Provide Comment on Extension Plant Pathologist Candidates

Four candidates for the Extension Plant Pathologist Faculty Position have visited University of Delaware and made presentations about their research and Extension experiences. These presentations have been recorded and may be viewed online. We value input from the Delaware agriculture community on these candidates. You can view the presentations and provide comments online at: Recordings and opportunity for comment will be available until June 15.

Considerations for Early Fertigation of Corn

James Adkins, Associate Scientist-Irrigation Engineering;

With the extreme wet weather holding up sidedress fertilizer applications, several farmers are looking towards fertigation to address immediate fertility issues. If you do not typically fertigate, or it been a few years since your fertigation system was used, you should be able to answer the questions below before considering injecting fertilizer through any irrigation system. If you decide to fertigate, turn off endguns where possible as the application rate ranges from 0.2 – 5 times what is applied under the main span. You should also check the pump frequently and mark the supply tank at regular interval to track the application rate and detect any injection pump calibration issues.

Here are some questions you should ask before fertigating:

Can you make a full circle with the pivot without getting stuck?
A stuck pivot while fertigating can be a disaster especially if the overwatering timer isn’t functioning the machine doesn’t shutdown automatically. If you typically have problems with machines getting stuck after an intense irrigation season an application on already saturated soil will only accelerate the rutting process, making the rest of the season more difficult.

Do you have the proper equipment?
Fertigation requires backflow prevention, calibrated injection pump(s), power supply at each pivot, and uniform irrigation application.

Is your fertigation pump and power supply functional?
Check valves stick, seals fail and electrical power supplies get bypassed with midseason emergency pivot repairs; check all your equipment before committing to fertigation.

Do you have an accurate pivot timer setting chart and good application uniformity?
In the over 780 center pivot evaluations performed by UD, 10-15% have either no timer setting chart that includes full circle time or have incorrect full circle estimates. Hitting your target application rate is not possible without accurate full circle time numbers. Furthermore over 50% of systems had a 20% or more variation of applied irrigation which will translate into a 20% +/- variance from the targeted fertilizer application.

Injection pumps should be calibrated with back pressure on the injection line. This is very easy with a venturi/mazzei type or small diaphragm pump that includes a built in flowmeter as the pump can be adjusted on the fly. Positive displacement pumps like the piston and diaphragm pumps made by Inject-o-Meter and AgriInject have a pulsed output and are nearly impossible to calibrate with a sight glass flowmeter (a few specific electronic flowmeters can read pulsed flow – $800+ each). The best way to calibrate a pulsed injection pump is with a graduated tube installed with proper valving on the suction side of the injection pump and measuring the injected volume over time with the irrigation pump running. In absence of other calibration methods, measuring the volume injected into a container from the pressure side of the injection will give a rough estimate of the flowrate but keep in mind that the flow will typically drop when injecting into pressurized water.

Use this equation when calibrating an injection pump to a target fertilizer rate at a given pivot timer setting:

Use this equation when adjusting the pivot timer setting to match a known injection pump rate:

Backflow Prevention – Chemigation Valve
Whenever any material is injected into an irrigation system a backflow prevention device is required by law. Backflow prevention is more complicated than just the basic check valve typically included on most irrigation pumps. A chemigation valve consisting of a spring loaded flapper type check valve, a low pressure drain and air/vacuum relief valve are all required in addition to an injection pump interlock switch (to prevent operation of the injection pump when the irrigation pump is off) and an injection line check valve (to prevent irrigation water from backfeeding through the injection pump into the supply tank).

More information regarding the calibration of fertigation systems can be found in this factsheet from South Dakota State:

Guess the Pest! Week #10

Bill Cissel, Extension Agent – Integrated Pest Management;

Test your pest management knowledge by clicking on the GUESS THE PEST logo and submitting your best guess. For the 2018 season, we will have an “end of season” raffle for a $100.00 gift card. Each week, one lucky winner will also be selected for a prize and have their name entered not once but five times into the end of season raffle.

This week, one lucky participant will also win A Farmer’s Guide To Corn Diseases ($29.95 value).

You can’t win if you don’t play!

What is this disease?

Guess the Pest! Week #9 Answer: Plum Curculio

Bill Cissel, Extension Agent – Integrated Pest Management;

Congratulations to Sylvie Childress for correctly identifying the ovipositioning scar and larva in the photos below as plum curculio 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 #9 Answer: Plum Curculio

The plum curculio is a pest of apples, peaches, plum, and other stone fruit. The adult beetles are about ¼” in length, dark brown with patches of white, and have a protruding snout. They belong to the weevil family, commonly referred to as the snout beetles.

Adult Plum Curculio

The primary damage to fruit is caused by the ovipositioning or egg laying behavior of the female plum curculio. The females lay eggs in the developing fruit and cut a crescent shaped slit beneath each egg to prevent the rapidly growing fruit from crushing the egg. As the fruit continues to grow, the slit the female beetle cut below the egg develops into the classic, crescent shaped scar that you see in the photo above. These scars are usually only cosmetic. However, if the egg hatches, the larva will bore into the fruit, which will usually cause the fruit to drop from the tree. After a couple weeks of feeding on the fruit, the larva will exit the fruit to pupate in the soil.

In addition to the crescent shaped ovipositioning scars and fruit drop, the adult beetles will also feed on fruit, creating numerous round puncture holes in the fruit skin. It has been estimated that a single beetle will average over 100 feeding and/or puncture wounds during its lifespan.

Fun Entomology Fact: The family Curculionidae (true weevils), are the largest family of insects with the most species described worldwide.

Use Caution when Selecting Adjuvants

Mark VanGessel, Extension Weed Specialist;

The weather patterns lately have resulted in situations where the risk of crop injury from postemergence herbicides is higher. Specifically, prolonged periods of overcast skies, cooler weather, and plenty of rain. If postemergence herbicide applications are made as the days turn hot and sunny, the risk of injury is greater. This is due to the wax layer on the leaves not having developed and the leave surface being “tender”. If spraying during these sensitive periods, switch to “softer” additives if the label allows it; for instance MSO increases the risk of injury over COC; and non-ionic surfactants (NIS or 80-20’s) reduces the risk further. Consider using the lower allowed rate of the surfactant or nitrogen. Be sure to read the label and see what is allowed by the manufacturer.

Corn Height Restrictions for Postemergence Herbicides

Mark VanGessel, Extension Weed Specialist;

Corn herbicides need to be applied at the correct timing to avoid crop injury; and the weeds need to be small (3-4 inches depending on the herbicide) for effective control. Many labels state from corn emergence to a certain size corn. However, some herbicides require the crop to be at a certain size before the herbicide can be applied (i.e. Status require corn to be at least 4 inches tall or V-2 stage). Almost all herbicides have a maximum crop size and this can range from V-2 to V-8. Maximum size depends on the herbicide and can vary based on whether the herbicide is applied over the top of corn or with drop nozzles. Applications after this time can result in crop injury and possibly yield reduction. Some labels refer to crop size based on height of corn in inches, collar stage, or leaf stage. Refer to the herbicide label to ensure applications are made at the appropriate crop stage. When corn height and collar number are given for the same herbicide, base decision on whichever feature is first attained. If tankmixing, use the guidelines based on the most restrictive herbicide.

Also, this year corn within the same field maybe at different stages. Evaluate the field and select the herbicide that allows application to the stages of corn that represent your field.

Heavy Rainfall is Making Nitrogen Management a Challenge

Amy Shober, Extension Specialist – Nutrient Management and Environmental Quality, University of Delaware,, Sydney Riggi, Extension Agent – Nutrient Management, University of Delaware,; Gurpal Toor, Extension Specialist – Nutrient Management and Water Quality, University of Maryland College Park,; Bob Kratochvil, Extension Specialist – Agronomy, University of Maryland College Park,; Jarrod Miller, Extension Specialist – Agronomy, University of Delaware,

Continued rainfall in our region has kept many growers out of the field again this week. These wet weather patterns are frustrating and serve as a reminder about the uncertainty and unpredictability of weather. Over the last four days, weather stations in Newark, Dover, and Georgetown report between 0.03 to 1.28 inches of rainfall and over the last month the Georgetown weather station has recorded 10.22 inches of rain. Not only has this excess rainfall affected the timely planting of corn (and even beans at this point), but has the potential to affect the crops that are already planted.

Nitrogen (N) management is one of the major challenges faced due to the wet conditions this spring. This type of wet weather is one of the main reasons we heavily advocate split applications of N to corn, despite the convenience of applying all N prior to or at planting. Early in the season, N uptake by corn will be about 1 lb of N per day per acre and N uptake will increase to approximately 3 lb N per day per acre during later vegetative stages. As such, if you applied 200 lb of N per acre prior to or at planting, uptake by the corn crop may be 30 lb per acre in the first 30 days; this leaves 170 lb of N per acre in the soil. This spring especially, N losses via leaching (movement of N below the root zone), runoff (movement of N in surface water flow), or denitrification (loss of N gas to the atmosphere) from fields that received all N prior to or at planting will be excessive, as shown in the below image.

In fields where only a small amount of N was applied prior to or at planting, the risk for early season N losses are reduced. This is because the bulk of the N is applied in a sidedress (or even one or more fertigation events) closer to the time that corn enters into rapid growth phase and the bulk of the N uptake occurs. The period of rapid N uptake occurs when corn is approximately 10 to 20 inches tall, which usually occurs sometime around 35 days after emergence. However, the N applied prior to sidedress, as well as residual N from past manure applications, cover crop biomass, or previous legume crops was susceptible to N losses this spring.

There is no way to know exactly how much N was lost from fields, as this requires precise measurement. While the extent of N loss will vary depending upon soil type, location, and amount of rainfall received, we can make some general assumptions about N losses. If we consider that 1 inch of rain adds 27,154 gallons of water to an acre and weighs about 113 tons, this 1 inch of rain will penetrate 6 to 15 inches in soil (depending on soil texture). This rain water will flush (leach) nitrate, which is the most common form of N in soils and is soluble and highly mobile (due to negative charge that prevents binding with soils), deeper into the soil profile. Since we received several inches of rain, we expect that nitrate has been leached below the current depth of plant roots. In addition, we have had several events where rainfall amounts exceeded the infiltration capacity of soils, resulting in ponding or even surface runoff. When runoff occurs, nitrate and N attached to soil particles is lost if you have erosion. In fields with ponding of water, conversion of nitrate to N gas, which will escape to the atmosphere, is highly likely.

Going forwards, growers should consider conducting a pre-sidedress nitrate test (PSNT) to determine if there is a need to add more N fertilizer. It is important to remember that the PSNT is most appropriate for fields where manure or other organic N sources were applied. Regardless of whether fields received all N up-front or a split application was planned all along, growers should contact their nutrient consultant to assure that in-season N application is in compliance with their nutrient management plan. As in season adjustments are made, Delaware nutrient consultants can write a justification for additional fertilizer applications. A copy of the addendum should be included with the original nutrient management plan before the end of the calendar year and will be essential if the farm is audited.

Sulfur Deficiency on Field Corn

Jarrod Miller, Extension Specialist – Agronomy,; Cory Whaley, Extension Agent- Agriculture, Sussex County,; Amy Shober, Extension Specialist – Nutrient Management and Environmental Quality; Phillip Sylvester, Extension Agent – Agriculture, Kent County,; Richard Talyor, Retired Extension Specialist – Agronomy

Our region has experienced plenty of cool, wet springs in the past, but the intensity and accumulated rainfall of the last few weeks presents additional challenges. Like nitrate, the plant available form of sulfur (S) in soils is easily leached below the root zone. Sulfur issues were quite rare on corn in the last few decades because crop need was met, or even exceeded, by deposition of S from the atmosphere. However, S deposition in Delaware has dropped from 25 to 5 lbs/acre between 1985 and 2010 due to stricter air emissions standards.

Sulfur doesn’t necessarily leave the soil profile, rather it accumulates deep in the soil. While corn roots will eventually take up this S, the roots may not reach these deeper S stores until after sidedress time. Past experiments conducted at the University of Delaware showed that gypsum application can provide an early growth response and mitigate S deficiencies in corn, but plants grown on control plots (no S) eventually caught up when roots reached deep S later in the season. This year is unusual though, and it is possible that the amount of rainfall could have moved more S deeper into the soil profile than normal.

Sulfur deficiencies are visually similar to N deficiency, with corn plants appearing light green or yellow in color. However, since S is not mobile in the tissue (unlike N), the whole plant may appear pale if you catch S deficiency early enough. In recent years, we have noticed that S deficiency shows as interveinal chlorosis of the recently mature (collar visible) leaves. These symptoms are also appearing this year, particularly on sandy, low organic matter soils.

Sulfur deficiency symptoms on corn

If possible, confirmation of a S deficiency should be performed with a tissue test. Apply additional S if the concentration of S in tissue falls below the critical level of 0.12% S or if the N:S ratio in tissue is greater than 15:1. In general, an application of 30 to 40 lbs S/acre is enough to supply the majority of the crop S need. If you believe that some S still remains deeper in the profile, or you already added S with your starter fertilizer, you could try applying S at a lower rate. However, remember that excessive application of ammonium sulfate (or a reduced form of S) can have a negative impact on soil pH and require more frequent application of limestone to neutralize the increase in soil acidity in the long-term.

Growing Degree Days Through May 29th

Jarrod O. Miller, Extension Agronomist,

It takes an average of 475 growing degree days (GDD) to reach V6, so anything planted the last two weeks of April should be in this range for all three counties. Be sure to scout your fields and check out our article on whether you need a PSNT:

Over the last week, the entire state has experienced about 21 growing degrees per day, so we can easily expect to reach V6 with any fields planted the first week of May if warm temperatures continue. It is possible that saturated conditions have slowed corn development, although there isn’t great information on how this may correlate to GDD. The warmer temperatures that came between rainfalls also could have promoted growth as well as denitrification in saturated soils.

Table 1: Growing degree days accumulated through May 29th from the beginning of each week.

  Sussex Kent New Castle
Sunday, April 22 588 582 526
Sunday, April 29 536 536 493
Sunday, May 6 427 426 395
Sunday, May 13 324 320 297