Poor Stands and Plant Vigor in Early Planted Fresh Market Sweet Corn

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Growers are reporting issues with stands and vigor in sweet corn fields in 2018, especially in early planted fields. There can be many causes for stand loss and weak seedlings: surface compaction and crusting, birds, soil insects, slugs, cold soils that delay emergence, soil diseases affecting seeds or seedlings, wet soils, fertilizer injury, deep planting, and herbicide injury are just a few examples.

When checking sweet corn fields with vigor and stand problems, it is important to dig up seeds and affected plants and examine the seed remnants, roots, and mesocotyl (stem that pushes the seed leaf to emerge above the ground). Corn seedling survival and early vigor is directly tied to a healthy seed kernel and mesocotyl from planting through the six-leaf stage. Any damage to the seed or mesocotyl during this period can lead to stunted or weak seedlings, and in severe cases, seedling death. This is because the corn seedling depends on the seed for food to grow for several weeks after emergence until sufficient leaf area has been produced and nodal roots have become established. The seed kernel provides the means for early roots to grow and these food reserves are also mobilized and transported through the mesocotyl to grow the first stalk and leaf tissue. The mesocotyl also serves to transport water and mineral nutrients from the seedling roots.

Sweet corn is more susceptible to stand loss and poor vigor problems than field corn because the seed has less food reserves. Shrunken types (supersweet, sugary enhanced, augmented shrunken, synergistic varieties) have even less stored food than “normal” types and therefore are more susceptible to stand problems.

I have looked at sweet corn fields with stand loss and vigor problems (uneven growth) over the years. Often, when digging up the seedlings and examining the seed remnants and mesocotyls, the kernels will be disintegrated and there will be darkening at the mesocotyl attachment. This means that the seeds deteriorated prematurely and the full content of the food reserves in the seed were not available for seedling development, leading to the stand and vigor issues. Premature seed deterioration and/or poor vigor seedlings can be due to diseases that cause seed rots, seedling blights and/or root rots. Soil insects can cause seed deterioration by feeding on seed contents or creating entrance wounds for disease organisms. In addition, certain soil insects and slugs can feed on the mesocotyl causing seedlings to collapse. Sweet corn that takes more than 10 days to emerge is at great risk of injury due to insects and diseases as seed treatments dissipate.

Cold stress and cold soils are common stress factors leading to poor stands. Often growers are pushing the limits and are planting sweet corn very early. In 2018 we had a cool April which further stressed early sweet corn. While field corn will start to germinate at 50°F, many types of sweet corn need much warmer soils. This is especially true of supersweet varieties and other shrunken types, which perform best at higher soil temperatures (above 60°F). When soil temperatures are below 55°F, germination is greatly extended. Food nutrients are mobilized in the seed but are not being utilized rapidly by the plant. The seed then becomes a perfect food source for many soil microorganisms. On a positive note, many of the newer sweet corn varieties have much more cold tolerance and emerge more rapidly in cold soils.

Stand issues are often related to the inherent poor vigor of sweet corn. Work with seed suppliers to obtain their best lots for early plantings with the largest seed sizes. Obtain varieties that perform better under cold stress. When possible, obtain reports from early planted sweet corn trials to assess which varieties are the most cold tolerant. Request seed treatment information and select treatments with the best protection potential for early plantings. There are in-furrow fungicide options; however, research is limited with sweet corn in our region.

Growers often face the decision on whether or not to keep plantings with poor stands. This is most often a marketing decision based on the need for and value of early sweet corn for that farm. An estimate of potential marketable ears will be based on stand counts of full vigor plants from 20-40 sites throughout the field. This stand count information then can be used to estimate the value of the field as is versus the value of a later planted full stand crop.

Guess the Pest! Week #5 Answer: Cutworm

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Congratulations to John Comegys for correctly identifying damage as cutworm 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!

By David Owens, Extension Entomologist

Corn planting is at full speed, and early season insect pest pressure is not far behind. The above photo is an example of small larval cutworm feeding injury to field corn. Small larvae cannot cut plants, they will chew holes through the whorl that, as the leaves unfold, leave a characteristic, symmetrical shot hole pattern. Cutworms can be a significant corn pest. Older larvae can bore through larger plants causing dead heart and cutting plants off at the ground level.

In general, there has been a trend towards decreasing cutworm activity, though not as strong as the trend seen with European corn borer. How can we manage cutworm, and what do the holes mean? First, many of the Bt varieties have suppressive efficacy on early instar cutworms. Please reference the Handy Bt trait table here: https://lubbock.tamu.edu/files/2018/01/BtTraitTableJan2018.pdf. There are traits that do not have cutworm efficacy, so know what you have! DoublePro, TriplePro, and Yieldgard corn do not have traits that are effective on cutworm. If you see this type of injury in fields planted without an effective trait, pay special attention to the field and be ready to treat it with a pyrethroid if you see 5% damaged plants and you find larvae. But what about those fields that do have one of the traits that does have suppression activity (Cry1F or Vip3A)? The Bt traits are stomach active toxins, and specific to target insects or groups, meaning they need to be ingested and there is specificity. Rootworm traits only work on rootworms, not on wireworms, caterpillars, or grubs. Larvae may feed on plants for short period of time before dying. If you see this type of injury in a field with Cry1F and Vip3A corn, monitor, preferably within a few days. It also means that traits are most effective on small larvae that require less of a dose. We are seeing large dingy and black cutworms out right now in some cover crops. These will be done with their life cycle by the time the field is planted and seed germinating. However, cover crops are attractive to ovipositing moths, so if a field is planted soon after burndown or into a green cover, pay attention to stand. A few holes in leaves will alert you to a population, but if that is all you see in consecutive field visits, the traits have probably done their work. My colleagues in western states will caution that if initial populations are REALLY high, the traits may need to be supplemented with additional control. It also pays to scout a field. There are seedling corn pests that are not managed by any Bt trait. If you see feeding injury, try to find the culprit as best as possible.

Supplemental control can be in the form of a non-neonic seed treatment. Right now, there is chlorantraniliprole, trade name Lumivia (same active ingredient as Prevathon and Coragen). It is rated by Auburn and University of Tennessee entomologists Dr. Kathy Flanders and Dr. Scott Steward as having good efficacy for cutworm. Neonics do not. You can find their efficacy charts here: http://blogs.ext.vt.edu/ag-pest-advisory/files/2014/10/Field-corn-insecticide-seed-treatment-chart.pdf and here: http://www.utcrops.com/cotton/cotton_insects/pubs/PB1768-Corn.pdf.

Additional control can be achieved with a pyrethroid. What scenarios are at greater risk for cutworm? LATE planted fields that have living weeds/cover within a week of or two of planting. Pay special attention to these fields and to fields planted green. As we wrote recently, there is a cost/benefit trade off with high seed treatment rates and a pyrethroid in furrow/over the soil. The closer you are to planting, the more likely a pyrethroid at burndown will hit an insect pest. However, if you are not planting right away, pyrethroids break down over time. Should another seedling pest move in, the original spray will not be effective. If your concern is soil pests such as wireworm, put the pyrethroid in the furrow. The pyrethroid should still have efficacy on cutworm.

Bird Repellent for Corn Seed

David Owens, Extension Entomologist, owensd@udel.edu

Some folks have recently asked about products to discourage birds from feeding on field and sweet corn seed. Avipel (9,10-Antraquinone), manufactured by Arkion Life Sciences LLC has a 24(c) special local needs label for use on corn seed in Delaware. Approved Delaware formulations are dry hopper box treatment and a liquid formulation for treating seed ONLY in commercial seed treatment equipment. It should also be used with a dye and cannot be used for other purposes than planting – standard language for seed treatments. Both registrations expire July 1, 2022. Registration was originally requested because of incidences of severe bird feeding damage in fields east of Route 1. Large flocks of blackbirds can quickly pull up a stand as they migrate north. You might want to consider this if you have had significant bird damage in the past and are seeing large numbers of blackbirds in your field. Should you use Avipel or have it applied to your seed using seed treatment equipment, you MUST have a copy of the 24(c) label in your possession when handling and planting. As always, consult the label for additional application guidance. The labels for Delaware can be found here: http://arkionls.com/av/states/delaware.html. Maryland and Virginia ONLY have the hopper box treatment approved, labels can be found here: http://www.arkionls.com/av/states/maryland.html, and http://www.arkionls.com/av/states/virginia.html.

Timings for Late Summer and Fall Harvested Vegetables Revisited

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Plantings for fall harvested vegetables are underway and will continue through August. Timing these plantings can be a challenge, especially where multiple harvests are needed. Plantings from early July through the beginning of September may be made, with cutoff dates depending on the crop, variety, and season extension methods such as row covers, low tunnels, and high tunnels.

These plantings can be divided into 2 groups: 1) warm season vegetables for harvest up to a killing frost and 2) cool season vegetables for extended harvest in the fall.

The three main factors influencing crop growth and performance in the fall are daylength, heat units, and frost or freeze events. A few days difference in planting date in the summer can make a big difference in days to maturity in the fall.

Warm season vegetables for fall harvest include snap beans, squash, and cucumbers. July plantings of sweet corn can also be successful to extend seasons for farm stands. Mid-July plantings of tomatoes and peppers also are made for late harvests, particularly in high tunnels.

Cool season vegetables for fall harvest include cabbage, broccoli, and cauliflower; the cole crop greens, kale and collards; mustard and turnip greens; turnips for roots; spinach; beets; lettuce; leeks; green onions; and radishes.

To extend harvest in the fall, successive plantings are an option. However, days between plantings will need to be compressed. One day difference in early August planting for a crop like beans can mean a difference of several days in harvest date.

Another option to extend harvest in the fall is by planting varieties that have different days to maturity at the same time. This is particularly successful with crops such as broccoli and cabbage where maturity differences of more than 30 days can be found between varieties.

Another way to get later harvests is to use row covers or protecting structures (high tunnels). This can allow for more heat accumulation and will aid with protection against frost and freezes. Decisions on what type or combination of covers/protection to use and when to apply the protection will influence fall vegetable maturation and duration of harvest. In general, plantings of cool season crops can be made 30-45 days later in high tunnels than in outside production.

A final factor for summer planting for fall production is on planting cutoff dates. For example, a crop such as cucumber may produce well with an August 2 planting but poorly with an August 8 planting; broccoli has a wider planting window than cauliflower; turnip greens have a wider planting window than kale.

Planting Window for Fall Harvested Warm Season Vegetables
(harvest September through Frost)

Snap Beans: July 10 through August 10

Lima Beans: June 15 through July 20

Cucumbers: July 10 through August 7 (high tunnel transplanted up to September 1)

Peppers: Transplant up to July 10 (high tunnel up to July 30)

Pumpkins and Winter Squash: Direct seed through June 30, transplant up to July 7

Summer Squash: Direct seed July 15 through August 15 (high tunnel up to September 1)

Sweet Corn: Direct seed July 1 through July 30

Tomatoes: Transplant up to July 10 (high tunnel up to July 30)

Planting Window for Fall Harvested Cool Season Vegetables
(harvest September – December)
For transplants, seed 3-6 weeks prior to desired planting date (8 weeks for leeks and onions).

Beets: Direct seed July 1 through August 10

Swiss Chard: Direct seed July 15 through August 20 (high tunnel up to September 30)

Broccoli: Transplants July 15 – August 20

Brussels Sprouts: Transplants through July 10

Cabbage: Transplants July 1 – August 10

Cauliflower: Transplants July 20 through August 15

Kale: Transplants July 15 through August 30

Kale: Direct seed July 1 through August 15 (high tunnel up to September 30)

Collards: Direct seed July 15 through August 15

Carrots: Direct seed through July 10 (high tunnel up to August 30)

Turnip Greens: August 1 through September 10 (high tunnel up to September 30)

Turnip Roots: August 1 through August 30 (high tunnel up to September 20)

Mustard Greens: August 1 through September 10 (high tunnel up to September 30)

Leeks: Transplant July 20 through August 10

Lettuce (full head stage): Direct seeded August 1 through August 20

Lettuce (full head stage): Transplants August 10 through August 30

Lettuce (baby stage and cut salad mix): Direct seed August 1 through September 15 (high tunnel up to October 15)

Onion (green bunching): Direct seed July 1 through August 30 (high tunnel through September 30)

Parsley: direct seed July 15 through August 15 (high tunnel through September 15)

Radishes (salad): Direct seed August 1 through September 30 (high tunnel through November 30)

Radishes (Daikon): Direct seed August 1 through September 10 (high tunnel up to September 30)

Spinach: Direct seed August 10 through August 30 (high tunnel up to September

Bacterial Stalk Rot in Sweet Corn

Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu; @Delmarplantdoc

With the recent stretch of hot, dry weather, many producers have begun to irrigate their sweet corn. Consequently, there have been reports of bacterial stalk rot in some areas where surface water or ponds have been used as water sources. Bacterial stalk rot, as the name implies, is caused by a bacterium that grows well on wet, decaying plant material. When water containing the bacterium is moved onto corn plants, particularly smaller plants in the vegetative stage, water containing the bacterium pools in the whorl and leaf sheaths and has easy access to natural openings or wounds on the corn plant. Once inside the plant, the bacterium can digest plant materials, and produce a slimy exudate that can clog water and nutrient conducting tissues. If infections are severe, a blight can occur. More often, we see an orange to brown, mushy rot near the shank or nodes randomly scattered on plants in the field. In addition, the rot has a characteristic, foul odor. Warm temperatures allow the bacterium to grow rapidly and produce more symptoms. This, in addition to the increased use of overhead irrigation when it is hot and dry, is why bacterial stalk rot tends to be an issue during this time of the year.

Bacterial stalk rot on corn is characterized by an orange to brown, slimy lesion and rot that gives off a characteristic odor.

Managing bacterial stalk rot can be achieved by: 1) Incorporating residue into soil will reduce local inoculum that may spread to plants through rain splash; 2) using well water to irrigate fields instead of ponds or surface water; 3) using a hypochlorite system to treat water if you must use surface water for irrigation. An example of such a system is Accutab™; 4) utilizing drip irrigation instead of overhead irrigation, which may be practical for small operations; 5) applying more water less frequently, which may reduce the amount of overall plant wetness and reduce potential opportunities for infection. Copper fungicides will not help with disease management.

Postemergence Herbicides for Sweet Corn

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Options for postemergence weed control in sweet corn is limited. Be sure to consider what weeds are present and how big they are, the size of the sweet corn, what is to be planted after the sweet corn, and how long until harvest. Most herbicides that have the most flexible herbicide rotations, also have fairly limited spectrum of control. Postemergence options for grasses include Armezon/Impact, Laudis, and Accent. For broadleaves, Cadet or Basagran will control a limited number of species and typically only very small plants. Armezon/Impact, Laudis, and Callisto will control a fair number of broadleaf weeds (and control is improved with addition of a little atrazine); all are safe on most processing sweet corn hybrids grown locally; and so check the rotational crops and consider the preharvest interval to help make the final selection.

Seed Vigor in Sweet Corn Revisited

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Uneven or poor stands may be caused by reduced seed vigor in a specific lot of sweet corn seed. This results in reduced yields in fresh market sweet corn and may cause losses in processing corn if differences in growth are significant.

By its nature, sweet corn has lower stored food reserves compared to field corn. With the advent of different endosperm types than the traditional sugary (su), such as homozygous sugary enhanced (se), shrunken supersweets (sh2), and the more recent augmented shrunken types, vigor became even more of an issue. In general, vigor of sweet corn rated from highest to lowest is: normal sugary su > se heterozygous > se homozygous > sh2 augmented > shrunken sh2. Synergistic sweet gene varieties may have seed with vigor characteristics of a se or a su sweet corn depending on the specific genetics (check with your sweet corn seed company for specifics on the vigor of these hybrids). Supersweet hybrids (shrunken sh2) are noted for having inherently low seed vigor due to reduced food reserves and it has been a standard recommendation to plant these varieties only when soil temperatures are above 60°F.

This inherent lower vigor of sweet corn becomes magnified when a specific seed lot has problems. Sweet corn seed vigor will be affected by seed growing conditions, seed conditioning practices, storage conditions, and length of storage. Seed companies spend significant resources evaluating sweet corn seed for quality (viability and vigor) prior to release and suspect lots are removed from sale. However, seed lots can decline between testing and sales. An acceptable seed lot can become problematic over time.

Sweet corn fields planted with reduced vigor seed will often have uneven stands with healthy plants next to smaller stunted plants.  This pattern will often be present across the field. Low vigor plants are less productive and can actually act as weeds in a field, taking resources away from healthy plants and reducing the potential for compensation (producing bigger ears or multiple ears as occurs with remaining plants in fields with stand losses). Reduced vigor seed will produce these field patterns in warm weather but the effects are most severe in cold soils.

Stand reductions are also common with reduced vigor seed. Often, when digging up the seedlings and examining the seed remnants and mesocotyls of stunted plants, the kernels will be disintegrated and there will be darkening at the mesocotyl attachment. This means that the seeds deteriorated prematurely and the full content of the food reserves in the seed were not available for seedling development leading to the stand and vigor issues.

Seed viability is measured with a germination test which is done under optimum temperature, moisture, and light conditions. However, germination tests do not directly measure seed vigor, and seed vigor declines before germination is reduced. Therefore, it is possible to have seed that will germinate in a field but be of low enough vigor that sweet corn plants do not grow properly.

If a seed lot is suspected of having low vigor, then seed vigor tests are recommended. Testing for vigor is also very important for carryover seeds or seeds stored for long periods in unfavorable conditions. Seed vigor testing is also useful when troubleshooting fields where seed vigor issues are suspected (testing left-over seed).

Tests that are used to evaluate seed vigor that are available from different state and private seed laboratories include:

The Cold Test – Seeds are germinated using a specific cold, moist treatment regime. This will be useful in selecting those lots that will perform the best under early cold soil conditions.

Seedling Vigor Classification Test (SVCT) – In this test seedlings from a normal germination test are rated visually according to vigor (strong or weak). Visual ratings are based on if the seedlings have normal developmental characteristics in all seedling plant parts. With sweet corn this would be the roots, the mesocotyl, and the coleoptile. In low vigor seed one or more of these parts will be abnormal. This is a good test to troubleshoot suspect low vigor seed lots.

Tetrazolium (TZ) Test – This is a quick biochemical test that essentially stains living tissue in a seed a red color. The more red staining, the more viable the seed. This test is good for spotting lots with significant differences in vigor between seeds.

Accelerated Aging Test (AAT) – In this test, seed is put under a high temperature and humidity regime for a period of time and then is evaluated using a standard germination test. This is often used to check the storability of seeds under less than ideal conditions but also will do a good job of evaluating seed vigor. Modifications to the Accelerated Aging Test have been made to do a better job of evaluating sweet corn types such as shrunken sh2 varieties.

Electric Conductivity Test – This test measures cell membrane integrity which is correlates well with seed vigor and sweet corn seed emergence. As seeds age and cell membranes deteriorate, cell contents leak, the more leakage, the higher the electrical conductivity and the lower the seed vigor. This is most useful in comparing different lots after extended storage.

Most seed companies also grow out sweet corn lots in field tests prior to sales (commonly in winter nurseries) to confirm results from germination and vigor tests that have been performed.

Guess the Pest!

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Congratulations to Howard T. Callahan for correctly identifying the seedling corn injury as bird damage 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 below to participate in this week’s Guess the Pest!

The damage to seedling corn from this past weeks Guess the Pest was bird damage. This was actually damage from a wild turkey that uprooted the seedling corn plant to feed on the corn seed. While turkey injury on seedling corn is unusual, bird damage from blackbirds and crows unfortunately isn’t.

In Delaware, we have a Special Local Needs (SLN) 24-C label for the use of Avipel Hopper Box (Dry) and a SNL 24-C label for Avipel liquid seed treatment (commercially applied only) for managing blackbirds and crows in field corn and sweet corn. These labels will expire on July 15, 2017 and a copy of the label must be in your possession to use. To obtain a copy of the 24-C labels and for use directions please visit Arkion’s website at: http://arkionls.com/av/states/delaware.html

Guess the Pest # 2

What caused this damage to wheat? Think you know the answer…. Click on the Guess the Pest Icon below to submit your best guess.

What’s your best guess? Guess correctly and your name will be put into a hat for a chance to win a $100.00 Visa gift card at the end of the season. Each week, one lucky winner will also be selected to have their name entered into the end of season raffle not once but 5 times. Click on the “Guess the Pest” logo or go to https://goo.gl/forms/pWjHQUpmjABFB0v32 to submit your guess.

Bird Management in Field Corn and Sweet Corn

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Avipel Special Local Needs (SLN) 24-C Labels for Management of Blackbirds and Crows in Field Corn and Sweet Corn in Delaware in 2017: I wanted you all to be aware that the 24C SLN (24-C) label for the use of Avipel Hopper Box (Dry) has been renewed for the 2017 season. In addition, there will also be a SLN (24-C) label for the Avipel Liquid Seed Treatment (commercially applied only) for the 2017 season. Both labels will expire July 15, 2017. If you have questions, please call me at 302-530-8948 or send an e-mail to jwhalen@udel.edu.

For more information on how to use these products as well as a copy of the 24C labels, which must be in your possession to use, please visit Arkion’s website at http://arkionls.com/av/states/delaware.html.

Agri-Mek SC (Syngenta) for Spider Mite Control

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

The Agri-Mek SC label contains revised use instructions or restrictions as well as new labeled crops (including dry and succulent beans, soybeans and sweet corn). Be sure to read the label before making an application for use rates and all restrictions including but not limited to adjuvant requirement that must be followed to avoid illegal residues. NOTE – this is the only labeled formulation of abamectin available for use on dry and succulent beans, soybeans and sweet corn for spider mite control http://www.cdms.net/ldat/ld9NL020.pdf