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

Agronomic Crop Insect Scouting

David Owens, Extension Entomologist; owensd@udel.edu

Insecticide Update
The following insecticides and seed treatments have been voluntarily removed by the registrants: Meridian 0.20G, Meridian 0.14G, Avicta Complete Corn 500, THX MXM FDL TBZ FS, Adage Deluxe, Adage Premier, Emesto Quantum, V-10170, Inovate Seed Protectant, Inovate Neutral Seed Protectant, Aloft GC, and Flower, Rose&Shrub Care III. More information can be found here: https://www.federalregister.gov/documents/2019/05/20/2019-10447/product-cancellation-order-for-certain-pesticide-registrations.

Early Season Moth Activity

Trap Location True Armyworm per night Black Cutworm per night
Willards, MD 0.1 0.1
Salisbury, MD 0 0.4
Laurel, DE 0 3.3
Seaford, DE 0.4 0.4
Bridgeville, DE 0
Harrington, DE 0.1 0.7
Smyrna, DE 0.3 2.3
Kenton, DE 0 0.3
Pearson’s Corner, DE 0.1 0
Sudlersville, MD 0.7

We have had lower armyworm activity in our blacklights this year compared to last, however you should still look to make sure that you don’t have a population that is clipping heads. If you have significant head clipping, worms are active, and smaller than an inch, you may want to consider an insecticide. Pay attention to pre harvest intervals, as most pyrethroids have a 30 day PHI. Mustang has a 14 day PHI, and Prevathon has a 1 day PHI.

Growing Degree Days (GDD) and Rainfall through May 14th

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

Since the rainfall this weekend, cooler temperatures have slowed the accumulation of growing degree days. Since April 14th we had observed a steady trend in GDD accumulation, keeping emergence on track between 7-10 days. Any planting done since May 6th may see delayed emergence, compared to previously planted fields. For those fields planted mid-April you may be at V4-V5. As temperatures warm back up, be prepared to sidedress between V6-V8.

The cooling trend is more apparent when you look at statewide temperatures and rainfall. Compared to nighttime lows, daytime temperatures had a significant drop since the weekend. Rainfall accumulation was between 1.5-2 inches, with the Harrington area still leading the state with 8 inches since April 14th. However, compared to last year these intermittent rainfalls are allowing fields to drain and not leaving all the duck ponds we had in 2018.

Figure 1. Growing degree days in the Dover area since April 14th.

Figure 2. Statewide temperatures since April 14th.

Figure 3. Statewide rainfall accumulation since April 14th.

Scouting Fields for Fusarium Head Blight

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

We had quite a few rain events during the flowering windows for barley and wheat, which kept us at high risk for Fusarium Head Blight (FHB). Once wheat has flowered, symptoms of FHB are visible in 18-24 days. Heads with FHB will have bleached florets or bleached sections of the head (Figure 1) and may have pink growth on spikelets. (Glume blotch typically has more of a grey appearance). You can follow these steps to assess the level of FHB present in your field.

Figure 1: Wheat heads with symptoms of Fusarium Head Blight

  1. For every 10 acres of field, randomly select one spot to survey.
  2. Keeping your line of sight above the wheat heads, walk 40-50 yards and randomly pick 10-20 heads to place into a bag. (You don’t want to be looking down and biasing the heads you select).
  3. Once you have randomly collected the heads, rate them for the percent of FHB on each head by looking at the visual symptoms (bleaching or pink growth on spikelets).
  4. After you have recorded values for each head, determine the average percent FHB severity by dividing the sum of disease severities by the total number of heads collected.

(Ex. You rate 10 heads with severity values: 0, 10, 30, 0, 0, 20, 10, 0, 0, 0. These add up to 70. 70/10 heads = 7% FHB severity)

Higher levels of FHB are typically associated with elevated levels of DON and possible issues with yield and test weight.

  1. Repeat this assessment as needed to get an overall rating for the field. Fields with greater than 10% FHB severity are at higher risk for yield losses or elevated DON. Fields with elevated DON should be harvest as early as possible and you may want to consider increasing combine fan speeds and shutter openings to reduce the amount of scabby kernels harvested.

Agronomic Crop Insect Scouting

David Owens, Extension Entomologist; owensd@udel.edu

Early Season Moth Activity

Trap Location True Armyworm per night Black Cutworm per night
Willards, MD 0 1.7
Salisbury, MD 0.1 2.0
Laurel, DE 0.1 6.0
Seaford, DE 0.7 1.3
Bridgeville, DE 0
Harrington, DE 0.1 1.7
Smyrna, DE 0.3 13.3
Kenton, DE 0.1 2.6
Pearson’s Corner, DE 0 1.6
Sudlersville, MD 0 0.3

 

Black cutworm larvae have been observed in some fields cutting plants. Most infestations are well below economic thresholds. Cutworms are large enough to cut plants once 300 DD have passed from a ‘significant moth flight’. This is defined as a trap catch of 9 moths or more for 2 consecutive nights. Since we only check our traps once a week, it is more difficult to determine when this happens, but a trap would have to catch at least 18 moths in a week. We had a large number of moths in Seaford and Laurel the week of April 10, spreading to Smyrna the following week. We have now accumulated enough degree days to see large worms. Scout your fields, and especially fields that had living cover crop or living weeds in the middle to end of April. Remember, moth counts in a trap do not mean that your field will have a damaging level of black cutworms. Also, black cutworm, while notorious, is not the only cutworm species present. Whether or not cutworms develop into large worms will depend, in part, on the Bt trait package present in your corn. Trait packages with Cry1F or Vip3A proteins should control cutworm. You can find all of the trait packages effective on cutworm and true armyworm here: https://lubbock.tamu.edu/files/2018/11/BtTraitTableNov2018.pdf

True armyworm developing in cover crop has been observed feeding on corn in nearby states, including Maryland’s eastern shore. Threshold are pretty high, 25% infested plants. True armyworm is only going to be controlled by Vip3A containing traits.

Soybeans
Be sure to watch emerging soybean stands like a hawk. There are three pests that we have to think about when planting soybean: seedcorn maggot, slugs, and bean leaf beetle. The first two do not have good rescue treatment options. We are at generation 1’s peak seedcorn maggot flight activity now. They are favored by cool, wet conditions with very recent cover crop incorporation (within the last few days). You can either delay planting about a week to avoid maggots or plant with an insecticidal seed treatment. With slugs, recent cool weather and rains will favor their activity. Early in the morning or at night, kick some field residue around and you may find them, or you can place a square foot shingle or board or cardboard out in location and count slugs underneath it the next morning. Generally, a good level of concern is 3 or more slugs. This doesn’t mean you will have problems, because slug activity is heavily influenced by environmental conditions. Hot, dry conditions are not favorable for slug activity. Slug bait is an option, but by the time stand loss is observed, damage has already been done. Vertical tillage or turbo-tilling can help disrupt the slugs. By the time a field is replanted, weather conditions may change and then it becomes a question of did the intervention work or the weather is just better? Planting without a neonicotinoid seed treatment can help preserve beneficial, slug-feeding ground beetles. Closing furrows and removing residue around the furrow with row cleaners can also help protect the seed and encourage faster germination. Once seedlings emerge from the soil, they become harder for slugs to kill. Bean leaf beetle are also active. Most feeding is cosmetic; the soybean plant will recover. Thresholds prior to 2nd trifoliate are 2 per foot AND 25% stand reduction. After this stage, thresholds increase dramatically to 2-3 per plant and 30% defoliation.

Nitrogen Fertilizer Additives

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

Nitrogen (N) can be found in many forms in the soil, some of which can be lost as a gas or through leaching. Some commercial products have been developed to keep N in available forms that are not easily lost until the plant can take them up. These include Agrotain Ultra, Nutrisphere-N, NZone, and Instinct. In a recent study on urea granules, only Agrotain Ultra was successful in slowing the transformation of urea into ammonia, which can be lost as a gas. Both Nutrisphere-N and NZone were not successful.

Nitrification is a process that transforms N into nitrate, a plant available, but easily leached form of N. This same study also observed nitrification with urea, SuperU granules, and urea treated with Nutrisphere-N, NZone, or
Instinct. Of these products, only Super U granules and those treated with Instinct had slower nitrification. Both Nutrisphere-N and NZone were not successful.

The study can be found in the Agronomy Journal and currently has open access to the public: https://dl.sciencesocieties.org/publications/aj/articles/111/3/1441 

Growing Degree Days (GDD) and Rainfall through May 7th

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

Compared to last year, our soils and weather have been more conducive to planting. Our no-till fields at the station have been the best to plant in this year, while rainfall this weekend has slowed some work in fields that received conventional tillage.

If you have been scouting your fields, there should certainly be some corn emergence across the state. It takes up to 120 GDD to see corn emergence, and fields planted by April 28th in Sussex County should all have emergence. Only New Castle County has been cool enough to slow emergence, but any field planted by last Sunday should still see corn pushing through in the next few days.

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

If you planted -> Apr 14 Apr 21 Apr 28
Sussex 304 214 132
Kent 285 192 117
New Castle 276 189 109

Emergence = 120 GDD, V6 = 475 GDD.

Similar to 2018, there is a variable amount of rain that has fallen across the state since April 14th. The region around Harrington has received the most, at almost 6 inches in the last few weeks, while Newark and Dover have received about 3.5 inches. In 2018, Georgetown had almost 5 inches at this point, similar to 2019. While the total amount of rainfall across the state between 2018 and 2019 is similar, the major difference is the size of the storms. In 2018 Georgetown observed an event that deposited almost 3 inches, while this year we have received several 1 inch events over several weeks, giving soils enough time to drain and dry out.

Irrigated Soybean Seeding Rates

Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu and Cory Whaley, Sussex Co. Extension Ag Agent; whaley@udel.edu
Research was conducted on irrigated soybean seeding rates at the Warrington Research Farm in 2017 and 2018. Soybeans were planted in 15” rows at rates of 80, 100, 140, and 170 thousand seeds per acre in irrigated and dryland plots. No yield advantage was observed at higher seeding rates, while the use of irrigation improved yields by an average of 7 bu/acre over both years. With no difference in yield by seeding rate, projected income was greater at the lowest seeding rate of 80k seeds. This represents only two seasons of research on one soil type, but does indicate that Delaware soybean producers could potentially lower their seeding rates and not see a decrease in yield.

To read the full report see: https://www.researchgate.net/publication/329811290_2017-2018_Irrigated_Soybean_Seeding_Rates

Wheat Disease Update

Alyssa Koehler, Extension Field Crops Pathologist; akoehler@udel.edu
We have had quite a few rain events with more in the forecast for this weekend. The rain and warm temperatures are maintaining another high-risk year for Fusarium Head Blight. If your wheat is close to flowering or, already flowering and you are considering a fungicide application, the optimal window is anthesis until about 5 days after. Once wheat has finished flowering, if your plants were infected, FHB symptoms will be visible in 18-24 days. Next week we will discuss steps for scouting and assessment of FHB damage. We have seen a few lesions caused by Parastagonospora nodorum (formerly Stagonospora nodorum). Symptoms include small brown lesions with a yellow halo (Figure 1). This fungus causes Septoria nodorum blotch, also be referred to as Stagonospora nodorum blotch, and it is the same causal agent of Stagonospora glume blotch. Since leaf blotch precedes glume blotch, lesions high in the canopy and on the flag leaf can indicate an elevated risk for glume blotch.

Figure 1: Septoria nodorum blotch lesions on wheat leaves