Weekly Crop Update

University of Delaware Cooperative Extension

Volume 5, Issue 14

July 3, 1997


Field Crops:

Field Crop Insects -

Joanne Whalen, Extension IPM Specialist.




Potato leafhopper adults and nymphs can be found in fields throughout the state. Fields should be sampled within a week of harvest for both adults and nymphs. Treatments should be applied if you find 20 leafhoppers 100 sweeps in alfalfa 3 inches or less in height, 100 per 100 sweeps in 7-11 inch alfalfa, and 150 per 100 sweeps in alfalfa 12 inches or greater in height. Ambush, Baythroid, dimethoate or Pounce will provide good control. If leafhoppers are reaching threshold and the field is 60% bud or flowering, early harvest will be the best control option.


Field Corn.

Be sure to check your earliest planted field corn for corn borer infestations. We have seen fields that are approaching 50% infestation. Be sure to also watch for larvae boring into the mid-ribs of leaves and stems. If at least one-third of the larvae have bored into the stems and/or leaves, a control would not be cost effective. The treatment threshold is 50% infested plants in irrigated and 75-80% infested plants in dry-land corn. A pyrethroid or Penncap-M will provide effective control if sprays are applied as the tassels are emerging from the whorls. NOTE - Penncap should not be used if bees are foraging in the area. Since many fields were planted later, you may begin to see fall armyworm larvae feeding in the whorls. If a BT hybrid was planted, research results indicate that you should see good fall armyworm control. During the early to mid-whorl stages, a treatment should only be considered if plants are stressed, larvae are less than one inch long, and 35% of the plants exhibit heavy feeding. In silage corn, a treatment should only be considered if plants exhibit heavy feeding on 50-75% of the plants. Treatment should be applied by ground using a minimum of 30 gallons of water per acre. Lannate or Warrior have provided control if the spray is directed into the whorls. Dipel 10G or Lorsban 15G have also provided good control.



Manganese Deficiency in Soybeans -

Richard W. Taylor, Extension Agronomist.



Delaware's light but variable soils make the maintenance of a consistent soil pH difficult. Often within a single field, soil pH can vary by 1 or 2 or more pH units. In the future, farmers will be better able to manage this situation with variable rate application and GPS/GIS (global positioning systems/geographic information systems) technologies. Until then in areas of high pH, manganese (Mn) deficiency can be a problem on soybeans.


Manganese deficiency symptoms occur when the level of soil Mn and soil pH combine to limit plant available Mn below that needed for crop growth. Sandy soils have less native soil Mn and less buffering capacity so applications of lime can cause large swings in soil pH and lead to Mn deficiencies. Deficiency symptoms for Mn in soybeans are:


* Pale green to yellow-red color between green veins.


* Mottled effect of yellowing with veins remaining dark green to olive green color


* Stunting with above leaf symptoms especially on newest leaves.


* In severe cases, the most recently expanded trifoliate leaves will be very pale yellow to nearly white in color with only a suggestion of veins.



Yes, this problem can be managed and, if caught early, full yield potential can be maintained. Most often this problem reoccurs in the same field year after year. Barley is very sensitive to low soil Mn levels and high pH and can be used to indicate where trouble is likely. Also, you should scout carefully fields that have recently been limed, have received heavy broiler litter application (temporarily raising the pH), or have been fertilized with layer manure.

When symptoms first appear and enough leaf area is present to intercept a foliar spray, apply 1 lb Mn/A (elemental Mn) as manganese sulfate (techmangam) or 0.75 to 1 lb Mn/A as chelated manganese. If symptoms reappear as often happens, apply a second application at first bloom (R1 growth stage).

When do plants have enough leaf area to intercept the spray? Usually, plants at the V5 to V7 growth stage have adequate leaf area to intercept enough foliar spray to solve the problem. If the deficiency is severe, a second application may be needed at R1 to maximize yield potential. Growth stage V5 is when there are five nodes on the main stem with fully developed leaves. A leaf is fully developed when the leaf immediately above it on the main stem has opened (the leaf has unrolled sufficiently that the leaf edges are not touching). The first node is where the two unifoliate leaves were attached and each node thereafter has one trifoliate leaf attached.

Late-planted beans or very early-maturing beans can begin blooming at about the V4 to V5 growth stage and must be scouted carefully to catch the problem as early as possible. Full-season late-group III, group IV, and group V beans begin blooming in mid- to late-July and should have canopied the ground by two weeks prior to bloom. Depending on planting date, these beans should have adequate leaf area by late-June if foliar Mn is needed.



Manganese Applications With Postemergence Roundup -

Mark VanGessel, Extension Weed Specialist.



Roundup Ready soybeans may require a postemergence application of Roundup and a manganese application about the same time. Roundup Ultra can be tankmixed with manganese with some precautions. The manganese products can bind with Roundup in the spray tank and reduce Roundup's effectiveness. The form of manganese had an impact on preformance. Manganese chelated with EDTA did not affect the performance of Roundup, but other forms did. The addition of ammonium sulfate overcame the problem. Thus, when tankmixing Roundup with manganese, use an EDTA form of manganese or add ammonium sulfate to overcome the reduced weed control.


Field Crop Diseases -

Bob Mulrooney, Extension Plant Pathologist.




Soybean cyst nematodes can be seen on the roots of susceptible varieties from 28-34 days after emergence. If you see stunting or irregular patterns in a field, you may want to check for the presence of SCN. White and yellow females can be seen if the stunted plants are dug and not pulled from the soil. The females are attached to the root and can be seen with the unaided eye. They are small, only 3/4 of millimeter, smaller than the nitrogen fixing nodules, but are often very numerous. The females will turn from white to yellow, then dark brown. Once they turn brown, they fall easily from the roots and are much harder to see without the help of a hand lens. They will usually produce three generations here in Delaware. Fact sheets are available from the county offices on SCN, taking soil samples for nematode detection, and selecting SCN resistant varieties.


Septoria brown spot can also be seen on the unifoliate leaves now. This is usually a minor fungus leafspot especially when seasons are dry. Septoria usually causes defoliation of the unifoliate leaves.


Soybean severe stunt virus was also diagnosed this week in a field with a history of SSSV. This is a unique virus of soybeans that is only known to occur in Sussex County, Delaware. Infected plants are very stunted in irregular areas in the field, the leaves will be abnormal, and often superficial reddish-brown lesions can be seen on the stems. Planting resistant varieties or rotating away from soybeans are the only controls. We are evaluating more varieties for resistance to this virus again this season, with a grant from the Delaware Soybean Board.


Upcoming Events:


Weed Twilight Tour


July 21, 1997

6:30 p.m. in the grove.

University of Delaware Research & Education Center, Georgetown,



Tour features weed control programs in corn & soybeans as well as several types of herbicide resistant corn and soybean varieties. One pesticide re-certification credit will be given.



Crop Diagnostic Field Day


July 31, 1997

7:30 a.m. - 1:00 p.m.

University of Delaware Research & Education Center, Georgetown, Delaware.


Participants can earn 3 continuing credits for the certified crop advisor program.

( 1.5 credits for integrated pest management; 1.0 credits for crop production; 0.5 credits for soil fertility.)

Registration is limited. Cost is $25 and pre-registration is required.


For more information, contact Mabel Hough at




Corn Troubleshooting For Growers Field Meeting


July 31, 1997

3:00 p.m. - 5:30 p.m.

University of Delaware

Research & Education Center, Georgetown, Delaware


Weeds, Insects, Nematodes, Fertility, Variety, Yield Potential


Test your diagnostic skills for solving corn production problems that can occur during the growing season. University of Delaware Extension Professional will provide hands-on training to improve your trouble-shooting skills in corn. As a group you will determine the cause of the problem and possible corrective and preventative solutions. The field scenarios have been set up by Joanne Whalen, Mark VanGessel, Richard Taylor, Bob Uniatowski, and Bob Mulrooney. This exercise will be challenging and fun. The meeting will conclude with dinner. Two pesticide re-certification credits can be earned for participation in the meeting.

This field meeting will provide a great opportunity to train younger members of your operation in the art and science of troubleshooting, and will be a great refresher for more experienced producers.


Corn Troubleshooting for Growers is open to everyone. Prior registration is required. Participation is limited to the first 60 applicants. The registration fee will be $ 15 and should be received by July 18, 1997.

Registration will begin at 2:30 p.m., with training starting at 3:00 p.m. The program will conclude by 5:30 p.m., followed by dinner. Hand lens and sweep nets will be available for use if needed.


For more information contact Derby Walker or Mabel Hough at 302-856-7303.


Vegetable Crops:

Vegetable Crop Insects -

Joanne Whalen, Extension IPM Specialist.



Lima Beans.

As soon as the earliest planted fields begin to flower and set pin pods, fields should be sampled for lygus and stink bugs. Treatment should be considered if you find 15 adults and/or nymphs per 50 sweeps. Dimethoate will only control lygus bugs. Lannate should be used if both species are present.



In areas where corn borer catches are above 3 per night, peppers should be sprayed on a 7-10 day schedule for corn borer control.



Low levels of aphids can now be found in fields where Admire was not used at planting. The treatment threshold is 4 per leaf until 2 weeks from harvest when this threshold increases to 10 per leaf. Monitor or Provado will provide green peach aphid control. Leafhopper adults and nymphs can also be found in these situations. Since damage can occur quickly, controls should be applied if you find 1 per sweep and/ or just 5 nymphs per 50 leaves. A pyrethroid or Provado will provide control. Colorado potato beetle larvae can also be found in a number of fields. In many cases, Kryocide or Provado will provide the best control at this time. For resistant management, Provado should not be used where Admire was used at planting. We recently conducted a field demonstration with Agri-mek. Results at four days after treatment indicate that we got good control of small larvae but fair to poor results on large larvae.


Snap Beans.

At this time corn borer catches are low in most traps throughout the state. In areas where corn borer catches are above 3 per night and/or you can easily flush moths out of fields , processing snap beans should be sprayed at bud and again at early pin with Orthene (1 1/3 lb/acre). After the pin spray, sprays will be needed on a 7 day schedule. In fresh market snap beans, Lannate should be applied on a 7 day schedule until harvest.


Sweet Corn.

Fresh market silking sweet corn should be sprayed on a 4-5 day schedule in Kent and Sussex counties and on a 6 day schedule in New Castle County. Sprays should be applied as soon as ear shanks are visible. We continue to see economic levels of corn borers in corn that is 18-24 inches in height. Treatment should be applied if you find 15% infested plants. Materials should be directed into the whorls using at least 30 gallons of water per acre to get effective control. Continue to watch for small fall armyworm on the latest planted field. Populations still remain low; however, moth activity and egg laying has started to increase in the southern part of the state.


Vegetable Diseases -

Kate Everts, Extension Plant Pathologist, Universities

of Maryland and Delaware



White mold, caused by the fungus Sclerotinia sclerotiorum, has been common in pea fields during the past two weeks. Before planting another crop into infested fields, consider that white mold has a wide host range including snap and lima beans, and to a lesser extent cowpeas (black-eyed Peas). In order to reduce disease on one of these susceptible crops, try to make the environment less favorable to disease development. Sclerotia (the overwintering structures of the fungus) need prolonged wet soil conditions to develop apothecia (the structure necessary for infection of the crop). Plant bean varieties which have more upright architecture and less dense canopy. Plant rows in the direction of the prevailing wind, and avoid high plant populations and narrow row spacing. Also, schedule irrigation so that the soil surface stays drier after flowering. These cultural modifications may make the environment less favorable to disease, and reduce disease damage in infested fields.


Vegetable Crop Diseases-

Bob Mulrooney, Extension Plant Pathologist, University of Delaware



Root Knot Nematode.

Be on the lookout for root knot nematodes. Along with soybean cyst nematode, root knot can be diagnosed in the field by the presence of the nematodes on the roots. Root knot produces swellings on the roots of susceptible crops such as cucumbers, cantaloupes, carrots, beans, and many other vegetable crops. At this time of the season the first indication may be some stunting in irregular areas in an infested field. Digging the plants carefully at this time can reveal the swellings on the roots called galls or knots that form around the developing female root knot nematode. Little can be done after galling is seen. Frequent irrigation can help minimize the damage to the infected root systems. Check for root knot by soil sampling fields to be planted to susceptible vegetables in the fall before planting the crop. Fumigants or insecticide/nematicides such as Vydate can be used before planting to reduce the nematodes.


Sweet corn.

It was mentioned before that common rust is present in some plantings of susceptible sweet corn. If rust is seen before whorl stage, fungicide control may be beneficial. Scouting young sweet corn for rust is highly recommended.



Ozone damage was diagnosed this week on the variety Snowden. The typical purple to black flecking was very easily seen on the younger leaves, while the oldest leaves were completely brown. On the lower leaf surface pitted areas could be seen producing what we call bifacial necrosis. Ozone damage had been reported earlier on the very susceptible red variety Norland.




Late Blight Report.


DSV accumulation as of July 1, 1997 are as follows:


Location/ Emergence date


DSV’s June 26


DSV’s June 30



Baldwin - 4/20



10-day, high rate

Jackewicz - 4/26



10-day, high rate

Zimmerman - 4/28



7-day, high-rate

Baker - 5/7 -5/9



10-day, mid-rate


Weather conditions continue to be unfavorable for late blight development. These next two days of possible showers and thunderstorms will probably provide favorable conditions which may impact some late planted fields. Late blight information hotline number 1-888-831-SPUD.


Weather Summary

University of Delaware,


Week of June 27 to July 3, 1997


 0.47 inches: July 3

Readings taken for the previous 24 hours at 8 a.m.




Ranged from 96 F on June 27 to 80 F on July 3.


Ranged from 73 F on July 3 to 57 F on June 29.


Soil Temperature:

74.3 F. Average for the week.




Complied & Edited By:

Tracy Wootten

Extension Associate - Vegetable Crops

Cooperative Extension Education in Agriculture and Home Economics, University of Delaware, Delaware State College and the United States Department of Agriculture cooperating, John C. Nye, Dean and Director. Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. It is the policy of the Delaware Cooperative Extension System that no person shall be subjected to discrimination on the grounds of race, color, sex, disability, age or national origin.