Vol. 5 No. 4 April 25, 1997

Weekly Crop Update

University of Delaware Cooperative Extension

Volume 5, Issue 6

May 8, 1997


Field Crops:

Field Crop Insects -

Joanne Whalen, Extension IPM Specialist.


Field Corn.

As soon as corn emerges, be sure to begin scouting for signs of cutworm damage. Small larvae, leaf feeding and cut plants have been found in fields in Kent and Sussex Counties. Once leaf feeding is observed, fields should be scouted twice a week to time insecticide applications. From the spike through the 5-leaf stage, a rescue treatment is recommended if you find 10% leaf feeding of 3% cut plants and live larvae are present. Treatments should be applied before larvae are one-inch long. Asana, Ambush, Pounce or Warrior will all provide effective control.

Small Grains.

Insect activity remains low in most fields throughout the state. Sawfly numbers are still low and an occasional cereal leaf beetle larvae can be found. As wheat heads emerge, be sure to watch for aphids feeding in the heads. The combination of a cool, spring followed by a period of warm weather resulted in aphid explosions in 1996. Treatment is recommended if you can find 20 to 25 aphids per head and beneficial activity is low. Dimethoate (wheat only), Disyston, Malathion, Penncap-M, and Warrior (wheat only) will provide control.

Companies Squabble Over Glufosinate-Resistant Corn Patent Right - Mark VanGessel, Extension Weed Specialist.


The most recent issue of Progressive Farmer (May 1997) ran an article of the legal woes associated with glufosinate-resistant (Liberty Link or GR) corn. Dekalb and AgrEvo (manufacturer of Liberty herbicide) have court action pending on the patent of this technology. As a result, AgrEvo will not warrant the crop safety of Dekalb hybrids resistant to Liberty (designated GR). AgrEvo will stand behind crop safety issues associated with Liberty Link hybrids. The bottom line for those who have planted or will plant glufosinate-resistant corn is be sure to keep record of which hybrid was planted and which herbicides were applied to that corn.

Acetanilide Herbicide Injury to Corn -

Mark VanGessel, Extension Weed Specialist.


The last few weeks have been ideal conditions for corn injury from acetanilide herbicides. This class of herbicide is the residual grass herbicides (i.e. Dual, Micro-Tech, Partner, Harness, Surpass, Frontier). Cool damp weather increases the chance of corn seedling injury. The seedlings may or may not emerge from the soil. The type of injury observed is emerged seedlings with the leaves not unrolling properly. The leaves seal together and the new tissue continues to emerge and tries to push itself out. The result is seedling with the tip of the leaves sealed and bent to the side and the lower part of the leaf loops upward. It is quite a distinctive symptom. Nothing can correct the problem once it occurs. If the corn emerges and this symptom is seen, the corn often grows out of the problem, although the first few leaves maybe torn. Under severe conditions, or in situations where the corn does not emerge, stand loss can occur and re-planting considerations come into play. If corn is having difficulty emerging, a light rotary hoeing can loosen the crust and assist the plants in emerging. I have seen severely injured seedlings produce a normal looking mature plant. Decisions must be made on a field by field observation.

Grain Marketing Highlights -

Carl German, Extension Specialist, Crops Marketing.


For the week ending May 9, it is imperative to note planting progress for U.S. corn and soybeans. It is duly noted to be well ahead of the five and ten year average, at 50 % for corn and 9 % for soybeans. Assuming normal 1997 crop development, the early progress made in planting portends to the need to get initial 1997 crop sales made, if not done so already. Next week, we will analyze the information forthcoming from the May USDA crop report.

Checking for Frost Damage -

Richard W. Taylor, Extension Agronomist.


Bob Uniatowski, Extension Associate-Field Crops.


Many small grain fields are heading early this season. With conditions favoring frost on May 8, we thought we would review with growers what to expect and look for in frost damaged small grains.

Frost is generally most severe in low areas called frost pockets. Since cold air is heavier than warm air, it flows downhill towards low places, where it accumulates and causes a frost pocket. Ridges and other terrain aspects can direct the flow to unexpected places too. Fields surrounded an several sides by woods are also susceptible. In these fields, the woods block the wind and allow areas of still air to form. Areas of still air lose heat rapidly and can result in frost areas even when other nearby fields are not affected. In these unaffected fields, the wind keeps the air stirred up, mixing warmer air from above with the colder air near the ground.

What symptoms do you look for? If the head is exposed when the frost occurs, the cold can cause pollen to be either non-viable or stop its formation and release from the anthers (the male portion of the wheat flower). This results in blank heads that with time bleach out to a straw color. These heads will contain no grain at all. If the head is still in the boot, the upper portion, of the stem directly below the head may develop a curlicue appearance. This will look a little like the twist seen, in the stem directly beneath the head in Barsoy barley. In this case, a portion of the head may be barren while the rest of the head contains normal-looking grain. The portion of the head that is barren will bleach out to a whitish or straw color and may vary from head to head based on the exact development stage of each head at the time the frost hit.

Vegetable Crops:

Vegetable Crop Insects -

Joanne Whalen, Extension IPM Specialist.



Small diamondback larvae can be found feeding in the hearts of plants. A treatment is recommended when 5% of the plants are infested. Monitor or a Bt will provide control. If the weather remains cool, Lannate, Thiodan, or a pyrethroid should be mixed with the Bt. A pyrethroid should not be used in areas were diamondbacks are resistant to pyrethroids.


In fields where Admire was not used, adult beetles can be found feeding on plants along field edges and egg laying has begun. Examine 5 plants in 10 locations throughout a field for beetles. Be sure to check around the base of plants and cracks in the soil, especially early in the morning. A treatment is recommended if you find 25 adult beetles per 50% plants and a 20% defoliation. Provado at 3.75 oz. per acre will provide excellent control. Agri-Mek (from Merck) has been recently labeled for Colorado potato beetle control on potatoes. The first application should be applied at 50% egg hatch. The use rate is 8 oz. per acre.

Sweet Corn.

Cutworm leaf feeding activity has also increased in sweet corn. The treatment threshold is 10% leaf feeding or 3% cut plants and live larvae present. A pyrethroid applied late in the day will provide the best control.

Crop Pest Hotline.

Statewide corn borer and corn earworm blacklight trap information will be updated once a week starting May 14. In-state: 1-800-345-7544; Out of state: 1-302-831-8851.

Correction in the Commercial Vegetable Guide -

Mark VanGessel, Weed Extensions Specialist.


In the weed control section for beans (page 66 of the 1997 Guide) the entry for Command failed to specify "for use only on snap beans in VA and MD". Only snap beans have a state label in VA and MD. Lima beans are severely injured by Command. Please note this correction in your guide.

Vegetable Crop Diseases -

Bob Mulrooney, Extension Pathologist.


Fungicide Update.

ISK Biosciences is no longer marketing Bravo 720 under this brand name. All new product is now called Bravo Weather Stik. All Ridomil products sold by Novartis, formerly Ciba Crop Protection, now contain the active isomer of the former product and have been renamed Ridomil Gold. Some older product is probably still around and bears the old name. The new formulation contains more active ingredient, so use rates have been reduced.

Section 18's Granted.

The Delaware Dept. of Agriculture received word from EPA that Delaware has been granted section 18 registrations again for four fungicides to control late blight on white potatoes. Acrobat MZ, a.i. 9% dimethomorph and 60% mancozeb, manufactured by American Cyanamid Co.; Curzate M-8, a.i. 8% cymoxanil and 64% mancozeb, manufactured by E.I. duPont de Nemours and Co.; Manex C-8, a.i. 8% cymoxanil and 64% mancozeb, manufactured by Griffin Corp; and Tattoo C, a.i. 30.5% propamocarb hydrochloride and 30.5% chlorothalonil, manufactured by AgrEvo USA Company may be used. All four products may be applied by ground or air and have 14 day pre-harvest intervals (PHI). A maximum of 5,000 acres may be treated. Be sure to check the new labels for use rates and restrictions.


By the time you read this the Late Blight Hotline should be in service. The phone line, 831-6400 in New Castle county and 1-888-831-SPUD for long distance callers is in service now. Late blight has been active in Florida since February and just this week late blight was discovered in an eastern North Carolina potato field. The North Carolina infection was introduced on seed and the seed source was reported to be from Maine. Growers need to be vigilant. Late blight is not going away anytime soon. Potatoes on the eastern shore of Virginia look good and are late blight free at the present time. They have accumulated 18 DSV values already and protective fungicides are being applied.


Fruit rot control. Straw mulch should be in place between the plants and between the rows. Other cultural controls to avoid fruit rot include: avoiding excessive nitrogen applications, controlling weeds, and managing irrigation properly. If fungicides are used, apply them during bloom. Begin applications at 5-10% bloom and repeat in 10 days. Research has shown that these two sprays using Ronilan or Rovral plus captan can be as effective as a season long spray program. Since gray mold gets established on the flower petals, early control is important. Sprays applied after bloom provide little fruit rot control, but help prevent foliage infections by leaf-spotting fungi.


Avoid excessive cutting pressure on beds to reduce the incidence of Fusarium root and crown rot. Fields should not be harvested until the third year in the production field, and those fields should be harvested for only two weeks. The next year harvest beds for 4 weeks, and all remaining years harvest from 6-8 weeks.

Compiled & Edited By:

Tracy Wootten

Extension Associate - Vegetable Crops


Weather Summary

University of Delaware,


Week of May 2 to May 8, 1997


0.11 inches: May 2

0.24 inches: May 4

0.15 inches: May 7

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



Ranged from 73EF on May 4 to 65EF on May 5 and 8.


Ranged from 51EF on May 4 to 34EF on May 8.

Soil Temperature:

57EF. Average for the week.


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.