Guess the Pest! Week #14

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).

What caused this damage?

Guess the Pest! Week #13 Answer: Ozone Damage

Bill Cissel, Extension Agent – Integrated Pest Management;

Congratulations to Keith McGowan for correctly identifying the damage in the photo as ozone damage 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 #13 Answer: Ozone Damage

Photo and article by Nancy Gregory

Ozone damage on vegetables is observed during hot and humid weather in our region. The most susceptible crops include potato, watermelon, cantaloupe, pumpkin, squash, snap bean, and lima beans. Symptoms occur on the top, younger leaves, and show up as dark spots on potato. On cucurbits, ozone damage shows up as yellowing with spots having a white crusty center. Watermelon is the most susceptible in this group. On beans, spots are small and bleached looking, and leaves may have a bronzed appearance on the top surface. Affected leaves may yellow and drop. Ozone is easily misdiagnosed and hard to accurately confirm, since an episode of air pollution cannot be chemically tested for. Inversion layers in hot humid weather result in trapped air at the field level that has a high level of air pollutants. Air remains at field level until winds pick up and disperse it, and four or five hours at levels of 80 ppb can result in damage. Some cultivars have some tolerance, and keeping plant stress low can help to avoid damage.

Checking Mid-Season Nutrient Uptake

Jarrod O. Miller, Extension Agronomist, and Amy Shober, Extension Specialist – Nutrient Management and Environmental Quality, University of Delaware,

With variable corn growth following our wet spring, you may be interested to know more about what may be going on in good and poor performing parts of your fields. While lack of nitrogen (N) has been a major reason for poor corn performance due to very wet conditions in May and early June, there are other nutrients that could be lacking and contributing to poor performance. Nitrogen, sulfur (S), and boron (B) are all potentially mobile in the soil and may have moved below the root zone, while manganese (Mn) can be more available for plant uptake when soils are saturated. Nutrients can also be antagonistic to each other during plant uptake. For example, plant uptake of the ammonium form of N can slow the uptake of Mg from the soil. So, any N applications made for crop recovery may have impacted plant uptake of other nutrients.

This may be a good season to evaluate mid-season fertility since corn was under duress, plus the results of corn leaf sample testing can help you plan for next year. When at least 50% of the ears are showing silk, sample the entire ear leaf from at least 15-20 plants and submit these samples for plant nutrient analysis. Choosing where in the field you sample depends on what you are looking for. To evaluate general field health, stay away from obviously distressed areas that show insect or equipment damage or small areas that show nutrient deficiencies. However, if the goal is to determine why there is an issue, take samples from both field “good” and “bad” areas of the field and label the samples as such when you submit them for testing. Be sure to gently clean any soil or dust off the leaves to prevent contamination of the tissue with soil material, which can skew results.

Once you get results of your tissue test back from the lab, compare the values to the sufficiency ranges in the following table. Any nutrient below these ranges is lacking and could prevent your crop from reaching maximum achievable yield. These are meant to be guides, and may not be accurate for all soil types and corn hybrids. Keep track of the area you sampled and match it to any soil samples or yield you obtain at the end of the season. This information may help plan for next year or whenever similar weather patterns arise.

(Campbell and Plank, 2000)

Growing Degree Days (GDD) Through June 25

Jarrod O. Miller, Extension Agronomist,

Many fields planted in late April (that survived) should be at or very close to tassel in at least Sussex or Kent county. Remember that ear leaf samples can be taken when corn starts to tassel (VT) so you can check for critical nutrient levels. Any replanting in early June should be close to V6, so think about side-dressing those parts of the field.

V6: 475 GDD
V12: 870 GDD
VT: 1135 GDD
R1: 1400 GDD

Table 1: Growing degree days accumulated through June 25th from the beginning of each week.

Sussex Kent New Castle
22-Apr 1190 1148 1088
29-Apr 1138 1101 1054
6-May 1030 992 956
13-May 962 886 858
20-May 815 779 764
27-May 657 620 616
3-Jun 505 469 466

Soybean Insect Scouting Update

David Owens, Extension Entomologist, and Bill Cissel, Extension Agent – Integrated Pest Management;

The usual defoliators are present in fields, albeit in low numbers this week. We are finding green cloverworms, bean leafrollers, grasshoppers, an occasional bean leaf beetle, thrips, Japanese beetles, and spider mites. First generation bean leaf beetle populations are dwindling, the second generation should emerge from the soil in late July – August.

Two spotted spider mites are showing up in more soybean fields. Mite populations are still low enough to warrant continued monitoring. With hot dry weather (didn’t think I would be saying that), populations can increase rapidly. As a reminder, most pyrethroids tend to ‘flare’ mites by removing natural enemies, although bifenthrin does have activity on TSSM. Zeal and Agri-mek are two other very effective, narrow spectrum products that have excellent spider mite activity. You can find our action thresholds and insecticide recommendations here: In R-stage soybean, we are also picking up green stink bug adults, egg masses, and first instar nymphs. We picked up our first Dectes stem borer yesterday in a corn field that was soybean last season, emerging from its overwintering chamber in the soybean stubble.

Field Corn Insect Scouting Update

Bill Cissel, Extension Agent – Integrated Pest Management; and David Owens, Extension Entomologist,

Japanese beetles began to be spotted late last week, and numbers are increasing. Japanese beetles will sometimes concentrate along the edges of fields, pivot tracks, and, this year, possibly mini-edges from drowned out spots. Japanese beetles damage corn by clipping silks prior to and during pollination. The pollination period is SHORT, so you need to be vigilant. Pollen shed for an individual tassel usually takes 2-7 days and 1-2 weeks for an entire field. As a general rule, treatment may be necessary if silks are clipped to less than ½ inch and less than 50% of the plants have been pollinated and 3 or more Japanese beetles per ear are actively feeding (information from Purdue University,

Japanese beetles typically do not move into the field interior, and border sprays may be all that is needed if you experience the pressure outlined above. Stink bugs can also sometimes be managed by border sprays. Please refer to our Field Corn Insecticide Recommendations if a control measure is necessary:

How can you tell when an ear has been pollinated? Here is an informative Youtube video by Dr. Bob Nielsen demonstrating how to perform an ear shake test to determine corn pollination progress:

If the ear has been pollinated, Japanese beetle silk feeding is of no consequence.

Bacterial Wilt Problems in Cucurbits this Year

Jerry Brust, IPM Vegetable Specialist, University of Maryland;

In a sentinel plot of cucurbits (cantaloupe, cucumber, watermelon, pumpkin, etc.) near Cambridge, MD on the Eastern Shore and in a few other cantaloupe and cucumber fields are some of the worst cucurbit bacterial wilt (Erwinia tracheiphila) infections I have seen in the past 5-7 years. Most of the infected plants are still small and were fed on by cucumber beetles 2-2.5 weeks ago. The first sign of bacterial wilt infection is when leaves near the base of the plant wilt and turn a brownish-gray/green and then dry up (Fig. 1). Then other leaves on the vine with those first dying leaves will begin to flag and wilt in the mid-afternoon (Fig. 2). In a few more days the entire vine will wilt. Sometimes that maybe the only problem but often another vine will start to wilt and then another until the plant is dead. After bacteria enter the plant it takes anywhere from 2-4 weeks for an infected plant to wilt and die.

So far this year about 18% of the cantaloupe and cucumber plants have begun to wilt. Normally I see 3-6% of plants wilt down at this plant size. I don’t think it was an unusually high striped cucumber beetle population (these beetles act as vectors for E. tracheiphila) although a few areas had very high numbers (15-20/plant). It appears that a greater percentage of beetles were carrying the bacteria than what we would normally encounter. In Figure 3 this level of feeding damage would usually lead to about 35-40% of the plants going down to bacterial wilt, this year it is 65-75% of plants like this going down to wilt.

Under this sort of pressure applying neonics to plants while they were in the tray or that were drenched at planting (which is usually sufficient) often will not be enough to hold back beetle transmission of the bacteria 7-10 days after treatment. Foliar sprays with pyrethroids would be needed. But how do you know when more beetles are going to act as vectors—you don’t. And that is the problem, next year do you over treat because of one outlier season or continue with what you have been doing? My guess is that this is a onetime blip that so many more beetles were infective than normal. If your cantaloupe or cucumber plants look good and do not have any more than the usual amount of bacterial wilt you can consider your striped cucumber beetle management to be good.

Figure 1. The base-leaves of an infected vine begin to wilt and then dry up and die

Figure 2. After 7-10 days leaves on the infected vine become flaccid

Figure 3. Heavy beetle feeding on cantaloupe plant

Vegetable Recovery from Temporary Flooding

Gordon Johnson, Extension Vegetable & Fruit Specialist;

On June 9, at our Georgetown research station, we received 4.2 inches of rainfall in a one-hour period. Many of our vegetable research plots were temporarily flooded and most of our plasticulture research area had water that topped the beds.

If this flooding would have lasted for more than 24 hours, most of our crops would have been lost. However, there was enough drainage early on so that the majority the area has come through, albeit with some complications. The following are some pictures that show the recovery and losses:

Pepper plants with leaf drop. Ethylene buildup in saturated soil conditions can cause leaf drop, flower drop, fruit drop, or early plant decline in many vegetable crops. The bed area around this plant was saturated for two weeks. For the plant to recover, the bed must dry out to allow the plant to regenerate roots. The pepper plant has additional sets of buds to allow for new leaves to form but the recovery, if it happens, will put the plant several weeks behind other non-affected plants. Another concern is potential for root rots. In severe cases, cutting away the plastic can save a crop by allowing for better drying out and aeration.

Watermelon plant in saturated bed conditions after flooding showing little growth. Oxygen starvation to vegetable roots will cause roots to cease to function resulting in plant stunting, or collapse, with limited recovery potential. For this plant to recover the bed must dry out and aerate enough for the plant to fully regenerate roots.

Flooded pickling cucumber planting. Soils in in this area stayed saturated for 3 days while seeds were germinating. Lack of oxygen caused seeds to deteriorate and plants did no emerger in much of the area.

Tomatoes fully recovered from temporary flooding. In this case the crop did not drop leaves and was able to regenerate new roots quickly.

Our watermelon variety trial stopped growing and aborted fruits after the flooding but has now fully recovered and plants are starting to fill in. The beds were allowed to dry out for 9 days after flooding and then the crop was fertigated with nitrogen and sulfur. New fruits are now being set but the trial will have few early watermelons.

Spring Broccoli Revisited in 2018

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Growing spring broccoli to acceptable commercial quality standards is a challenge on Delmarva. We are currently repeating for a second year a spring broccoli variety trial at the University of Delaware Georgetown research farm with 24 commercial varieties and several experimental lines to evaluate adaptation of these varieties for spring planting under high density for

Temperature variability is the main challenge for spring broccoli. Broccoli is a cool season plant and is best adapted to areas with consistent temperatures during head development where days are warm, not hot, and nights are cool. On Delmarva, high temperatures during head initiation in the spring leads to abnormal floret development. Temperatures more than 88°F can cause damage to florets in sensitive varieties. More heat tolerant varieties have been developed and are being tested again our spring trials.

This spring has been cool in general; however, there was a 3-day heat spike over 88°F on May 2, 3 and 4 that may have challenged some broccoli plantings. In our early April transplanted broccoli, we saw significant variability in head quality and head disorders between varieties. A second planting was made in mid-April and quality so far has been good on many varieties. Harvest has started on a late April transplanted test with quality being remarkably good.

June has been relatively cool and development has been slowed in broccoli, avoiding another common problem of rapid bolting in high temperatures in spring broccoli in our region. High temperature in later head development causes broccoli to advance so quickly that the crop often flowers before it can be harvested. This reduced harvest window can be a challenge to manage as heads can go from marketable to unmarketable in as little as 24 hours when temperatures are in the 90s.

Lieutenant broccoli has performed well in 2018 spring trials.

A Bejo variety doing well in a late planting

A Seminis variety also performing well in the late April planted trial in 2018.