Denitrification and Corn Fertilizer Programs

Richard Taylor, Extension Agronomist; rtaylor@udel.edu

As a follow-up article to my article last week, I thought I should review what I’m seeing a lot of in northern Delaware and southern New Jersey.

When you look at many of the fields of corn in the northern part of Delaware and southern New Jersey, the impact of denitrification is readily visible (Photos 1-6). Many fields are showing typical N chlorosis symptoms that indicate that most, if not all, of the soil available nitrate-N has been lost as N2, NO or N2O gas. In some cases, the economic loss of N was minimal as the growers applied only small amounts of N as starter. However in other cases the amount of applied N was much greater and done to allow greater flexibility in the timing of sidedress N. In other cases, not only was commercial fertilizer as urea ammonium nitrate solution (UAN) applied but past efforts at building soil organic matter had been expected to add to the timing buffer via the effect of soil organic matter mineralization. In still other cases where growers were successful in planting corn early, sidedressed N application could have taken place before the last period of heavy rain.

The unusual weather pattern of 2013 (cold, followed by cold and wet, followed by dry and hot, and then this last period of moderate temperatures and heavy rainfall) led to slow mineralization of soil organic matter but complete denitrification of any soil N that had been mineralized and nitrified. In addition, in those fields that had been sidedressed with UAN early, a substantial portion of the urea and ammonium that had been nitrified to nitrate-N could have been lost to denitrification. The difficulty now will be getting on these wet fields with either sidedress N or additional N before the lack of N lowers the corn’s yield potential.

At this point in the growing season, we all probably wish for a crystal ball to tell us how much N remains in the soil, what the weather will be like for the rest of the growing season, how best to get N on our crops, and a host of other questions. Unfortunately, that type of crystal ball just doesn’t exist and the answers will likely vary from field to field. Each grower or consultant will need to evaluate the individual fields and try to make the best guess possible as to what needs to be done to produce the maximum economic return from each field. As a guide, you should keep in mind that N is a mobile element. What this means is that the plant will cannibalize the N from the older, lower leaves first and send that to either the growing point or later, after silking and pollination, to the grain that is being filled. The severity of N deficiency can to some degree be estimate by how many corn leaves on a plant are showing N deficiency symptoms. The more leaves affected the greater the severity.

Another question often asked is how much of the required N is taken up by the crop by a particular growth stage. An Iowa State Extension Special Report No. 48, “How a Corn Plant Develops”, by Ritchie, Hanway, and Benson indicates that by tassel emergence to silking about 60 to 70% of the uptake of N has occurred with two thirds of the 60% taking place between V10/V12 and tassel emergence (R1). For potassium (K), about 90% of K uptake is completed by R1 with almost all of that occurring between V6 and R1. Phosphorus, on the other hand, has a steady and almost linear uptake that is not complete until after R5 (dent stage).

Again from a nutrient management viewpoint, you need to consult with you nutrient management plan writer to be sure your plan is modified for this year’s unusual weather. In addition to the proposed changes in your N management plan, be sure your plan writer includes the justification for the additional N you may need to apply to corn this year.

Photos 1 – 6 below are illustrative of the impact of denitrification loss of N on corn in southern New Jersey (June 2013, Photos by R. Taylor).

cornNloss1

cornNloss2

cornNloss6

cornNloss5

cornNloss3

cornNloss4

 

Agronomic Crop Insects – June 21, 2013

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

Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. We are starting to see a significant increase in populations. Once plants are yellow, yield loss has already occurred. The treatment thresholds are 20 per 100 sweeps on alfalfa 3 inches or less in height, 50 per 100 sweeps in 4-6 inch tall alfalfa and 100 per 100 sweeps in 7-11 inch tall alfalfa.

Field Corn
Over the past couple of weeks, we have observed the movement of cereal leaf beetle adults from adjacent small grains into corn fields. Although they are generally found on field edges, there have been recent reports of field wide infestations. Although we do not have any firm thresholds for this insect on corn, as a general guideline controls may be needed on corn for adult feeding damage if you find an average of 10 beetles per plant and 50% of the plants exhibit feeding damage. In the Midwest, it has been reported that the adult beetle is a vector of maize chlorotic mottle virus (MCMV) that causes corn lethal necrosis disease. Thresholds would be much lower if this disease is an issue. We still have not found this virus in Delaware; however, be sure to let us know if you find potential problems.

Soybeans
Although insect activity remains generally light on early planted soybeans, you should continue to sample fields on a weekly basis for bean leaf beetles, potato leafhoppers, grasshoppers, green cloverworm and spider mites. As barley and wheat are harvested and soybeans are planted, these fields will be susceptible to attack and grasshopper feeding can often cause stand loss. The following link provides a guideline for decision making on the above insect pests:
http://extension.udel.edu/ag/2012/06/05/ipm-soybean-insect-thresholds/

Strawberry Renovation

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

Matted Row Systems
In matted row strawberries, the goals in renovation are to reduce plant numbers by narrowing the rows, remove old foliage (reduces diseases), control weeds, reduce insect and mite pests, and promote new runner development (production of daughter plants). After renovation, regular irrigation and weed control are essential. High yields next year depend on having large, healthy, vigorous plants when fruit buds are initiated in late summer.

With matted rows, renovation starts with an application of 2,4-D amine herbicide (Formula 40) after the last harvest. If grasses are a problem a sequential application of sethoxydim (Poast) or clethodim (Select) may be necessary (do not tank mix with the 2, 4-D). After the last herbicide application, wait 3-5 days and then mow off the strawberries to just above the crown (do not damage the crown). Apply nitrogen fertilizer (25-60 lbs N/acre) at this time. Using a split N application half at renovation and half 4 weeks later is preferable. If other nutrients were low or deficient (as indicated by tissue tests prior to fruiting) then apply at this time. Subsoil fields with compaction from equipment or heavy foot traffic between the rows (U-pick plantings for example).

Next, narrow the rows with a cultivator, coulters/discs, a rotary tiller/multivator or other devices to 12-18 inches at the base. Matted row strawberries are edge bearers and benefit greatly from this narrowing. Strawberries produce new roots higher on crowns each year so try to throw about 1 inch of soil over the row (without covering the crowns). This will also help new daughter plants root (runners produced from mother plants).

After narrowing the rows apply preemergence residual herbicides. Apply 2-4 ounces of terbacil (Sinbar). This is one half the annual rate. Sinbar can injure some varieties and attention should be paid so as not to have overlaps. If Sinbar is not used, napropamide (Devrinol) or DCPA (Dacthal) may be applied at this time. These materials require adequate rainfall or overhead irrigation for activation. Devrinol and Dacthal benefit from being lightly incorporated (possible in row middles). During the summer, cultivate between rows to remove weeds and to sweep runners into the row. From late summer on, cut off any additional runners during cultivation (discs or coulters work best).

Weeds in the rows must be controlled throughout the summer. Sethoxydim (Poast) or clethodim (Select) may be sprayed over the top to control grass weeds. Clopyralid 0.12-0.25 lb (Stinger 0.33-0.67 pt/A) has a 24c label for use in MD, NJ, VA, and PA for over the top control of some broadleaf weeds. Hand hoeing will be necessary for removal of remaining weeds.

Irrigate strawberries so that they receive 1.5 inches of water (combined rainfall and irrigation) each week during the summer. Irrigation during late July and August are very critical to produce large plants as flower buds will be initiated starting in August. Continue irrigation through the fall until dormancy (at reduced rates). Strawberries may benefit from low amounts of additional nitrogen fertilizer (25 lbs of N/acre) later in summer depending on the vigor.

Plasticulture Systems
With the high cost of establishing strawberries planted on plastic mulch, many growers choose to carry them over for another year. First, evaluate the disease pressure on the planting. If anthracnose was a major problem, you should not carry the planting over. If disease pressure was low, then renovation can proceed.

Goals in renovating plasticulture strawberries are to remove old foliage, remove any runners formed, remove diseased plant material from the field, control weeds, reduce insect and mite pests, and reduce crown size of very large plants.

Mow the strawberries as close to the crowns as possible without damaging them. Remove any diseased plant material from the field. Plants with more than 5 branch crowns will benefit from thinning. Using an asparagus knife, remove one half of the crown. Apply weed control measures between plastic beds (herbicides, cultivation, or combination) being careful not to apply herbicides over the plastic beds. Irrigate strawberries so that they receive 1.5 inches of water (combined rainfall and irrigation) each week during the summer. Fertigate with 40 to 60 pounds of nitrogen per acre in late August and add any additional nutrients as suggested by tissue tests. Continue irrigation as needed throughout the fall.

The key for carryover strawberries on plastic is not to have too many crowns going into the fall. Excessive crown numbers will reduce berry size greatly. Carry over beds should not be row covered until winter to avoid excessive growth and may not need row covers in mild winters until the frost protection period in March and April during flowering.

SWD Found in Cherries on the Eastern Shore of Maryland

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

There has been another report of spotted wing drosophila (SWD) in cherries, this time from an orchard in Caroline County, Maryland. We have a number of berry crops and soft fruits currently ripening (blueberries, raspberries, blackberries, early peaches) that may be at risk. Refer to the article by Jerry Brust for control measures: http://extension.udel.edu/weeklycropupdate/?p=5727

Brown Marmorated Stink Bug Update

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

Although we do not have a fruit scouting program, we do have a few pheromone traps out near peach and apple orchards in the state and we have found a few adults in traps. Reports from neighboring states indicate that brown marmorated stinkbug adults, egg masses and early stage nymphs can be found in apples and peaches. In general, they feel that populations are still generally low but at higher levels than seen last year at this time. For more information on management in fruit, you will want to consider subscribing to Rutgers Plant Pest Advisory http://plant-pest-advisory.rutgers.edu/.

High Tunnel Tomatoes With Mite Infestations

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

Last week I saw several high tunnel tomatoes with moderate to severe two-spotted spider mite (TSSM) Tetranychus urticae infestations (Fig. 1). These pests vary in color from white to yellow to red. Mites feed by sucking sap from the plant causing small white stippling marks on the top surface of the leaf (Fig. 2). This damage may look like other problems early on and because the mites are difficult to see on the underside of the leaf at this stage of damage their build-up can go unnoticed. The feeding damage then progresses to yellowing, wilting, browning, and eventual death of the leaves or whole plant. They occur in the highest numbers on the undersides of leaves, but when their populations get very high they can move to the top-side of a leaf or onto tomato fruit. In most of the HTs there were only 1 or 2 cultivars (out of 4-7 cultivars) that the mites were found on in heavy numbers. Growers need to be sure to check all the different varieties of tomatoes they are growing for mite infestations.

The warm, dry conditions in a high tunnel favor rapid mite development and reproduction, which enables them to complete a generation in as little as 5 to 7 days. Heavily infested leaves may be covered with very fine, irregular webs in which mites and eggs can be found (Fig. 3). Once the spider mites begin to make these webs they are even more difficult to control. This is because the mites hide under the protective layer of webbing making it that much more difficult to get spray coverage to the underside of the tomato leaf. For a heavy infestation, one miticide application usually will have to be followed by another about 7-10 days later to be sure all the adults, immatures and eggs are controlled. Check the Commercial Vegetable Production Recommendations Guide for miticides.

TSSMtomato1

Figure 1. High tunnel tomatoes with a severe two-spotted spider mite infestation

 TSSMtomato2

Figure 2. Early TSSM feeding causes white stippling (specks) on tomato leaf

TSSMtomato3

Figure 3. TSSM webbing on tomato leaflet. Dark specks are TSSM

New Invasive Pest Reported in Maryland: The European Pepper Moth

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

Stanton Gill has identified a new invasive pest of ornamentals and vegetables in Maryland: the European pepper moth, Duponchelia fovealis. The European pepper moth was found on petunia and geraniums from a wholesale greenhouse operation in Ann Arundel County in the last 10 days. The moth has spread around the U.S. and Canada (but only in greenhouses) since first being reported in 2004. The Government considered putting it under quarantine, but it spread so rapidly the idea was abandoned. It is not known yet if this pest has or will become established in the landscape, or is just found in the nursery and containerized vegetable trade. Based on the climate of its native habitat (Southern Mediterranean), this pest has a chance of becoming established in states along the west coast and the southeastern U.S. How far north the pest could move up the southeastern U.S. coastline is not known at this time.

EuropeanPepperMoth

This caterpillar feeds on a wide range of plants including ornamentals but also on several vegetables including tomato, pepper, squash and strawberries. The larvae feed on roots, leaves, flowers, buds and fruit. On leaves, this feeding damage appears as rounded or crescent-shaped holes on the outside of the leaves, but eventually the whole leaf is eaten. Usually the leaves that are attacked are at the base of the plant. Late instar larvae also can burrow into the plant stem (pepper, tomato, squash). I do not think this insect will become established in our area, but it may be a problem for some vegetable growers during the summer when it escapes from a greenhouse operation. If you see any odd leaf feeding or stem boring and the larva looks something like the picture above give me a call at 301-627-8440 or jbrust@umd.edu.

Bean Diseases

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

Pythium Blight (Cottony Leak)
Pythium blight has been a problem on the Delmarva during periods of rainy weather and cool temperatures. The disease causes damage on several plant species, including beans. The presence of Pythium blight is obvious by the profuse mycelia on infected fruit or stems. To reduce the damage from Pythium blight, improve air movement in and around the plants. Also, applications of Prophyte, K-Phite, Phostrol or Rampart have been effective in recent trials. A Section 24c label is available in Maryland and Delaware for use of Ridomil Gold Copper.

White Mold
We are at risk of white mold on green beans that are near or at flowering. The spray guidelines are that if the soil has remained wet for 6 to 10 days in your field and the bean crop is at 10 to 20% bloom, a fungicide should be applied. A second spray should be made 7-10 days after the first spray if the soil remains wet and blossoms are still present. Check labels for details on fungicide timing. The following are effective on white mold: Endura, iprodione, Omega thiophanate-methyl (Topsin-M), Switch, or Switch plus thiophanate methyl].

Remember that there is a very effective biological alternative to fungicide sprays. Contans is an outstanding alternative for organic and conventional growers alike. Contans is a formulation of the fungus Coniothyrium minitans, which parasitizes the survival structures of the pathogen Sclerotinia sclerotiorum. In trials throughout the U.S. Contans has worked well at reducing white mold severity. The challenge with using Contans is that it should be applied two months before the disease develops. Contans is OMRI listed from SipCam Advan, LLC. Because the product is living, it must be handled carefully prior to use.

whitemold

Signs of Sclerotinia sclerotiorum (apothecia and sclerotia) on the soil surface.

Bacterial Leaf Spot of Pepper

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

Bacterial leaf spot (BLS) is a common and widespread disease of peppers in Maryland and Delaware. The bacterial pathogen that causes BLS is Xanthomonas campestris pv. vesicatoria. The disease may be seed borne or may become established on the crop after overwintering on Solanaceous weeds or volunteer plant hosts in the field. Tomatoes also are a host of this disease. BLS spreads by splashing rain, irrigation, or mechanically on machinery or the hands of field workers, etc.

Symptoms of BLS on pepper are small irregular spots that often have a yellow halo or border. As the disease progresses, infected leaves turn yellow and drop off the plant resulting in extensive defoliation. Disease progress in the crop is favored by high moisture and high nighttime temperatures (optimum is about 75°F).

Several tactics should be employed to reduce disease. Host resistance is available in several cultivars. Plant cultivars with resistance or tolerance to races 1, 2 and 3, which are the most common races in the Mid-Atlantic. Clean seed is important – be sure seed is treated or use hot water seed treatment. Use only clean transplants, practice crop rotation, and do not plant peppers after tomatoes.

During transplant production, apply a streptomycin spray (for example, Agri-Mycin 17 or Agri-Strep) every 4 to 5 days. Maintain good fertility in the field to help plants tolerate disease. Spread of disease can be minimized with fixed copper applications or, alternatively, a Section 2ee is available for use of Quintec for Maryland and Delaware.

Finally, the following practices will help reduce carryover of BLS to future years: control solanaceous weeds, disk the field immediately after final harvest, and do not save seed from BLS infected crops.

 BacterialLeafSpotPepperBacterial leaf spot (Xanthomonas vesicatoria) on sweet pepper
Gerald Holmes, Valent USA Corporation, Bugwood.org