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.

Time to Scout for Weeds

Kurt M. Vollmer, Postdoctoral Researcher – Weed Science, University of Delaware; kvollmer@udel.edu

With most of the corn and soybeans planted, now is the time to start scouting for weeds. Doing so will prevent major headaches later in the growing season. While scouting, be sure to note the weed species present, height, life-cycle, and severity of the weed infestation. When looking at fields this year, pay attention to those areas that were drowned out last summer. The weeds in many of those spots produced seed and now have very high seed banks. So while weed pressure in the rest of the field may not be too heavy, weeds present in these spots may be at unacceptable levels.

In particular, Palmer amaranth can quickly become unmanageable if not spotted early. Many herbicide labels suggest spraying this weed when it is less than 4 inches tall, but the UD Weed Science program recommends applying postemergence herbicides before its 3 inches tall. Our research with soybean shows that the best time for this second application is no later 28 days after applying a residual herbicide. Furthermore, Palmer amaranth can quickly exceed 4 inches, and research at the University of Maryland has shown that delaying the postemergence application to 32 days or longer can result in reduced levels of control. Remember, the earlier Palmer amaranth is spotted the better. Furthermore, keep in mind there could be several days between scouting and actually getting the sprayer into the field, allowing Palmer amaranth to reach heights that prevent complete control.

Broad Mites on High Tunnel Tomatoes

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

It is unusual that I see or hear about broad mites (Polyphagotarsonemus latus) being a problem in our tomato high tunnels. A grower was having symptoms of twisted growth and browning/bronzing of their tomato leaves this spring and guessed they might have broad mites. They did, with some plants severely damaged while others were fine. The grower had a late fall crop of cherry tomatoes that they kept into December but did not clean up until 2 weeks before they planted their spring crop of tomatoes. Unfortunately, the grower had a small infestation of broad mites in the fall crop of tomatoes that was able to overwinter. I wrote an article earlier this season about the necessity of cleaning a high tunnel or greenhouse well in advance of another crop in case there was a small infestation that had started in the last crop or on weeds left in the high tunnel. Sanitation is key to keeping pest problems out of a high tunnel or greenhouse.

Broad Mite Description and Biology
Female mites are oval, 0.2 mm long and are yellow or green with a light, median stripe that forks near the back end of the body. Males are similar in color but lack the stripe. The translucent, colorless oval eggs are firmly attached to the surface of a leaf. The eggs are very distinctive and are usually used to identify whether or not broad mites are present. (Often times adults or immatures cannot be found on a sample, but the eggs will be.) The eggs are covered with scattered white tufts on their outer surface that look like round dots (Fig. 1). Immature broad mites are white and slow moving. After just one day, the larva becomes a quiescent nymph which is clear and pointed at both ends. When females emerge from this quiescent stage, males immediately mate with them. Adult females lay a total of 30 to 76 eggs on the undersides of leaves and in the depressions of small fruit over a 9-14-day period and then die. Adult males may live 5-10 days. While unmated females lay eggs that become males, mated females usually lay four female eggs for every male egg. Males and females are very active, but the males account for much of the dispersal of a broad mite population when they carry the quiescent female to new leaves.

Figure 1. Broad mite egg greatly magnified

Hosts
The broad mite has a wide range of host plants: apple, avocado, cantaloupe, castor, chili, citrus, coffee, cotton, eggplant, grapes, guava, jute, mango, papaya, passion fruit, pear, potato, sesame, string or pole beans, tea, tomato and watermelon. Broad mites also infest many ornamentals, including African violet, ageratum, azalea, begonia, chrysanthemum, cyclamen, dahlia, gerbera, gloxinia, ivy, jasmine, impatiens, lantana, marigold, peperomia, pittosporum, snapdragon, verbena, and zinnia. Their ability to attack both vegetables and ornamental plants make them especially troublesome in greenhouses that grow both.

Damage
The damage caused by broad mites can look similar to the damage caused by viruses, herbicides or nutrient deficiencies. They feed on plant cells within the leaf epidermis using their piercing-sucking mouthparts. Early feeding is mainly concentrated near the growing point on the underside of a leaf near the stalk, which tends to cause the leaf to curl and become twisted and distorted (Fig. 2). More serious infestations cause leaf bronzing leaving the main veins green against the brown leaf tissue that eventually turns black, shrivels and dies (Fig. 3). Corky patches frequently appear on fruits that often crack at the site of deformation (Fig. 4). Extensive damage can be caused by relatively low populations. Commonly, the lower leaves of a plant can remain unaffected while the younger leaves are badly damaged. Symptoms of feeding damage can remain visible several weeks after the mites have been removed. Therefore, after treatments the plants need to be checked again for the presence of the mite, even though damage may still be apparent.

Figure 2. Leaves of tomato twisted and deformed by broad mite feeding

Figure 3. Broad mite feeding causing bronzing of leaves–leaving green veins

Figure 4. Damaged and aborted cherry tomato fruit due to broad mite feeding

Management
Once the mites have been positively identified as the cause of the tomato deformities horticultural oils or sulfur can be used that produce results similar to synthetic chemical applications. The most important aspect of the application is thorough coverage. The material needs to get down into tightly wrapped growing points and to the underside of leaves. Be careful when applying the oils or sulfur as they can cause phytotoxic problems under hot humid conditions. Portal XLO has been found to control broad mites in tomato and is classified as a mitochondrial electron transport inhibitor (METI) (IRAC subgroup 21A) and should be rotated with other miticides (hort oils, sulfur, Oberon, Agri-Mek) that have a different mode of action (i.e., a different IRAC No.). As in the case of oils and sulfur, Portal is a contact miticide and for best performance uniform and thorough spray coverage is needed. The addition of a nonionic wetting or penetrating adjuvant to the synthetic chemicals is recommended to improve their performance.

Root Knot Nematode on Watermelon and Other Cucurbits

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

All cucurbits, including watermelon and cantaloupe, are hosts of root knot nematodes (RKN). RKN are plant pathogenic roundworms, which live in the soil. Although many other species of nematodes are nonpathogenic to plants or even beneficial, RKN can invade host roots and result in yield losses to cucurbits. Symptoms of RKN are often overlooked because the stunting, reduced vigor and wilting of the host plant can be caused by many other biotic or abiotic causes. To determine if plants are infected with RKN, observe plant roots for large galls, knots, or swellings. Also, look for damage that occurs in patches in the field. Plants are most susceptible to damage from RKN at the seedling stage.

Preventative management of plant-parasitic nematodes, using rotation, cover crops, transplants that are free of nematodes, nematicides and soil fumigants, is an important measure and more effective than trying to manage an outbreak. Use of green manure and soil amendments is also beneficial. We have found the soil incorporation of large amounts of organic matter, such as sorghum-sudangrass green manure in combination with poultry compost, reduces populations of root knot nematodes. Some rapeseed cultivars, such as ‘Dwarf Essex’ and ‘Humus’ also are suppressive to nematode populations.

As noted above, management of RKN is best done prior to planting. Where damage has been observed in the past, several soil samples should be taken from soil within the root zone, mixed together, and sent to a diagnostic lab for identification. If RKN is present in damaging levels, be sure that appropriate cultural practices are in place. If chemical management is necessary, it is best conducted preplant with fumigants such as Telone or Vapam.

Once the crop has been established the available options are less effective. However, the following can be used, Vydate L can be applied at 0.5 to 1.0 gal/A and incorporated into top 2-4 inches of soil, or at 2.0 to 4.0 pt/A apply 2 weeks after planting and repeat 2-3 weeks later. Velum Prime, which is in a different chemical class, can be applied at 6.5 to 6.84 fl oz/A through drip irrigation at 5-day intervals (see label for details).

I discussed this problem with Dr. David Langston at Virginia Tech, who has worked extensively on nematodes. His opinion is that although both Vydate and Vellum Prime may have some effect on nematodes when applied after transplant, the effect will be modest. If a grower is committed to a rescue treatment, keep in mind that watermelons are relatively tolerant to RKN and either forego treatment, or apply the least expensive option.

No matter what the treatment decision, remember that the damage from RKN can be mitigated, to some extent, by providing plants with adequate nutrition, moisture, and protection from stress.

Pollinator Strength

David Owens, Extension Entomologist; owensd@udel.edu and Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Honeybees are used extensively to ensure adequate pollination for vine crop vegetables (cucurbits) and for many fruit crops (apples, berries, etc.). Without good pollination, poor fruit set or misshapen fruit can occur. Most of the honeybees used for pollination are rented from beekeepers. Questions have come up how to know if a colony is strong enough to provide adequate pollination service. A good resource on pollinators, colony strength, and farmer best management practices for pollinator health (including water sources can be found in the MidAtlantic Vegetable Production Recommendations, Section A, pages 21 – 27.

It is important to ensure having enough bees (managed and wild) to avoid having problems with fruit set and misshapen fruit. There are two ways to check the strength of a colony: in-hive inspection and assessing hive traffic at the entrance. In the hive, bees should cover 6 to 8 frames, have 4 to 6 frames of brood and (eggs, larvae, and capped) fill 1.5 to 2 boxes. This is considered a ‘minimum standard.’

An easier, but less accurate method of assessing colony strength is to watch colony entrances in late morning to early afternoon on a calm day. During a 1 minute interval, 50 – 100 bees should be arriving and leaving the colony. While counting bees, be sure to note the presence of bees carrying pollen. They will have large yellow ‘sacs’ on both back legs.

Farmers should work with their beekeeper to ensure that only strong colonies are placed in fields. This has become more difficult in recent years due to higher winter mortality caused by bee pests and pathogens. Stronger colonies provide much more pollination service than one or two weaker colonies. Beekeepers should work with the state apiarist, Meghan McConnell to assess colonies. On the farm side, farmers should read labels carefully and avoid making applications when bees are active in fields. Several insecticides and miticides have pollinator advisory language on them. The fastest way to find it is to download the label from a website such as cdms.net and search the label for ‘bee’ or ‘pollinator’ using Ctrl + F. Insecticides of special concern have a bee in a red diamond to indicate pollinator protection language. Bees can also be affected by fungicide applications. Bees feed their larvae fermenting pollen, and bees rely on the microbes living with them to fend off diseases; fungicides can disrupt the beneficial microbes in the colony. Thus, even fungicides should be timed for periods when bees are not active in the crop. On warm days, bees also forage for water to cool the colony. Having a clean water source within a ¼ mile will benefit the bees. This doesn’t necessarily mean flowing water; large puddles should suffice.

Boron in Vegetable Crops Revisited

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

Boron has important roles in vegetable plants. It is needed for protein synthesis, development of cell walls, carbohydrate metabolism, sugar translocation, hormone regulation, pollen grain germination and pollen tube growth, fruit set, and seed development. Boron is mobile and readily leached in sandy soils and regular additions are necessary for many vegetables, but only in small amounts.

Boron (B) is a micronutrient required in very small amounts and there is a narrow range of safety when applying boron as toxicities can occur if too much is applied.

Vegetables vary considerably in their B requirements.

High B requirement crops include broccoli, cabbage, cauliflower, beets, spinach, turnips, and rutabaga. Apply 3 lbs/a of B for these crops.

Medium B requirement crops include asparagus, carrots, cucumbers, eggplants, leeks, muskmelons, okra, onions, parsnips, radishes, squash, strawberries, sweet corn, tomatoes, and potatoes. Apply 2 lbs/a of B for these crops.

Low B requirement crops include peppers and sweet potatoes. Apply 1 lb/a of B for these crops.

Very low B requirement crops include beans and peas. No additional boron is usually needed for these crops (snap beans actually are very sensitive to high B levels which will cause toxicities).

Boron deficiency symptoms in plants include the death of growing points resulting in a stunted or rosette appearance; leaves with a yellowish or reddish cast, and in members of the cabbage family most boron deficient cole crops develop cracked and corky stems, petioles and midribs. The stems of broccoli, cabbage and cauliflower can be hollow and are sometimes discolored. Cauliflower curds become brown and leaves may roll and curl, while cabbage heads may be small and yellow. Of all the cole crops, cauliflower is the most sensitive to boron deficiencies.

It is recommended in broccoli and kale to apply 3 pounds of boron (B) per acre in mixed fertilizer prior to planting. In Brussels sprouts, cabbage, collards and cauliflower, boron and molybdenum are recommended. Apply 3 pounds of boron (B) per acre and 0.2 pound molybdenum (Mo) applied as 0.5 pound sodium molybdate per acre with broadcast fertilizer.

Boron may also be applied as a foliar treatment to cole crops if soil applications were not made. The recommended rate is 0.2-0.3 lb/acre of actual boron (1.0 to 1.5 lbs of Solubor 20.5%) in sufficient water (30 or more gallons) for coverage. Apply foliar boron prior to heading of cole crops.

Boron toxicity is common in western states where boron levels in soils or irrigation water are high. In the east, we do not have high boron soils or high levels in irrigation water. In addition, boron leaches readily from soils. Boron toxicities therefore occur only when excess boron is applied in fertilizers. The margin of safety for boron application is small and excess application or improper blending in fertilizers may lead to toxicities – deficiencies show up at 1 ppm and toxicities appear at 5 ppm of available boron in the soil (leaf tissue levels between 20 and 100 ppm are sufficient with tissue levels over 200 ppm being excessive leading to toxicities).

The vegetable crops most sensitive to excess boron are beans, particularly snap beans. Boron is generally not recommended for snap bean production and boron should never be included in starter fertilizer for snap beans. Boron toxicity often occurs where starter fertilizer containing boron for other crops, such as corn, is applied to snap beans.

Boron toxicity in beans commonly appears as yellowing in unifoliate leaves with burning of leaf edges and yellowing of leaf edges of the older trifoliate leaves that can progress to edge burn. In severe cases, plants will develop a scorched appearance and leaves may prematurely drop off.

Vegetable Crop Insect Scouting

Cucurbits
Spider mites have been observed in a few fields at low numbers. Most mites have been observed near field edges immediately adjacent to other crops or woodlines. With the warm weather, I am beginning to find spider mites on edge-weeds such as pokeweed. Last year, every pokeweed plant I looked at by mid-July had large spider mite populations on it, and quite a few predators too. Spider mites can also ‘balloon’ into fields, meaning that a population can start in the field interior.

Our lab examined rye strips from several fields at the beginning of May looking for spider mites. We collected 20 row feet of the windbreak foliage, washed it in soapy water, and filtered the water. We did not observe spider mites in the rye samples. However, this is like looking for a tiny needle in a large haystack. One of the fields we sampled had spider mites early, likely coming in from the greenhouse. This can be reduced by keeping greenhouses weed-free when not in use, and especially in late summer through fall.

The table below lists the miticide active ingredients and their mode of action group for watermelon. There are generic formulations of some of the chemicals, this list is not meant to serve as an endorsement.

 

Miticide Active Ingredient MOA Group Life Stage Active Applications per Season
Agri-Mek Abamectin 6 Mobiles (translaminar) 3-5
Gladiator Zeta-cypermethrin + avermectin 3 + 6 Mobiles (translaminar) 3
Acramite Bifenazate 25 Eggs and mobiles (contact) 1
Kanemite Acequinocyl 20B Eggs and mobiles (translaminar) 2
Oberon Spiromesifen 23 Eggs and juveniles (contact) 3
Portal Fenpyroximate 21A Mobiles (contact) 2
Magister SC Fenazaquin 21A Eggs and mobiles (contact) 1
Zeal Etoxazole 10B Ovicidal, juveniles (translaminar) 1

 

 

Please note that avermectin is in the same mode of action class as abamectin. Do not apply one right after the other. Also, some of these products will stop mite feeding quickly, but the mite will take a few days to die. If you have sprayed a field and see mites a couple of days later, wait another few days and resample. Also, many of these products have a long residual activity, meaning that if it is not active immediately on eggs, it should still be around once the eggs hatch. This year, we have plans to test these products at our research station. As always, read the label thoroughly for further guidance. Some products have restrictions on reapplication interval, and restrictions on consecutive applications. There are also requirements on some for spray adjuvants to avoid illegal residues. Good coverage is key for miticide efficacy, even on those that are translaminar. Also, be mindful of products that are in a tank-mix. For instance, Zeal may have some reduced efficacy when mixed with Boron, and stickers or ‘sticky’ fungicides can interfere with Agri-mek. Both Agri-Mek and Magister can be very toxic to pollinators, while Portal and Kanemite are not generally considered toxic to bees. All of the others advise caution.

Cucumber beetles are still moving into new plantings.

Sweet Corn
Sweet corn pheromone and blacklight traps are checked twice weekly on Mondays and Thursdays. By Tuesday and Friday morning, data is uploaded to our website: https://agdev.anr.udel.edu/trap/trap.php. For reference, action thresholds based off of blacklight and pheromone trap can be found here: http://extension.udel.edu/ag/insect-management/insect-trapping-program/action-thresholds-for-silk-stage-sweet-corn/. Moth counts continue to be elevated, and seem to be higher and a little earlier than last year. Thursday’s trap capture is as follows:

Trap Location BLT – CEW Pheromone CEW
3 nights total catch
Dover 1 2
Harrington 0 63
Milford 1 21
Rising Sun 1 45
Wyoming 2 29
Bridgeville 0 65
Concord 1 8
Georgetown 2 38
Greenwood 0
Laurel 3 18
Seaford 0 8
Harbeson 0
Trap Pond 4 12
Lewes 3 63

 

Beet and Spinach Leafminers

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

In high tunnels and in the field, I have seen spinach and beet leaf miners Pegomya hyoscyami and P. betae respectively in swiss chard and spinach. These leafminers are a type of blotch leafminer, creating irregularly shaped mines. These flies attack crops and weeds in the plant family Chenopodiaceae, which includes chard, beets, and spinach and the weed lamb’s quarters. These fly species are very similar, but the spinach leafminer may also feed on Solanaceous crops such as peppers.

Adults are small flies about 1/3 inch in length and gray to brown. Larvae are whitish and cone-shaped. Flies of both species overwinter as pupae in the soil. In April and May, flies emerge and lay white eggs in groups of 4-8 on the underside of leaves (Fig. 1). Eggs hatch and larvae begin feeding between leaf tissues creating mines (Fig. 2). As the larvae feed and develop, they create areas of dead tissue where they have fed. These areas are opaque at first and then later turn brown (Fig. 3). Once inside the leaf tissue larvae are difficult to control. The larvae are active for about two to three weeks, before dropping to the ground and pupating in the soil. The entire life cycle is 30-40 days. There are three to four generations per season. Once the summer is over, leafminers will overwinter as a puparium in the soil emerging in early spring the next year to start the cycle again.

Figure 1. Leafminer eggs are white and laid on underside of leaf

Figure 2. Leafminer eggs have hatched and larvae are mining between leaf layers

Leafminer feeding has little impact on overall plant growth but can be quite damaging to vegetables grown for edible greens. So, a crop such as Swiss chard or spinach that you are trying to sell the leaves of are greatly impacted while something such as turnips or beets that you are selling the bulbs of are less impacted (unless you are selling the tops too).

The damage to the Swiss chard and spinach I saw probably could have been far less if the first infested leaves with leafminers or fly eggs had been removed or destroyed. Any additional plantings of spinach or chard this season (or next year) on this farm should be planted in a different area of the field because of pupae still in the soil. Once the spinach or chard is planted in a new area a row cover or chemicals can be used to protect the plants and keep the leafminer flies that emerged from previously infested sites from laying eggs.

Figure 3. As larvae grow their damage becomes more pronounced

Because these leafminers feed mostly on one crop group and some weeds that include chickweed, pigweed and lamb’s quarters, weed control and crop rotation are important management tools. Chemical controls such as dinotefuran, thiamethoxam and spinetoram (spinetoram also has translaminar activity and if combined with an adjuvant is more effective against larvae) are foliar and soil controls for use in spinach. Chemical controls for leaf miners in other crops are more limited, so check the 2019 Mid-Atlantic Commercial Vegetable Production Recommendation guide and always follow label instructions. For organic production spinosad can provide good control if used at or before egg laying and has only minor impacts on natural enemies. Neem oil can be used to prevent egg laying but is not as effective as spinosad. As always thorough coverage is necessary for good control which includes getting the material to the underside of the leaf.

Monitoring Nutrient Status in Vegetable Crops

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

June means warm weather and long days and spring planted vegetable crops are growing rapidly. Monitoring the mineral nutrient status of vegetable plants is important to evaluate fertility programs and to make adjustments. Recommended fertility programs for vegetable crops are given in the Commercial Vegetable Production Recommendations publication for Delaware and surrounding states. See http://ag.udel.edu/extension/vegprogram/publications.htm for an electronic version.

While these recommendations should be the base of a fertility program, additional monitoring of plant nutritional status is recommended, especially for highly managed crops such as those grown in plasticulture where fertilizers can be injected through the drip irrigation system. Tools that are available include tissue testing, petiole sap testing, or the use of instruments such as a chlorophyll meters or NDVI sensors to monitor plant nutrient status.

Tissue testing involves taking samples from the plant (most commonly leaves) at various times during the growth period and sending them to a laboratory for mineral nutrient analysis. Petiole sap testing involves taking leaf petioles and expressing the sap which is then tested for nitrate and/or potassium using portable meters. Chlorophyll meters are used to measure “greenness” of individual leaves and NDVI sensors are used over top of crop canopies to measure the amount of green foliage.

When taking tissue samples, specific procedures should be followed to obtain reliable results. For whole leaves, the sample should not have any stem material. For sweet corn or onions, the leaf is removed just above the attachment point to the stalk or bulb. For compound leaves (beans, tomatoes, etc.), the whole leaf includes the main petiole and all the leaflets. With heading vegetables like cabbage take the outermost whole wrapper leaf. For young plants, the whole above-ground portion of the plant is sampled.

Most tissue tests are done using the most recently matured leaves (MRML) for analyses. Most-recently-matured leaves are leaves that are full size and have changed from the young leaf light-green color to a darker green color.

For each sample take 25 to 50 individual leaves. More accuracy in determining the actual nutrient status is derived from a larger sample size. Leaves of the same age (physiological age and position) should be removed from each sampled plant (the MRML). Plants damaged by pests, diseases, or chemicals should be avoided as well as plants with dust accumulation. Samples should be air-dried before shipment and paper (not plastic) bags should be used to ship or samples to the testing lab.

Tissue test results are interpreted using critical value tables. Results are commonly placed in the following categories:

Deficient – nutrient levels are below a critical value and plants are being affected. Corrective measures will be needed with additional fertilization.

Low – nutrient levels are below a critical value and plants may be affected. Corrective measures may be needed with additional fertilization.

Adequate – nutrient levels are in a range for normal growth

High – nutrient levels are above the range for normal growth and may indicate over-fertilization

Very High – nutrient levels are above the range for normal growth may be damaging to the plant or may indicate luxury consumption

In some lab results low and deficient categories are combined and very high may not be used unless a toxicity is detected.

Petiole sap testing is useful for monitoring nitrogen and potassium and can give very quick results with the use of portable meters. For sap testing, petioles collected from most recently matured leaves (MRML) are used for analyses (see above). A random sample of a minimum of 25 petioles should be collected from each field or zone of 20 acres or less. Leaves with obvious defects or with diseases should be avoided. Sampling should be done the same time of day (best between 10 AM and 2 PM).

To take petiole samples, collect whole leaves from the plant and then remove the leaf blades and leaflets. A petiole of several inches in length remains. Petioles are chopped into about one-half inch segments, crushed in a hand press, and sap is collected in a cup. Follow the instructions for the specific meter you are using to analyze the sap.

Petiole sap results are normally given in the expected range for good growth at a given crop stage.

We have added critical tissue test values and petiole sap test values to the Commercial Vegetable Production Recommendations for many vegetable crops. These can be found at:http://extension.udel.edu/ag/vegetable-fruit-resources/commercial-vegetable-production-recommendations/ online.

The chlorophyll meter is a tool that is useful to monitor nitrogen status. Test plants are fertilized with extra nitrogen so they become fully green. These test plants are then compared with the crop with normal fertilization. Again 25-50 MRML leaves are tested by clamping the sensor head to the leaf and recording the reading. The sensor should be placed in the portion of the leaf blade without large veins, midrib, or folds. Major differences between test plants and normally fertilized plants indicates lower nitrogen status and that additional nitrogen may be necessary.

NDVI sensors have been used for on-the-go sensing of crops for nitrogen status. High nitrogen test strips are used to compare with sensor readings in the field. There is the potential for on-the-go nitrogen sidedressing of crops such as sweet corn using this technology.

 

Vegetable Crop Insect Scouting

David Owens, Extension Entomologist; owensd@udel.edu

Cucurbits
We are still in need of your cucumber beetles. If you have a population that you are about to treat or treated within the past 24 hours, please contact me, owensd@udel.edu if you don’t mind me lifting a couple hundred beetles out of the field. Thanks.

With the hot weather this week, be on the lookout for spider mites, not just in watermelon but also tomato. We may get a bit of a reprieve if the weather cools down, but once mites get started in a vegetable field, weather only slows them down but will not remove them. Tomato thresholds from North Carolina are 4 mites per upper canopy leaflet (not leaf).

Sweet Corn
Sweet corn pheromone and blacklight traps are checked twice weekly on Mondays and Thursdays. By Tuesday and Friday morning, data is uploaded to our website: https://agdev.anr.udel.edu/trap/trap.php. For reference, action thresholds based off of blacklight and pheromone trap can be found here: http://extension.udel.edu/ag/insect-management/insect-trapping-program/action-thresholds-for-silk-stage-sweet-corn/.

We observed a significant increase in moth activity in several locations throughout the state.

Thursday trap counts are as follows:

Trap Location BLT – CEW Pheromone CEW
3 nights total catch
Dover 1 7
Harrington 0 48
Milford 2 45
Rising Sun 1 27
Wyoming 1 11
Bridgeville 5
Concord 1 71
Georgetown 2 47
Greenwood 1
Laurel 2 94
Seaford 0 4
Harbeson 2
Trap Pond 3 8
Trapwoods 3 8