Guess the Pest! Week 6 Answer: Soil Compaction

David Owens, Extension Entomologist, and Gordon Johnson, Extension Vegetable & Fruit Specialist;

Congratulations to Will Carlisle for correctly answering soil compaction. Will will receive a sweep net and be entered along with all correct guessers for the end of season raffle. Unfortunately, this is one case where a sweep net is not going to do much to alleviate the problem, unless you put a shovel or soil corer on the end of the handle.

This from Gordon Johnson:

Peas do not perform well in soils that are worked when they are too wet. Compaction will lead to poor emergence and reduced growth. Wet soil conditions, compaction, and poor drainage are also associated with higher rates of infection of root rots in peas such as Aphanomyces root rot, or common root rot. Soil compaction limits root development and root function and will reduce yield potential in vegetable crops such as peas.

There are two processes at play when soils are compacted by equipment. The first is destruction of soil structure. In most Delaware soils, our surface soil structure is granular or crumb in nature and consists of small aggregates. It takes considerable time and good cropping practices to build up soil structure. When compacted by equipment, structure is destroyed, making soils denser. Excessive tillage also destroys soil structure.

A second compaction process is the compression of soil particles, pushing them closer together. This happens with equipment traffic across fields. The heavier the loads carried by equipment passing over soils, the more the compaction. With large equipment and heavy axle loads, significant soil compaction is expected; the heavier the weight on an axle, the more the compaction. Other equipment factors affecting compaction include tire size, tire pressure and operating speeds. Wider tires or dual tires will distribute weight over larger areas, reducing deep compaction but increasing the amount of area with shallow compaction. Higher tire pressures will result in more deep soil compaction and slower speeds will also result in more compaction.

In wet soil, there is less resistance to soil particle movement and soil is more “plastic”. This means that potential for compaction is greater in wet soils than dry soils. It is important to wait until soil conditions are favorable for tillage. Waiting a day or two for soils to dry will improve yield potential by reducing compaction.

Subsoiling in the fall is a short-term solution to deep compaction. The use of forage radish cover crops has shown great potential to reduce shallow and deep compaction. Research in Delaware has shown that peas can be no-tilled after a winter-killed radish cover crop successfully with equivalent or better yields than conventionally tilled peas.

Yellowing in Peas

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Pea harvest is nearing and we are seeing yellowing and poor growth in many pea fields due to wet conditions. Peas do not perform well in soils that are worked when they are too wet or when they receive heavy rainfall after planting. Compaction and crusting over will lead to poor emergence and reduced growth. This is evident in many Delmarva pea fields in 2019.

Recently, heavy rains have caused some pea fields or parts of fields to turn yellow, particularly were there was compacted soil or poor drainage. Peas are effective at fixing nitrogen; however, we normally apply 40-80 lbs/a of fertilizer nitrogen (N) prior to planting thus reducing N fixation contributions from Rhizobium nodules on the roots. With the frequent rainfall, some fields have remained saturated and denitrification has occurred, reducing available N from the initial fertilizer application. In addition, root function and Rhizobium nodulation is further impaired in saturated soils, thus limiting any potential N fixation contributions.

In pea fields that have had a past history of root rot, we have the potential to see problems in 2019. According to the Crop Profile for Peas in Delaware: “Aphanomyces root rot, or common root rot, is one of the most destructive diseases of peas. It occurs in most pea producing regions of the U.S., including the Mid-Atlantic. In the Northeast, average annual yield loss to this disease is about 10%, though losses in individual fields may be up to 100%. Wet soil conditions and poor drainage are associated with higher rates of infection. The disease is most damaging in years when a cool, wet spring is followed by an early, warm summer with low rainfall.”

Good pea growth and development.

Yellowing in peas in wet soils

Vegetable Crop Insect Scouting

David Owens, Extension Entomologist; 

Scout peas for aphids. We have seen unusually high numbers of aphids in other crops. Paraphrasing from my predecessor: “On small plants, sample 10 plants in 10 locations, on larger plants, 10 sweeps…5-10 aphids per plant or 50 per sweep is a good action threshold for aphids in peas.”

Some of the first melon transplants are going out soon. Be advised that striped cucumber beetle may be active earlier this year than in recent years past due to the unusually warm weather.

Pea Herbicides

Mark VanGessel, Extension Weed Specialist;

Weed control options remain limited for processing peas. Pursuit can be used as a pre-plant incorporated or preemergence treatment and is used primarily for broadleaf weeds. Preemergence applications of Command or Dual are labeled for control of annual grasses and some broadleaf weeds. Crop safety with these herbicides improve as the air and soil temperatures increase.

Be aware that if you intend to plant lima beans after peas, Command cannot be used in peas. The Command label states a 9-month rotation between application and planting lima beans

For postemergence applications, Basagran and Thistrol are labeled for broadleaf weeds. Apply Basagran after peas have more than three pairs of leaves. Do not add oil concentrate. Select, Assure II, Targa, or Poast can be used for postemergence grass control.

For no-till plantings of early peas, controlling all weeds present before planting can be challenging. The cool weather can result in poor performance with glyphosate. There are no products that can be tankmixed with glyphosate to improve control in this situation. In some situations, use of glyphosate 10 to 14 days prior to planting and then a sequential application of Gramoxone maybe need at planting to completely kill weeds prior to planting.

Insecticide Trial Results for Vegetable and Agronomic Crops

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

Summaries of last season’s insecticide trials in peas, sweet corn, watermelon, field corn, soybean, and wheat can be viewed at

Results from 2018 Vegetable Variety Trials Available Online

Emmalea Ernest, Associate Scientist – Vegetable Crops;

The Extension Vegetable and Fruit Program conducted several variety trials in 2018. Crops that we evaluated for yield and quality characteristics were: seedless watermelon, mini seedless watermelon, baby lima beans, peas, beets and snap beans. Reports on these trials are now available online at:

Challenging Pea Season, Requesting Information on Pea Varieties Affected with Powdery Mildew

Gordon Johnson, Extension Vegetable & Fruit Specialist;

As pea harvest enters the last 2 weeks, reports are that yields are down significantly this year. Average yields are below 3000 lbs. in many fields. While weather variability has been a factor, high incidence of powdery mildew has been the main cause leading to reduced yields. Powdery mildew has also caused wider variability in crop maturity, making harvest decisions difficult, and has reduced pea quality.

Many pea varieties have powdery mildew resistance. The UD vegetable program is surveying processors and growers to see what varieties were affected and what varieties did not have powdery mildew. This information will be matched to seed company listings for powdery mildew resistance. We hope to have a completed list for industry to use in decisions for 2018 variety selections.

There also have been shifts in the powdery mildew pathogens in other regions and this will need to be investigated on Delmarva.

Information requested for all pea varieties planted is the following: Variety, Planting Date, Harvest Date, Presence of Powdery Mildew, and if present, When First Observed, Field Yield, and Estimated Yield Reduction. Please email this information to Gordon Johnson ( and Emmalea Ernest (

Powdery Mildew in Peas

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland;

Pea powdery mildew is currently widespread on Delmarva. This disease occurs throughout the US. The pathogen can overwinter in debris, on alternate hosts, or (less commonly) be seedborne. Because the pathogen is widespread, high disease severity occurs when conditions are favorable. Powdery mildew is favored by nights with dew, moderate temperatures (68-75F), and low light intensity. These conditions have prevailed in Maryland and Delaware for the last several weeks. Powdery mildew infestations can lead to uneven ripening of the crop, and yield loss (fewer peas per pod, lowered pea weight, etc.). Quality losses also occur. Several pea cultivars that are resistant to powdery mildew are available, and should be used if possible. Fungicides that are registered for management of powdery mildew include sulfur, which needs to be applied at 3 to 10 lbs/A, and Endura and Priaxor. These fungicide treatments can be costly because two applications at a 10 day interval may be needed.

Reduced Seed Set in Peas

Gordon Johnson, Extension Vegetable & Fruit Specialist;

There have been a number of early pea fields with reduced seed set. Pods developed but only one or two seeds were formed.

Reduced seed set is often related to flower development and pollination. Peas are self-pollinated. As the flower opens, the pollen from the anthers is released to the stigma of the pistil of the same flower. Once on the pollen is on the stigma, the pollen germinates and a pollen tube is formed and then grows down the style and when it reaches the ovule, the egg is fertilized by one of the two sperm cells, the other fuses with polar nuclei to become the seed endosperm. During the development of the pollen tube, plant hormones are released which are also essential for seed set.

Seed set problems therefore may be related to lack of pollen formation, pollen that does not release to the stigma, reduced pollen germination, abnormal pollen tube development, abnormalities in the stigma or stile, or abnormalities in the ovule. Lack of Gibberellin hormone release has also been shown to reduce seed set or lead to early seed abortion in peas. Stress to peas just after flowering has been shown to cause seed abortions.

What are the potential causes of reduced seed set in peas? Frost or freeze when flowers are opening has the potential to injure pollen or directly damage flower parts. Peas are very cold tolerant normally but are susceptible to injury at flowering. Our last freeze event on Delmarva was on April 9 where temperatures dropped to below 30°F at some locations. Fortunately, early peas were not in flower during that time.

Research has shown that peas under temperature and moisture stress produce fewer seeds. Experiments have shown that temperatures at 93°F or above can also reduce seed set in some varieties of peas. Dry soil conditions will magnify this effect. We had temperatures at 90°F in some locations on April 29 with upper canopies approaching critical temperatures during flowering.

Another factor to consider is timing of chemical applications to peas – applications near and at flowering may damage pea flowers under certain weather conditions and reduce seed set.

Pea pods with reduced seed set. Affected fields yielded 1000 lbs/a while nearby unaffected fields yielded near 4000 lbs/a.

Pea Harvest Season is Underway

Gordon Johnson, Extension Vegetable & Fruit Specialist;

The processing pea harvest season started on May 17 this year, about one week early than normal with February planted peas. Some of these early fields had freeze damage that resulted in erratic pod development and lower yield potential. With the higher than normal temperatures this week pea progress has advanced quickly making harvesting decisions challenging. Under high temperatures, peas can go from ideal harvest condition to overmature in a day, with some varieties increasing by more than 30 tenderometer units over 24 hours. Saturday will bring cooler weather and more seasonable temperatures are expected for the remainder of May, which will favor slower pea development.