Field Crops Disease Management Resources and 2019 Guides

Alyssa Koehler, Extension Field Crops Pathologist;

The 2019 Fungicide Efficacy for Control of Corn Diseases table is now available through the Crop Protection Network This table is produced annually by the Corn Disease Working Group and provides efficacy ratings for fungicides to manage corn foliar diseases. A number of other resources for disease management in corn, soybean, and small grains are available through the Crop Protection Network

Insecticide Trial Results for Vegetable and Agronomic Crops

David Owens, Extension Entomologist, and Bill Cissel, Extension Agent – Integrated Pest Management;

Summaries of last season’s insecticide trials in peas, sweet corn, watermelon, field corn, soybean, and wheat can be viewed at

Potential Hurricanes and Flooding

Jarrod O. Miller, Extension Agronomist,

We avoided most of the flooding seen in the Carolinas with Florence, but hurricane season lasts until the end of November. Some later planted corn is still drying down, so saturated soils and winds may cause lodging, but there are no hurricanes on the horizon that may cause those issues. Full season and double crop beans are more likely to have issues if another storm heads for the Delmarva. Depending on development stage, storm conditions could increase disease pressure, cause lodging and shattering. For more detailed information, check out NC State extension as they dealt with the aftermath of Florence (

For fields along tidal streams and shorelines, hurricanes could bring salt water across fields. It may be necessary to perform soil tests in these fields to check for salt levels prior to next year’s crop. In general, if Na makes up more than 15% of the cation exchange capacity, lower yields could be observed. Total salts (which can include Ca and Mg) may also cause issues in fields flooded with tidewater. Gypsum works well if Na is the only issue, but irrigation is needed to leach soils high in Ca, Mg and Na.

Field Corn Insect Damage

David Owens, Extension Entomologist, and Bill Cissel, Extension Agent – Integrated Pest Management;

Corn earworms in field corn are not normally associated with yield loss in timely planted corn. This year, our timely planted corn had between 2 and 10% injured ears. However, because of the weather, we had a fair amount of late planted corn that was silking when our major earworm flight began. It wasn’t uncommon to find 2 – 4 earworms in one ear, and I have to think that at these numbers, they could contribute to lower yield through their direct feeding as well as opening the ear up to disease. We sprayed some of our late planted field corn 5 days after silk to no avail. To keep late corn clean chemically, it would require just as many sprays as sweet corn. Once larvae are in the silk channel, they are protected.

At the beginning of the season I shared the link to the handy bt trait table ( This table lists the BT trait packages and what pests they have good efficacy on. If you regularly plant late corn and don’t want to see damaged ears, this may be worth keeping in mind as you select among varieties for next year.

Guess the Pest! Week #24 Answer: European Corn Borer

Bill Cissel, Extension Agent – Integrated Pest Management;

Congratulations to Grier Stayton for correctly identifying the insect as a European corn borer and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!

Guess the Pest Week #24 Answer: European Corn Borer

It’s hard to believe that a pest that once caused an estimated annual economic loss of $1 billion dollars in the United States is now a rare occurrence. The European corn borer (ECB), as the name implies, is actually native to Europe and was introduced into North American in the early 1900s. In addition to being a pest of corn (field corn and sweet corn), it is also considered a pest of many vegetable and field crops. Since the adoption of transgenic corn hybrids in the mid-1990s, losses due to ECB have been virtually eliminated in Bt crops and significantly reduced in other vegetable and non-Bt field crops. This is one of the pests that the UD Insect Trapping Program monitors with black light traps. The reason we continue to monitor ECB populations throughout the state is because even though generally speaking, populations have been low, there are still local pockets where ECB is causing damage. The photo above of the ECB larva was taken on the Eastern Shore of VA by Helene Doughty from a non-BT sweet corn plot that was 100% infested with ECB.

For information on the benefits of Bt adoption, read this article: Regional pest suppression associated with widespread Bt maize adoption benefits vegetable growers

2018 Summer Rain and Temperatures

Jarrod O. Miller, Extension Agronomist,

Corn shelling has been rolling where fields have been dry enough. Due to the highly variable rainfall and temperatures we have had since April, every field will probably be a little different. Many corn fields planted in late April should have started silking around July 9th, with other fields following. During the month of July, daytime temperatures fluctuated above 86°F several times, which may have reduced pollination. This may be more prevalent in fields that were planted in early May. Fields planted late May through mid-June may have been luckier; they should have been pollinating near the end of July, when daytime temperatures stayed out of the 90s.

When temperatures at night remain above 72°F, corn plants are stressed and consume some of the energy they had built up from the sun. There are several nights where this occurred over the summer, typically also associated with high daytime temperatures. This will reduce the amount of energy corn plants have to develop grains. This is not a doom and gloom statement, it is simply a way for Delaware farmers to asses any differences they may find across all of their planting dates.

Georgetown represents the southern end of the state, and typically had the highest night and daytime temperatures, but not by much. There are a few times (April 30, July 16) that Dover was a little warmer. There is a lot more variability in rainfall across the state than temperatures.

As most readers are aware, we are receiving high rainfall again, some which may have come from tropical storm Gordon over the weekend. Unlike temperature, there is high variability across the state. The earlier predictions from Hurricane Florence projected that our region may get 2-6 inches, but after this summer that seems normal. Current tracking as put us in a better position that our southern neighbors, but we are not done with hurricane season yet.

Assessing Nitrogen Management in Corn After this Challenging Season

Gordon Johnson, Extension Vegetable & Fruit Specialist,; Amy Shober, Extension Nutrient Management and Environmental Quality Specialist,; Jarrod Miller, Extension Agronomist;

Excessive rainfall that occurred early and late in the season made N management for corn (among other things) a challenge for many growers in our region. Early season rainfall resulted in significant N losses via leaching or denitrification, and in some cases delayed mineralization of manure N. We think it is safe to say that no one likely knew how much N was lost or how much additional N should have been applied. Throughout the season, we tried to provide some guidance to help growers and consultants answer those questions. While there is no way to turn back time and change the N management decisions that were made this season, we can take a look back to assess if the decisions to forego or apply additional in-season N were warranted.

The end-of-season corn stalk nitrate test (CSNT) makes use of the fact that corn plants either remove N from, or accumulate N in, the lower stalk based on soil N availability. Studies over a wide range of conditions have found remarkably similar relationships between the amount of N found in the lower stalks late in the growing season and the likelihood that corn has been under or over-fertilized. The test allows growers to identify N deficiencies or surpluses that may not be apparent upon visual inspection of the plant.

Collect stalk samples between ¼ milk line and 3 weeks after black layer from areas with uniform soil type and and management history, avoiding areas with severely damaged or diseased stalks. Purdue Extension provides a good review on how to identify corn reproductive stages. Cut an 8-inch segment of stalk starting 6 inches above the soil soil surface. Remove leaf sheaths from the stalk samples. Collect 15 stalk segments for every 10 acres of corn and submit them as a single sample. Store stalk samples in paper bags (not plastic) to allow for some drying and to minimize mold growth. Ship samples (within 24 hrs) for analysis or refrigerate until shipping is possible.

Factors that limit crop yield, like the unusually wet weather we experienced this year, can increase stalk nitrate concentrations. As such, we suggest taking a cautious approach to interpreting the results of the CSNT for this season. However, we believe that growers can still learn a lot about how well they fared with N management based on the results. In general, stalk samples with less than 700 ppm nitrate indicate the crop was under-fertilized with N, while samples with more than 2000 ppm nitrate indicate that the crop was likely over-fertilized with N.

The CSNT does not directly indicate how much N rates should be increased or decreased for a given stalk nitrate concentration. However, use of the CSNT for several years will allow corn producers to identify N management practices, including rates, forms, and times of application, that tend to result in optimum amounts of plant-available N. After appropriate consideration of weather and other factors, growers should consider making adjustments to N fertilizer and/or manure rates based on results of the CSNT over multiple years. In addition, the CSNT can also help growers make in-season N management decisions when faced with excessively wet conditions in future years.

For additional information:
Hansen, D.J., G. Binford, and J.T. Sims. 2014. End-of-season corn stalk nitrate testing to optimize nitrogen management. University of Delaware. Newark, DE.

Growing Degree Days (GDD) and Rainfall Through September 4th

Jarrod O. Miller, Extension Agronomist,

There is some shelling going on in Sussex County this week, with word that many fields will start on Monday. Any field planted between April 22 and May 20 should be in blacklayer, and the high temperatures this week should really help in reducing moisture content. If you planted over a range of dates, you should be able to roll through your fields week to week, depending on your variety. Most fields planted in early to Mid-June should be in R5 (dent), which you can check by watching the milk line (

R5 (Dent): 2190-2450
R6 (Blacklayer): 2700

Table 1: Accumulated growing degree days based on planting dates through September 4th.

If you planted
Sussex Kent New Castle
22-Apr 3179 3109 3000
29-Apr 3126 3062 2966
6-May 3018 2952 2868
13-May 2914 2846 2771
20-May 2803 2740 2676
27-May 2645 2580 2528
3-Jun 2492 2429 2378
10-Jun 2370 2312 2265
17-Jun 2234 2185 2136

To match most of the season, rainfall around the state over the last week was scattered with variable intensity. From the graph below, it appears that Dover took a pretty hard hit (2.47” on Aug 31st), but most of the state received about an average of 0.5” over the weekend. On DEOS I did observe at 5.39 inch rainfall in Oakley and 4.20 inches at the Prime Hook Wildlife Refuge.

Options for Harvest-Aid Treatments for Field Corn

Mark VanGessel, Extension Weed Specialist;

There have been several questions regarding options for harvest-aid treatment. A harvest-aid may be a consideration to dry down vegetation prior to harvesting and to reduce foreign matter in the harvested grain. There are a number of things to consider including: what weeds need to be treated; how will harvest-aid be applied; stage of the crop, and size of the weeds.

Be realistic on expectations of these products; while they are herbicides they were developed to control much smaller weeds than the size treated as harvest aids. Most of the products listed below will not kill plants; they are likely to burn off leaves, but not impact lower stems or vines. These products will not affect weed seed production. Reducing leaf material and foreign matter entering the combine should be the goal, not killing the weeds present.

A few more considerations:

  • With the hot temperatures, the risk of drift will increase with these products
  • Spray coverage is important for the contact herbicides (Defol and Gramoxone), so be sure to apply in 20 gpa or higher
  • If morningglory is present, can a combine run through the patches without pulling down the corn?
  • This time of year will favor translocation, so glyphosate is likely to cause more damage to desirable plants
  • Read the product label carefully for all instructions and restrictions

Products labeled:

Defol (sodium chlorate) is labeled for applications 14 days prior to harvest. Defol will dry down plants but it does not have herbicide activity. Dry down is slow; expect at least 14 days for dry down. Control of morningglory can be erratic.

Glyphosate is labeled but must be used with care do to potential injury to desirable vegetation. Apply glyphosate at 35% moisture or less and black layer has formed. Allow 7 days between application and harvest. Refer to the glyphosate label for rates.

Gramoxone Inteon is labeled for broadcast treatments. Application rates are 1.2 to 2 pts/A plus a non-ionic surfactant, and must be applied at least 7 days prior to harvest. Be sure to read the label for all precautions.

Aim is labeled for applications up to 3 days before harvest. Aim will only effect a few weed species and will not dry down grasses.

2,4-D amine is labeled but due to volatility and off-target movement, use of 2,4-D is not recommended. Applications with air temperatures above 85 degrees increases the likelihood of off-target movement. Application timing is after the hard dough or dent stage.