Vol. 5 No. 4 April 25, 1997

Weekly Crop Update

University of Delaware Cooperative Extension

Volume 5, Issue 8

May 22, 1997


Field Crops:

Field Crop Insects -

Joanne Whalen, Extension IPM Specialist.



Field Corn. Cutworm leaf feeding remains high, especially in mid to later planted fields. Although the number of cut plants is low, we should see an increase in damage as larvae increase in size and the weather turns warmer. Fields should be scouted from spike to the 4-leaf stage for cutworm damage. We have started to see small armyworm feeding in the whorls of the earliest planted corn, especially in no-till fields. Treatment will be needed if 25% of the plants are infested and larvae are less than 3/4 inch long. A pyrethroid will provide effective control.


Small Grains. Economic levels of cereal leaf beetle continue to show up in scattered fields throughout the state. Since the egg laying period appears to be over, you should be able to make a final treatment decision for this insect during the next week. Research from North Carolina shows that controls will be cost effective through the soft dough stage. Grass Sawfly and armyworm can now be found in small grains throughout the state. In general, the predominant species in barley is the armyworm whereas both species can be found in wheat. Since armyworms will clip heads faster in barley, the treatment threshold is one per foot of row. In wheat , the armyworm threshold is 2 per foot of row. We have already received reports of sawfly head clipping in wheat. Treatments should be applied when you find 2 larvae per 5 foot of row innerspace ( 0.4 per foot of row). If the number of heads clipped is already 2 times the worm count and most larvae are large, it is generally too late to treat since most of the damage has already occurred. Remember, Warrior has a 30 day wait until harvest in wheat and is not labeled on barley. As we get closer to harvest and both insects are found in a field, Lannate ( 7 day PHI) or parathion (15 day PHI, Aerial application only) will be the best control options.


Field Crop Diseases -

Bob Mulrooney, Extension Plant Pathologist.



Wheat. Disease levels in general continue to be low. Wheat spindle streak mosaic virus or wheat yellow mosaic as it is referred to now, was confirmed in a sample from Sussex county last week. This virus is transmitted by a soil fungus, so it is soilborne and usually is very uniformly distributed throughout infected fields. Leaves have yellow-green mottling, dashes and streaks. Larger spots can become necrotic and reddish streaking and necrosis at leaf tips sometimes occurs. It has been reported that take-all was seen in wheat this week. Infected plants are stunted, produce bleached heads with no grain, and the plants are pulled easily from the soil. Infected plants will have a shiny, black coating on the stem at the ground line. Peel back the leaf sheath and check out the lower stem. Roots will be rotted and few. Symptoms appear as stunted irregular spots in the field. Rotation out of small grains for several years is the best control. Corn, soybean, and vegetables are good rotation crops. Continuous cropping of wheat or barley and double-crop soybeans can be risky because of the possibility of take-all.

Soybeans. It is still not too late to check fields for the soybean cyst nematode. Soil sample bags are available from the county Extension offices. Turn around time should be just a few days of receiving the samples. The first issue has all the details as well as announcing the revised fact sheet PP-2 Management of the Soybean Cyst Nematode.


Moisture Needed to Activate Herbicides -

Mark VanGessel, Extension Weed Specialist.



The dry weather of the past few weeks could result in poorer weed control from soil-applied herbicides. The chloroacetamides (Lasso, Dual, Frontier, Harness, Surpass etc) need moisture to be "activated". Moisture is needed for two reasons. First, moisture moves the herbicides into the top one to two inches where the herbicide can be taken up by the shoot as it emerges or the roots, depending on which herbicide was used. Second, enough moisture needs to be present that allows the herbicide to be in the soil moisture and is taken up by the plant as it takes up the soil moisture. Without one-half to two-third inches of water, the herbicides are not "activated". As a result the herbicides will not provide the level of control that is needed. In that situation, be sure to scout early those fields that were planted up to three weeks ago and determine if a postemergence herbicide may be needed.

The chloroacetamides will become activated as soon as there is enough rain or irrigation, but these herbicides do not control weeds that have already emerged. They will control those weeds that will emerge after the rain. Atrazine will be taken up by the roots of small weed seedlings and control these small weeds. But weeds over one to two inches will not be controlled.

Be sure to scout and see if lack of rainfall has resulted in poorer weed control and if a postemergence spray is needed. Also, making that decision early will often allow you to use lower rates and still get good control.


Comments on Postemergence Herbicides -

Mark VanGessel, Extension Weed Specialist.



There are a number of products for postemergence weed control available. Each herbicide will have its strengths and weaknesses. Be sure to choose the best product for your situation. Permit and Exceed are similar, Permit is better on yellow nutsedge and Exceed is better on burcucumber and perennial species. Neither product will control grasses. Both perform better with 2 to 4 oz of Banvel.

Emerged grasses will require Accent or Basis Gold (which contains Accent). Note, Accent is not very effective on crabgrass. If johnsongrass or shattercane need to be controlled, Beacon is another option.

Resource is a new herbicide that will provide good velvetleaf control. It is weak on most other species, and so it needs to be tankmixed if more than velvetleaf is present.

Scorpion III will provide control of a number of problem weeds. Scorpion III contains 2,4-D so an early application (less than 8 inch corn) is needed to avoid crop injury.

With all the new herbicides available, we forget about some of the old standards. Banvel, 2,4-D, Marksman, and atrazine are all still available and will provide cost-effective control in many situations.


Considerations on Preemergence Herbicides in Soybeans -

Mark VanGessel, Extension Weed Specialist.



Three years of research supported by the Delaware Soybean Board has shown that reduced rates of soil-applied herbicides can be a cost-effective approach in many situations. Rates as low as one-half the labeled rate for the soil type were used alone and in combination with postemergence herbicides. When used alone, the reduced rates often did not provide as good of weed control as the full rate. However, if the population of difficult to control weeds is high (cocklebur, morningglory, ragweed etc.), a postemergence spray is needed. When the postemergence herbicide is used, treatments with the lower soil-applied herbicide rates were as good as the full rate. If you know there are fields that will require a postemergence spray, lower your costs by reducing the rate of soil-applied herbicides. Be sure to keep an eye on those fields with reduced soil-applied herbicides to get a timely postemergence spray.


Sorghum Planting -

Richard W. Taylor, Extension Agronomist



Soil temperatures remain fairly low. Grower's trying to get a jump on sorghum planting or planting no-till sorghum should remember that unless treated with a fungicide, sorghum emergence from cold soils is very poor. Fungicide seed treatments will increase germination in soils under 75E F. by a substantial amount and help ensure a stand, other factors being favorable.

I've noticed a lot of sorghum sold has been treated with one of the fungicides but check the label to be sure. All sorghum seed comes treated with a seed protectant (Concep or Screen) to safen the seed to Dual or Lasso but these protectants are NOT fungicides. If your seed is not treated with a fungicide, hold off planting until soil temperatures reach about 70 to 75E F. and are on the way up.

Once soil temperature reaches 80E F. and higher, sorghum emergence is close to the germination percentage and occurs very soon after planting (sometimes in 3 to 4 days). Apply your weed control program as soon after planting as possible.

If planting no-till, be certain the seed slot has been closed by your press wheels. The cast iron press wheels do a nice job of slot closure. Slot closure will reduce damage from slugs and will preserve moisture and improve germination and emergence.

Once sorghum emerges especially when planted early and in cold soils, you should evaluate the stand to be sure it is adequate. Always base your evaluation on the average number of plants per foot of row (target figures are available in our grain sorghum production manual). Do not base your evaluation on how the stand "looks". The small seed size of sorghum results in a very small plant so even when the population is ideal, the stand can "look" way to thin. Count and be sure.


Some Thoughts on Irrigating Small Grains -

Richard W. Taylor, Extension Agronomist



Although past research has not shown much response to irrigation by wheat, irrigation of wheat and barley planted in fields under center pivot irrigation systems should be considered. A number of small grain fields are rapidly running out of available soil water. Irrigation may boost yields for these fields or at the very least improve conditions for double-cropped soybeans. When irrigating small grains, keep in mind a few common sense suggestions.

  • Irrigate during the early part of the day so the foliage can dry before evening. Avoid irrigating in late evening or at night since wet foliage potentially can increase disease pressure.


  • Apply the maximum amount of water that can infiltrate into the soil at each irrigation. Do fewer, heavier irrigations rather than many, light ones.


  • Aim to replenish or prevent the depletion of subsoil moisture.


  • Irrigate up to the dough stage but let top soil and subsoil moisture carry the crop on to harvest. This should help prevent man-made reductions in test weight.


A later article will address differences in irrigation strategies for full-season and double-cropped soybeans.


Vegetable Crops:

Vegetable Crop Insects -

Joanne Whalen, Extension IPM Specialist.



Cabbage. With the recent warmer days, diamondback egg hatch has increased and small larvae can be readily found feeding in the hearts of plants. The treatment threshold is 5% infested plants, especially if larvae are feeding in the hearts of the plants. Since the temperatures are warmer, a Bt insecticide should provide effective control at this time. If you are growing a susceptible variety, be sure to start watching for thrips feeding in the leaf axils. The treatment threshold is 20% infested plants. Dimethoate, metasystox-R, Provado or Warrior will provide control.


Cucurbits. Cucumber beetles can be found in fields throughout the state, especially if Furadan was not used at planting. Since cucumbers and muskmelons are very susceptible to bacterial wilt, check fields soon after emergence or transplanting for beetles. Sevin or a pyrethroid will provide control.

Potatoes. Colorado potato beetle activity has increased in fields where Admire was not used at planting. Egg hatch has been found in areas of fields near last years potatoes. The treatment threshold for small larvae is 4 per plant. The threshold for adults is 1 per every 2 plants. In most cases, both stages will be present at the same time so the threshold for each stage should be reduced by one-half. Cryolite(Kryocide or Prokil cryolite), Agrimek (applied at 50% egg hatch), or Provado will all provide CPB control. European corn borer moth activity has started to increase in all potato growing areas. The counts for the Rt 9 and Bridgeville area have reached 15 per night and in the Rising Sun/Magnolia area counts have reached 40 per night. All early planted fields should be scouted for infested terminals ( which will appear as "flagged "or wilted leaves). Research from North Carolina indicates that a treatment should be applied when 25% of the terminals are infested. If you use this threshold, Monitor (1 qt/a) or Furadan (1.5 pt/a) are the best control options. Fields should be re-scouted and a second application will be needed if new infestations are still occurring. If you are unable to scout for infested terminals, then sprays should be applied within a week of finding 20 moths per night in your local blacklight trap. If this approach is used, generally 2 Furadan applications (1.5 pt/a) or 3 pyrethroid applications are needed.


Sweet Corn. Begin watching your earliest planted fields for European corn borer. Since most corn planting was delayed, the earliest planted fields will be very attractive to egg laying. Small true armyworms have been found in the earliest planted fields. The treatment threshold is 15% infested plants. A pyrethroid will provide effective control.


Vegetable Crop Diseases -

Bob Mulrooney, Extension Plant Pathologist.



Potatoes. Conditions continue to be unfavorable for late blight development. DSV=s (disease severity values) have not changed since last week. The cool temperatures and lack of humidity are the factors in our low DSV=s, which is good. The same situation is true for our neighbors in New Jersey, they have not accumulated any DSV=s as of May 21.

With some of the earliest fields approaching bloom, growers are reminded to make applications of Ridomil Gold MZ if pink rot and Pythium leak have been a problem in the past. Two applications are recommended, the first when tubers are nickel-sized or at flowering, then repeated two weeks later. A protectant fungicide application is suggested between those two sprays.


Tomatoes. Apply Bravo C/M, ManKocide, or a copper fungicide plus mancozeb shortly after transplanting and 7-10 days later for the prevention of bacterial diseases.


Cucumber and Cantaloupe. To prevent bacterial wilt continue to make insecticide applications until flowering to control cucumber beetles that transmit the bacteria that plug the water conducting tissues. Early control is essential since symptoms do not appear until vines run and fruit form. No control is possible then.



Correction from Last Week=s Issue

The date of the Crop Consultant Diagnostic Day is July 31, 1997 not July 3 as it was printed. Sorry for the error.


Upcoming Events...

July 31, 1997 -

Crop Consultant Diagnostic Day, U of D Research & Education Center, 9 AM


No-Till Vegetable Transplanter Demonstration Day and Twilight Meeting


Thursday, May 29, 1997


University of Delaware Research and Education Center, Georgetown, DE


During the Day, anytime between 10 a.m. to 4 p.m.

Twilight Demonstration begins at 6:30 p.m.


All Delaware vegetable growers and any other interested individuals are invited to attend a demonstration of a sub-surface tiller/transplanter. This machine was devised by Dr. Ron Morse of VPI to place transplants in a no-till production system. We will be transplanting tomatoes, peppers, cantaloupes, watermelons, and eggplants into rye cover crop that has been flattened with a Buffalo roller/chopper prior to the demonstration.

The no-till vegetable transplant production system has great promise for late vine crops such as watermelon, cantaloupe and pumpkins. It also has been proven to be an ideal system for tomato and cole crop production.

For more information or for special needs in accessing the demonstration, contact Gordon Johnson at the Kent County Extension Office at 302-697-4000 or e-mail at gcjohn@udel.edu



Crop Update for Delaware

Delaware growers are irrigating crops. Very little rainfall during the month of May (1.24" total) and prevalent drying winds have made field conditions very dry.

Wind damage has been seen in newly emerged fields of sweet corn and carrots, and newly transplanted vegetable crops.

Cool temperatures and wind has slowed the growth of cantaloupe and watermelon transplants.


Planting and Harvest

Fresh Market:

Strawberry harvest began this week in Sussex County.



Pea harvest will begin late next week.

Carrots in Delaware are 2" tall. Snap bean planting is in it=s third week and will continue through August.

Lima bean planting will begin the first week of June.

Sweet Corn is still being planted in Delaware. Out-of-State Processors have just finished planting with Local Processors having 50-75% of their acreage in.

Did You Know?


Question - When is a potato not a potato?


Answer - When it is a peanut.


APeanuts that are used in packaging of products shipped through the mail can now be made from potato starch.

This environmentally friendly packaging does not require the use of chlorofluocarbons (chemicals that deplete the Earth=s ozone layer) for its production. The materials aren=t manufactured from plastic, so the Apeanuts@ you receive in shipping boxes won=t just sit around in landfills, without degrading. Instead, they quickly dissolve into safe, organic by-products. The same manufacturer also produces potato-starch golf tees that disintegrate in the rain.

Weather Summary

University of Delaware,


Week of May 16 to May 22, 1997


0.03 inches: May 16

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



Ranged from 90EF on May 20 to 65EF on May 17.


Ranged from 68EF on May 20 to 40EF on May 17.


Soil Temperature:

61.9EF. 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.