Guess the Pest! Week 9 Answer: Sulfur Deficiency

David Owens, Extension Entomologist, owensd@udel.edu and Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu

Congratulations to Ben Coverdale for correctly answering sulfur deficiency on corn. Ben is going to be the proud new owner of a sweep net, to which all sorts of useful equipment could be attached on the handle, like a soil probe or a knife to take nutrient samples. Now if a sweep net could be included with a swiss army knife… All other correct guessers will be entered for an end-of-season raffle.

From Jarrod Miller
Sulfur deficiencies have been observed in the last couple of weeks across the state. Sulfur deficiency starts on the new growth because S is not mobile in the plant. In fact, S deficiency can cause the whole plant to be lighter in color. Another symptom of S deficiency is the appearance of stripes (interveinal chlorosis), as seen in this photo. While these stripes may also indicate a micronutrient or magnesium deficiency (and those who guessed magnesium are also entered for the end of season raffle), the most likely cause of this striping is a lack of S. We feel confident that S is likely the cause of this symptom, as we have observed it in similar conditions; corn grown on sandy, low organic matter soils. Plus, we have confirmed S deficiency with tissue testing in past seasons. Crops used to get more than enough S from the atmosphere. However, S deposition has been greatly reduced as technologies have reduced S release to the atmosphere when we burn fossil fuels. Now, the primary source of S to growing crops is soil organic matter. Unfortunately, Delaware soils are typically low in natural organic matter. In addition, the sulfate form of S is easily leached below the root zone; S leaching is also more likely in sandy soils. We recommend tissue testing to confirm S deficiency for sandy soils, especially if the field has not recently received manures or S containing fertilizers. Sample the whole plant up to 45 days after emergence or the 3rd leaf between 45-80 after emergence. If S in tissue is below 0.18% or if the N:S ratio in tissue is greater than 15:1, the corn is S deficient. If caught early in the season, apply 30 to 40 lb/acre of S. Apply a lower rate if you have evidence of S deeper in the soil profile (deep soil sample), or if you already added S with your starter fertilizer. However, remember that excessive application of ammonium sulfate (or a reduced form of S) can have an acidifying effect, resulting in lower soil pH. Soils receiving regular applications of acidifying fertilizer will require more frequent application of limestone to manage soil acidity in the long-term.

Growing Degree Days through June 2

Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu

Steady warm weather and periodic rainfalls have moved corn along pretty well. Most corn planted here in Georgetown in late April is at V6-7 and has been sidedressed. Fields planted through mid-May are at V5/V6, matching the predicted GDD pretty well.

The next milestone to watch for would be tasseling (VT), which occurs at 1135 GDD. At this stage just consider how weather may determine pollination and later grain fill as another determinant of final yield.

Table 1: Accumulated growing degree-days based on planting dates through May 20th.

If you planted

Sussex Kent New Castle
Apr 14 805 760 723
Apr 21 713 669 636
Apr 28 632 593 556
May 5 532 512 484
May 12 431 414 393
May 19 361 350 333

V6 = 475 GDD, V12 = 870 GDD, VT = 1135 GDD, R1 = 1400 GDD

Statewide Temperatures Since April 1st

Statewide Rainfall Accumulation Since April 1st

Guess the Pest! Weeks 7 & 8 Answer: Wireworm!

David Owens, Extension Entomologist, owensd@udel.edu

Entomologist, owensd@udel.edu

The last Guess the Pest was a bit of a headscratcher in a picture that, if seen in a field, probably would’ve been easier to figure out. Especially if a knife is fixed to the end of a sweep net handle to dig up suspect looking plants.

In several grass and grain crops, wireworm damage is called ‘dead heart’ where the whorl or emerging leaf dies because the wireworm has destroyed the growing point under ground. Larger plants might not be fed on entirely or the wireworm does not hit the growing point. These plants are more likely to show the blotchy yellowing of the leaves. Wireworms are susceptible to seed treatments and to pyrethroids in the furrow. Our northern neighbors in Canada are required to scout fields before a seed treatment can be legally applied. This past spring, I sampled a few fields with bait traps to assess wireworm presence. Bait traps can be time consuming, and the field shouldn’t be disturbed for the 2 weeks the bait is out, a tall order when soil temperatures warm. Cover crops may also interfere with bait attractiveness. Another scouting technique is the compact soil sample method, performed in either fall or spring. This consists of digging an 8 inch x 8 inch x 6 inch deep hole and relating numbers to action thresholds. The other reason I like this method is that a hole has to be dug if the field is being baited, so if there is an economic threshold, odds are you are going to identify it in the process of digging bait holes in addition to when the bait is checked about 2 weeks later.

Sulfur Deficiency on Field Corn

Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu and Amy Shober, Extension Nutrient Management and Environmental Quality Specialist; ashober@udel.edu

As corn has reached V3-V5 across the state, we have started to observe visual symptoms of sulfur (S) deficiency. Like nitrogen (N), sulfur deficiency can cause plant tissue to appear light green or yellow in color. However, one can tell the difference between N and S deficiencies by where they occur on the plant. Nitrogen deficiencies start on the lower (older) leaves because N is mobile in the plant. In contrast, S deficiencies start on the new growth because S is not mobile in the plant. In fact, S deficiency can cause the whole plant to be lighter in color. Another symptom of S deficiency is the appearance of stripes (interveinal chlorosis), as seen in this photo. While these stripes may also indicate a micronutrient or magnesium deficiency, the most likely cause of this striping is a lack of S. We feel confident that S is likely the cause of this symptom, as we have observed similar conditions that is grown on sandy, low organic matter soils. Plus, we have confirmed S deficiency with tissue testing in past seasons.

Crops used to get more than enough S from the atmosphere. However, S deposition has been greatly reduced as technologies have reduced S release to the atmosphere when we burn fossil fuels. Now, the primary source of S to growing crops is soil organic matter. Unfortunately, Delaware soils are typically low in natural organic matter. In addition, the sulfate form of S is easily leached below the root zone; S leaching is also more likely in sandy soils.

This year we have observed S deficiencies statewide, including in finer textures soils around Middletown. This may be explained by the cold temperatures observed mid-May. With nighttime temperatures in the lower 40s, release of S from organic matter was slower than normal, causing S deficiencies to appear when temperatures increased, and corn growth increased. We suspect that crops growing in soils with no history of S deficiency will grow out of the deficiency with time as warmer temperatures increase organic matter mineralization and roots begin to reach S that is held in subsoils.

However, corn growing on sandy soils with low organic matter may not grow out of their S deficiencies with warmer temperatures and increased rooting depth. We recommend tissue testing to confirm S deficiency for sandy soils, especially if the field has not recently received manures or S containing fertilizers. Sample the whole plant up to 45 days after emergence or the 3rd leaf between 45-80 after emergence. If S in tissue is below 0.18% or if the N:S ratio in tissue is greater than 15:1, the corn is S deficient. If caught early in the season, apply 30 to 40 lb/acre of S. Apply a lower rate if you have evidence of S deeper in the soil profile (deep soil sample), or if you already added S with your starter fertilizer. However, remember that excessive application of ammonium sulfate (or a reduced form of S) can have an acidifying effect, resulting in lower soil pH. Soils receiving regular applications of acidifying fertilizer will require more frequent application of limestone to manage soil acidity in the long-term.

Corn Leaf Stages and Growing Degree Days

Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu

With the fluctuation in temperatures since mid-April, corn emergence and growth has shifted week to week. At the research station, we have observed corn emergence take up to ten days planted April 24th, but only five when it was planted May 8th. Rising temperatures accumulate growing degree days (GDD) in less time, so that would be expected. Looking across the region, any corn planted on May 12th should have already emerged in Sussex, or be close to emerging in New Castle (as of May 20th).

Following emergence, the next important stage to manage is V6-V8, where you would typically sidedress corn. The V stage means six leaf collars, which can be identified as the white circle around the base of a corn leaf (Figure 1). Emerging, or recently emerging corn leaves will not have a collar yet. In Figure 1, counting the collars puts this field at V3. As you scout fields, some plants may be at the next stage, while the rest will catch up in a day or two.

Statewide temperatures and rainfall since April 1st can be seen in Figures 2 and 3. The rapid increase in temperature over the last week should have sped up emergence as well as advancing corn to the next stages. Rainfall over the weekend mostly hit the southern part of the state, increasing totals around Dagsboro and Delmar, but we have seen total rainfall of 6-9 inches since April 1st across the state.

Figure 1. Locating leaf collars on corn (left). Counting these collars will get you the corn stage (V3 in this case) to compare to GDD (right).

Table 1. Accumulated growing degree-days based on planting dates through May 20th.

If you planted

Sussex Kent New Castle
14-Apr 498 465 441
21-Apr 407 372 354
28-Apr 325 297 274
5-May 225 216 202
12-May 125 118 112

Emergence = 120 GDD, V6 = 475 GDD.

Figure 2. Statewide temperatures since April 1st.

Figure 3. Statewide rainfall accumulation since April 1st.

Pythium Causing Damping Off in Corn

Alyssa Koehler, Extension Field Crops Pathologist; akoehler@udel.edu

Over the past week, post-emergent damping off of corn has been present across the state. In most cases, this damping off has been caused by Pythium sp. Symptoms can include stunted, slower growing plants, to severely infected, dead plants (Figure 1). Infected plants typically have brown, rotted roots and mesocotyl. In severely infected plants, the top of the plant may be completely separated from the root system, resulting in plant death (Figure 2). Damping off from Pythium is common in low field areas that hold more moisture, but wet, cool spring conditions have favored development across entire fields this season.

Figure 1: Damping-off of corn caused by Pythium

Pythium is a soilborne fungal-like organism that is able to survive in the soil for many years as oospores. Under favorable environmental conditions, the oospores are able to germinate and produce small zoospores that swim in soil water following root exudates to infect emerging seedlings. Once root systems have developed, seedlings can usually survive mild to moderate Pythium infections. Seed treatments with oomycete activity can provide some protection for 10-14 days after planting, and can be helpful for improving seedling emergence and reducing pre-emergent damping off. This year most issues have occurred as post-emergent damping-off. Multiple species of Pythium are able to infect corn, with each species having a different optimal temperature. We are currently collecting samples to identify which species have been involved in infection this year.

Figure 2: Corn seedling with damping-off caused by Pythium

Field Crops Disease Management Resources and 2019 Guides

Alyssa Koehler, Extension Field Crops Pathologist; akoehler@udel.edu

The 2019 Fungicide Efficacy for Control of Corn Diseases table is now available through the Crop Protection Network https://cropprotectionnetwork.org/download/5214/. 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 https://cropprotectionnetwork.org/library/.

Insecticide Trial Results for Vegetable and Agronomic Crops

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

Summaries of last season’s insecticide trials in peas, sweet corn, watermelon, field corn, soybean, and wheat can be viewed at https://cdn.extension.udel.edu/wp-content/uploads/2013/12/15092741/Delaware-Field-and-Vegetable-Crop-Insect-Pest-Management-Trials.pdf.

Potential Hurricanes and Flooding

Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu

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 (https://soybeans.ces.ncsu.edu/2018/09/soybean-considerations-following-hurricane-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.