Weekly Crop Update

University of Delaware Cooperative Extension

Volume 5, Issue 16

July 17, 1997

 Field Crops:



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



Field Crop Insects -

Joanne Whalen, Extension IPM Specialist.

Derby Walker, Extension Ag Agent





Mite populations have exploded this last week with the hot dry weather that we have experienced. Damage is first noticed at the base of the leaves appearing as white stippling on the top surface of the leaves. The treatment is 20-30 mites per leaflet and 10% of the leaves with feeding damage. Dimethoate, Lorsban, Penncap and Parathion will provide control. Although dimethoate is the only systemic insecticide available for mite control in soybeans, it should be applied before populations explode and is generally less effective on drought stressed beans. The addition of crop oil or an organosilicone has greatly improved control, especially when using aerial application.



Does a Heavy Rain Help With the Mite Population?

Derby Walker, Extension Ag Agent


No, Rainfall will not wash the mites off the plants, but does help lower the temperature. Under cool temperatures and high humidity a fungus disease can develop which destroys the mites in a few days. The fungus can be very effective in controlling mites. However, if we continue to have very hot and humid conditions, the mite population will continue to expand. Drought stricken fields are more attractive to mites because the nutrients in the plant juices are more concentrated.


Options for Late-Season Weed Control in Soybeans -

Mark VanGessel, Extension Weed Specialist.


Single-cropped soybeans are now canopying across the rows in most areas. There are some situations that are needing a late postemergence herbicide for some weeds. At this point it is often difficult or very expensive to control more than one or two weed species. It is best to determine which weeds are the most troublesome and design a program around the dominant species. For example, Cobra is the best product for large common ragweed but it is ineffective on large lambsquarters. Here are some products for large weeds (rescue type scenarios): Pinnacle for lambsquarters and pigweed; Cobra for large common ragweed; Blazer for jimsonweed; Blazer plus 2,4-DB for morningglories and common ragweed 8 to 12 inches tall; Resource for velvetleaf (Classic and Basagran will also injure large velvetleaf). Postemergence grass herbicides rates vary by grass size so refer to the labels. All of the products listed are labeled with the addition of liquid nitrogen. Liquid nitrogen will improve control but it will also increase crop injury. In all situations do not expect complete control, these herbicides will cause enough injury to the weeds to allow soybeans additional growth to better compete with weeds.

STS soybeans are safe with the maximum rates of Synchrony (Pinnacle plus Classic) and Roundup Ready soybeans are safe with Roundup Ultra. STS and Roundup Ready soybeans are an asset when a rescue treatment is needed.


Field Crop Diseases -

Bob Mulrooney, Extension Plant Pathologist



Wheat Disease Summary. Disease incidence was low overall. We did identify barley yellow dwarf and wheat spindle streak mosaic this past season. Powdery mildew levels were higher than last season. Wheat varieties that were susceptible to powdery mildew in our variety plots this year included Glory, Tyler, Pioneer 2568, Hazen, SR 205, Penmore, and Elkhart. Septoria leaf spot and glume blotch were very low compared to last year. We identified take-all at several locations, which can be avoided by rotating away from small grains for 1-2 seasons. In one case, sharp eyespot caused by Rhizoctonia was present in addition to take-all. This disease is not common here but does occur. Usually lens-shaped lesions form at the lowest nodes. These lesions can girdle the stems and cause premature ripening of the head and lodging. Rust appeared very late, after it could cause any effect on yield. Scab was almost non-existent but was identified in a field of minimum till wheat. In this case it caused no blank heads and less than 5% kernel infection in the infected areas which were near a fence row which provided some shade and wind protection.


 A New Field of Soybean Severe Stunt Virus (SSSV) Was Confirmed in Millsboro Last Week.

Derby Walker, Extension Ag Agent


 The beans in the infected field were very stunted with very small leaves. This disease is commonly mistaken as 2,4-D damage or herbicide damage. At this point in time, the disease is concentrated in the center of Sussex county. It is very important to identify infested fields so corrective action can be taken. To control SSSV, crop rotation and soybean variety selection are the only choices a grower has (there are only a few known resistant varieties to this disease). If nematodes are also a problem, there are a couple of varieties that have resistance to both problems.


Crop Management Tour

Thursday, August 21, 1997

9:00 a.m. - 12:30 p.m.

Free barbecue lunch to all participants.


The Tour Includes Stops on the Following: 

  • Variable Rate Poultry Manure Spreader with Precision Farming 
  • Warrington Farm

C 15" vs 30" row lima beans

C High available phosphate corn

C Deer management

C Irrigation scheduling for corn and lima beans

C Bt corn and insect resistance

C Nitrogen management on corn

C Weed management in lima beans

C Weed management for irrigated corn 

  • Corn Production and Poultry Manure Utilization

 To reserve a spot --- call us at 856-7303. Ask for Charlotte or Marlyn.


Vegetable Crops:

Vegetable Crop Insects -

Joanne Whalen, Extension IPM Specialist.


Tracy Wootten, Extension Associate - Vegetable Crops



Lima beans and Snap Beans.

Potato leafhopper adults and nymphs continue to be very active in beans throughout the state. Weather conditions continue to be favorable for damage, especially in drought stressed fields. The treatment threshold is 5 per sweep during prebloom and 10 per sweep during bloom. In snap beans, this threshold should be reduced by one-half if thrips are also present, especially if the field is drought stressed. Asana, Lannate or Orthene will provide control of both pests.



Mites have exploded due to the hot, dry weather. The treatment threshold is 10-15% infested leaves. Dimethoate is still the first choice for mite control but needs to be applied before mites explode. If treating by air, the addition of an organosilicone has improved control.

 Continue to check for melon aphids in cucurbit crops. They can be seen in very hot dry weather conditions.


Peppers and Snap Beans.

Continue to monitor for corn borer. Check the Crop Pest Hotline for black light trap catches and spray schedules for each crop. Materials and rates can be found in last weeks issue.


Fresh Market and Processing Sweet Corn should be monitored for corn earworn and fall armyworm. Check the Crop Pest Hotline for activity and spray schedules.


U of D Crop Pest Hotline:

In State: 1-800-345-7544

Out-of-State: 302-831-8851


Vegetable Crop Diseases -

Bob Mulrooney, Extension Plant Pathologist




Be sure to apply Ridomil Gold PC11G in the seed furrow at planting to prevent damping-off caused by Pythium and Rhizoctonia, especially in double-cropped situations. If Pythium is the only concern apply Ridomil Gold in a 7 inch band over the row at seeding.



Apply Bravo or maneb on a 7-10 day schedule for control of Alternaria leaf spot.


Cucumbers, Pumpkins, Squash.

Be scouting for powdery mildew, conditions have been ideal and other areas are reporting powdery mildew earlier than usual this year. Apply Bayleton, Reach, Benlate, or Topsin for control (consult the 1997 Commercial Veg Production Guide for details). Note: Weather conditions are very similar to 1995 when we experienced an outbreak of virus problems in cucurbits. Watermelon mosaic was widespread that year, but nothing can be done once symptoms appear. Check for aphids which transmit the viruses and apply controls when necessary. Maintain fungicide sprays with Bravo for control of foliar diseases.



Late Blight Report.

DSV accumulation as of July 14, 1997 are as follows:


Location/ Emergence date


DSV=s July 10


 DSV=s July 14




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, high-rate


Weather conditions continue to be unfavorable for late blight development. For many fields, fungicide spraying has ceased as harvest approaches. Late blight information hotline number 1-888-831-SPUD.


Weather Summary

University of Delaware,


 Week of July 11 to July 17, 1997


1.07 inches: July 17

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




Ranged from 98 F on July 15 & 17 to 80 F on July 11.


Ranged from 71 F on July 15 to 51 F on July 11.


Soil Temperature:

75 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.