Sulfur Deficiency in Sweet Corn

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

For the last couple of years, I have talked about and conducted research on sulfur (S) deficiencies in watermelon. At several meetings I was asked if the deficiency also appears in (field) corn. I could only relate the comments I have heard from several corn growers that they have often found their corn crop to be low or deficient in sulfur. I bring all this up again because I have seen several sweet corn fields lately that have sulfur deficiencies (Fig. 1). One of the possible reasons we are seeing more S deficiency is because less sulfur is being deposited into the soil from the atmosphere due to reductions in acid rain (Fig. 2). In 1986 about 24 lbs/a of sulfate were deposited in Maryland soils per year, however in 2011 it was closer to just 8 lbs/a each year. Organic matter supplies most of the sulfur to the crop, but sulfur must be mineralized to sulfate-S to be taken up by crop plants.

Because mineralization is carried out by soil microorganisms, soil temperature and moisture primarily determine when and how much sulfur is made available to the crop. Excessively wet (like we have had most of this summer) or dry conditions reduce microbial activity and reduce S availability from soil organic matter. Sulfate is relatively mobile in most soils and easily can be leached from soils, especially sandy soils. Soils low in organic matter are much more likely to be deficient in S.

In the field sulfur deficiency can be highly variable since soil sulfur availability varies considerably with soil organic matter and texture. Sulfur deficiency is often seen in sandier, lower organic matter, hillier areas of a field while low lying, greater organic matter areas usually have sufficient levels of S. Sulfur can be added to the crop in combination with several other nutrients such as ammonium or potassium and spray-grade ammonium sulfate is a good choice for foliar applications.

There are other deficiencies (and problems such as root diseases) that can cause striping (although the one pictured here was S) and only by conducting a tissue test can you be sure. Magnesium deficiency may cause striping and/or reddening of corn leaves. The yellow areas between the veins often appear as ‘beaded’ lines rather than solid stripes. Zinc deficiency may cause striping that begins at the base of the leaf and progresses to the tip. These stripes often coalesce to form a white band along the edge of the leaf or the midrib. Manganese deficiency causes striping that is olive green or dark yellow in color with veins remaining green. High pH, high organic matter, and dry soil conditions can cause Mn deficiency.

Figure 1. Striping in sweet corn leaf caused by S deficiency

 

Figure 2. Amount of sulfate deposited in soil from rainwater in 1986 vs. 2011.

Vegetable Insect Updates

David Owens, Extension Entomologist, owensd@udel.edu and Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Sweet Corn
Bill Cissel and David Owens
Corn earworm populations are higher than last week, and yesterday’s trap capture was higher in all but one location from Monday’s trap capture. At the research station, our trap captures increased significantly in the last two nights. Blacklight trap captures are also increasing. Focus more on the state-wide trends. Monday trap capture can be found at (http://agdev.anr.udel.edu/trap/trap.php). As a reminder, what is reported on the web is on a per night basis, the table below is cumulative over Monday, Tuesday, and Wednesday night. These trap captures correspond to a 3-day spray schedule, though some states recommend a 2-day schedule.

Joe Ingerson-Mahar at Rutgers in New Jersey recently wrote about the need for product rotation, citing pyrethroid resistance monitoring Virginia has been doing (https://plant-pest-advisory.rutgers.edu/corn-earworm-control-in-sweet-corn/). We have been using Virginia’s vial testing method and have seen similar results, near 30% of tested moths can fly after 24 hour exposure. Does this mean pyrethroids are ineffective? No, but it does highlight the need for chemistry rotation. We also do not know how vial tests translate to sweet corn efficacy where we apply products several times in a narrow time frame.

Trap Location BLT – CEW Pheromone CEW
3 nights total catch
Dover 2 17
Harrington 1 28
Milford 2 41
Rising Sun 0 6
Wyoming 4 65
Bridgeville 3 63
Concord 3 56
Georgetown 4 32
Greenwood 2
Laurel 2 132
Seaford 5 52

Watermelon
David Owens
Spider mite populations have generally been decreasing, but can still be an issue with the warm weather. High humidity favors fungal pathogens of spider mites, but fungicide sprays can suppress them. Infected mites are going to look crumpled and may even be brown in color and a slightly fuzzy appearance as mite-killing fungi sporulate. Cucumber beetles are very active, as are various members of the rindworm complex. Yellow striped armyworms are present in fields.

Sweet Corn Insect Update

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu and David Owens, Extension Entomologist, owensd@udel.edu and

A whorl-stage sweet corn field on station had an economic infestation of fall armyworms, whorl thresholds are 15%. Be wary of fall armyworm in tassel-push corn, worms dislodged by the emerging tassel may go to the developing ear. Pyrethroids will not give complete worm control, scout fields soon after treatment. Other alternative mode of actions that are softer on beneficials include diamides (Coragen) methoxyfenozide (Intrepid), indoxacarb (Avaunt) and spinetoram (Radiant). Be sure to read the labels for use restrictions (indoxacarb cannot be used after tassel-push) and restrictions on the number of applications. A commonly used earworm product is Besiege which has chlorantraniliprole (Coragen) in it; earlier use of chlorantraniliprole may limit later use.

Corn earworm populations are higher than last week. Drier evening weather favors moth flight, and worms that developed in field corn are starting to emerge as adults. I expect moth flight activity to continue increasing state-wide until early-September. You may notice some trap locations that had been catching a lot of moths are now catching fewer; in some locations traps were adjacent to sweet corn that has since been harvested. However, other traps have been catching many more moths than they had been, especially in the Monday Laurel data. Blacklight trap captures are also increasing. Focus more on the state-wide trends. Monday trap capture can be found at (http://agdev.anr.udel.edu/trap/trap.php), and Monday trap captures were much higher from nearly all sites. As a reminder, what is reported on the website is on a per night basis, the table below is cumulative over Monday, Tuesday, and Wednesday night.

Trap Location BLT – CEW Pheromone CEW
3 nights total catch
Dover 0 7
Harrington 1 4
Milford 5 11
Rising Sun 7 5
Wyoming 7 25
Bridgeville 2 23
Concord 4 20
Georgetown 2 14
Greenwood 6
Laurel 0 78
Seaford 1 21

Guess the Pest! Week #15 Answer: Stink Bug

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

Congratulations Chris Leon for correctly identifying the damage in the photo as stink bug 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 to participate in this week’s Guess the Pest challenge!

Guess the Pest Week #15 Answer: Stink Bug

The damage on the corn stalk is stink bug feeding injury. Stink bugs will use their piercing-sucking mouthparts to probe into the stalk of the plant, removing plant fluids. If the stink bug hits the ear at this stage, the ear will often fail to develop kernels at the feeding site. This causes the ear to develop into the classic “C”-shaped or boomerang-shaped ear. This is why the greatest damage and yield loss potential due to stink bug feeding is prior to pollination. This is also why waiting until after tasseling (pollination), to control a stink bug infestation in field corn is too late. Here is a link to last week’s article discussing stink bug management in field corn: http://extension.udel.edu/weeklycropupdate/?p=12194

Guess the Pest! Week #14 Answer: Corn Rootworm

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

Congratulations to Kathleen Heldreth for correctly identifying the damage in the photo as corn rootworm 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 to participate in this week’s Guess the Pest challenge!

Guess the Pest Week #14 Answer: Corn Rootworms

The corn plants in the photo are damaged by corn rootworm larvae. As you can see, the larvae feed on the roots and root tissue of the plants causing the plant roots to be “pruned”. Older larvae will tunnel into the roots leaving visible entrance holes and blackened root tips. Plants with excessive root pruning will usually lodge and in reaching for the sun, become “goosenecked”. Corn rootworm infestations are unusual for Delaware and not something we typically have to manage for. Crop rotation is the preferred method of control in regions with sporadic populations. Corn rootworm females prefer to lay eggs in corn fields in August and September. The eggs do not hatch until the following spring. If the field is rotated out of corn, the larvae will starve to death in the absence of a suitable host plant.

Vegetable Insect Update – July 6, 2018

David Owens, Extension Entomologist, owensd@udel.edu and Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Sweet Corn
by Bill Cissel and David Owens
Sweet corn trapping data is updated by Tuesday and Friday mornings and can be accessed here: http://agdev.anr.udel.edu/trap/trap.php. If there is one thing insects are really good at, it is making liars out of people. That said, I expect low earworm numbers until mid-July. Trap catches are as follows:

Trap Location BLT – CEW Pheromone CEW Corn spray schedule
3 nights total catch
Dover 3 0 4 day
Harrington 0 0 No spray
Milford 0 0 No spray
Rising Sun 1 0 6 day
Wyoming 1 0 6 day
Bridgeville 0 0 No spray
Concord 0 0 No spray
Georgetown 0 1 6 day – no spray
Greenwood 0 0 No spray
Laurel 0 6 4 day
Seaford 0 1 6 day – no spray

 Cucurbits
by David Owens
With the warm weather, it is not surprising that low levels of spider mites can be found in most fields. Check field edges and near wood lines. The action threshold we use as a benchmark is 1-2 mites per crown leaf, 20 – 30% of the crown leaves infested. During hot dry spells, try to limit mowing as much as possible, spider mites feeding on the grasses will be forced to look elsewhere for food once the plant they are on has been cut. I have also noticed a slight uptick in cucumber beetle activity.

Sweet Corn Insect Management

David Owens, Extension Entomologist, owensd@udel.edu and Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Sweet corn trapping data is updated by Tuesday and Friday mornings and can be accessed here: http://agdev.anr.udel.edu/trap/trap.php. As we suspected last week, trap captures have plummeted in part, I am sure, because we are trying to capture sizeable earworm moths for pyrethroid resistance monitoring. We typically experience a lull in moth activity until mid-July. Trap catches are as follows:

Trap Location BLT – CEW Pheromone CEW Corn spray schedule
(3 nights total catch)
Dover 1 3 5 day
Harrington 1 0 No spray
Milford 0 0 No spray
Rising Sun 0 15 4 day
Wyoming 0 1 6 day – No spray
Bridgeville 0 1 6 day – No spray
Concord 0 0 No spray
Georgetown 0 0 No spray
Greenwood 0 0 No spray
Laurel 1 1 6 day – No spray
Seaford 0 1 6 day – No spray

Sweet Corn Insect Management

David Owens, Extension Entomologist, owensd@udel.edu and Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Sweet corn trapping data is updated by Tuesday and Friday mornings and can be accessed here: http://agdev.anr.udel.edu/trap/trap.php . I expect earworm counts to start decreasing; you can look at our historical trapping data

Trap Location BLT – CEW (3 nights total catch) Pheromone CEW

(3 nights total catch)

Corn Spray Schedule
Dover 1 8 4 day
Harrington 0 4 4-5 day
Milford 0 3 5 day
Rising Sun 0 22 4 day
Wyoming 0 0 No spray
Bridgeville 0 12 4 day
Concord 0 3 5 day
Georgetown 0 0 No spray
Greenwood 0 0 No spray
Laurel 0 7 4 day
Seaford 0 0 No spray

The spray schedule listed in the table is based off our sweet corn action thresholds which can be found here: http://extension.udel.edu/ag/insect-management/insect-trapping-program/action-thresholds-for-silk-stage-sweet-corn/ .

Sulfur and Vegetable Crops

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

With the recent heavy, leaching rains, we are seeing signs of sulfur deficiency in some vegetable crops. Sulfur is considered one of the secondary macronutrients that vegetable crops require for growth. Sulfur is a component of four amino acids and is therefore critical for protein formation. It is also a component of certain glycosides that give pungency to mustard family crops (greens, cole crops) and Allium crops (onions, garlic).

In the last 25 years, as industrial air pollution has been reduced (especially pollution from coal fired power plants) we have had less sulfur deposition from rainfall. Sulfur deficiencies are more common and sulfur additions in fertilizers or manures is being required for many crops to produce high yields.

Most of the sulfur in the upper part of the soil is held in organic matter. Upon mineralization, sulfur is found in the soil as the sulfate ion (SO42-) which has two negative charges. The sulfate ion is subject to leaching, especially in sandy textured soils (loamy sands, sandy loams). It does accumulate in the subsoil but may not be available for shallow rooted vegetables.

Sulfur can be added by using sulfate containing fertilizers such as ammonium sulfate, potassium sulfate, and K-mag (sulfate of potassium and magnesium). It is also a component of gypsum (calcium sulfate). In liquid solutions, ammonium thiosulfate is often used as the sulfur source. Sulfur is also found in manures and composts. For example, broiler litter has about 12-15 lbs of sulfur per ton.

In vegetable crops, sulfur removal is generally in the 10-20 lb/A range. Mustard family crops (cole crops such as cabbage and broccoli, mustard and turnip greens, radishes) remove between 30 and 40 lbs/A of sulfur. Research in our region has shown response to added sulfur for sweet corn and for watermelons. In Florida research it was shown that adding 25 pounds of sulfur per acre boosted yields by 1.7 tons per acre in tomatoes. Similar results were found with strawberries.

Our general recommendations are to apply 20-30 lbs of sulfur per acre on sandy soils for most vegetable crops. Remember to take credit for any sulfur being added with fertilizer sources such as ammonium sulfate (24% sulfur).

One vegetable where we want to limit sulfur is with sweet onions. Because sulfur increases onion pungency, and sweet onions are sold based on their low pungency, we limit sulfur applications to this crop.

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.