergillus ear rot is common in hot, dry years and is most prevalent on drought-stressed plants. This ear rot is caused by the fungus Aspergillus flavus, which also produces a mycotoxin known as aflatoxin. Aflatoxin is a very toxic carcinogen, and livestock that consume contaminated grain may be at risk for many health problems. Aspergillus ear rot appear as stunted ears that have an olive-green dusty mold under the husk on plants that are severely affected by drought stress and warm overnight temperatures. (Figure 1). Low levels of Aspergillus ear rot were detected in 2010 in southern IN, and conditions are once again favorable for disease development in 2011. More information about this disease can be found at: http://www.extension.purdue.edu/extmedia/BP/BP-83-W.pdf.
Figure 1. Aspergillus ear rot of corn (Photo by Burt Bluhm)
Fusarium ear rot, caused by Fusarium verticillioides, is also more severe under hot and dry weather conditions. This fungus also produces a mycotoxin known as fumonisin, which is especially toxic to horses and swine. Symptoms of Fusarium include white, pink, or gray kernels scattered across the ear and are often associated with insect feeding damage. In some hybrids, a white streaking can appear in kernels, which is known as a "starburst" pattern.
Figure 2. Fusarium ear rot of corn (Photo Charles Woloshuk)
Diplodia ear rot, caused by the fungus Stenocarpella maydis, is a common ear rot across IN, but usually appears first in southern regions. The fungus infects corn at and during silking, and infection and disease development are favored by wet weather. April planted corn in Indiana experienced wet, humid weather at silking, which may have favored infection in susceptible hybrids. Infected ears often have bleached husks with tiny black specks on the outer layer (Figure 3). Removal of the husk of an infected ear will reveal white fuzzy growth of the pathogen between the kernels, which often starts at the base of the ear (Figure 4). The cob can also appear rotted. More information about this disease can be found at: http://www.extension.purdue.edu/extmedia/BP/BP-75-W.pdf.
Figure 3. Corn husk with symptoms of Diplodia er rot (Photo by Paty Romero)
Figure 4. Ears with white mold at the base of the cob, which is indicative of Diplodia ear rot
At this point in the season, there are no management strategies for reducing ear rots if detected. Producers should scout fields prior to harvest and determine the level of incidence of the disease in the field. If any Aspergillus ear rot is observed in a field, affected areas should be harvested early and grain segregated to avoid aflatoxin contamination of non-infected grain. Fields affected by Diplodia and Fusarium ear rots should also be harvested prior to other non-affected fields. All grain contaminated by any ear rot fungus should be stored separately from good grain, and stored below 15% moisture to prevent further growth of fungi.
An Example of "Recovery" From Severe Root-Lodging – (Bob Nielsen)
A thunderstorm with strong winds flattened hundreds of acres in eastern Indiana late in the afternoon of 22 July as a result of plants being partially uprooted. The appearance of damaged fields the morning after was demoralizing to growers and casual observers alike.
However, because many of these fields were planted extremely late due to a wet planting season, the plants were still in the late vegetative phase of development (1 to 2 weeks before tasseling) and, most importantly, still in the process of stalk elongation. Root-lodged stalks that are still elongating can respond to such root-lodging by slowly bending or "goose-necking" in an attempt to regain an upright stance. Such "goose-necking" is the result of changes in the distribution of plant growth hormones in the stalk tissue that cause more rapid elongation on the bottom side of the nearly horizontal stalks than on the top side.
As long as root damage caused by the lodging is not extreme and there is adequate soil moisture to foster additional root development during the recovery period, flattened fields of corn at these growth stages can "recover" fairly well. I put the word "recover" in quotes because severely root-lodged fields will usually not recover completely. However, if the damaged plants can goose-neck sufficiently and quickly enough by the time the field moves into the critical tassel/silk pollination period such that the portion of the stalk containing the silked ears is again upright, then pollination will likely be successful.
The three photos accompanying this article are from a 30-acre field at the Davis-Purdue Ag Center in Randolph County. The storm moved through the area late in the afternoon of 22 July and severely flattened hundreds, if not thousands, of corn acres in the area. The field had been planted 3 June, much later than desired because of the frustratingly late, wet planting season. However, the good news was that plant development was consequently delayed relative to the calendar and the field was still about one week away from tasseling and pollination. I say "good news" because the plants were still in their rapid growth phase with stalks still rapidly elongating.
The first photo shows the appearance of the damaged field the morning after the storm. I estimated 80-90% of the plants were root-lodged and nearly flat to the ground. The second photo shows the same field six days late on 28 July and the change in appearance is amazing. Dramatic bending of the horizontal lower stalk tissue resulted in "goose-necked" plants and, more importantly, enough upright growth to place the silking ears in a position to be exposed to pollen from the tassels.
Time will tell to what extent yield will be decreased and, unfortunately, there is no good comparison to even determine how much yield will be lost. With the hope that weather conditions during grain filling improves, the next big challenge will be harvesting the crop because of the difficulty of moving the combine header through the yet root-lodged lower portions of the crop canopy.
Nielsen, R.L. (Bob). 2011. Prospects of Recovery for Root-Lodged Corn. Corny News Network, Purdue Univ. Extension. [online] Available at http://www.kingcorn.org/news/articles.11/FlatCorn-0726.html [URL accessed July 2011].
Kernel Set Scuttlebutt – (Bob Nielsen)
"Scuttlebutt": The cask of drinking water on ships was called a scuttlebutt and since sailors exchanged gossip when they gathered at the scuttlebutt for a drink of water, scuttlebutt became U.S. Navy slang for gossip or rumors. A butt was a wooden cask, which held water or other liquids; to scuttle is to drill a hole, as for tapping a cask.
Nautical Terms and Phrases, NAVAL HISTORICAL CENTER, Washington DC 20374-5060.
Online at http://www.ussbrainedd630.com/terms.htm [URL accessed Aug 2011].
The post-pollination scuttlebutt overheard in coffee shops throughout Indiana during late summer often revolves around the potential for severe stress that might reduce kernel set or kernel size in neighborhood cornfields. Growers' interest in this topic obviously lies with the fact that the number of kernels per ear is a rather important component of total grain yield per acre for corn.
Poor kernel set, meaning an unacceptably low kernel number per ear, is not surprising in fields that are obviously severely stressed by drought, but can also occur in fields that otherwise appear to be in good shape. Good or poor kernel set is determined from pollination through the early stages of kernel development; typically 2 to 3 weeks after pollination is complete.
Problems with kernel set stem from ineffective pollination, kernel abortion, or both. Distinguishing between these two symptoms is easy. Determining the exact cause of the problem is sometimes difficult.
Pollination is the movement of pollen from the tassels to the silks.
Fertilization is the actual union of the male and female gametes once the pollen tube reaches the ovule.
One of the causes of incomplete kernel set is unsuccessful fertilization of the ovules during pollination. Unsuccessful fertilization results in ovules that never develop into kernels and, subsequently, ears with varying degrees and patterns of incomplete kernel set. Many factors can cause incomplete pollination and distinguishing between them can be very difficult.
Certain insects like corn rootworm beetles and Japanese beetles can interfere with pollination and fertilization by their silk clipping action. These insects feed on pollen and subsequently clip silks as they feed on the pollen that has been captured by the silks. Unusually early or late pollinating fields are often particularly attractive to these insects.