Weekly Crop Update

University of Delaware Cooperative Extension

Volume 5, Issue 13

June 26, 1997

 Field Crops:

Field Crop Insects -

Joanne Whalen, Extension IPM Specialist.




Grasshopper populations continue to increase, especially in no-till situations and along field edges. As a general guideline, non-crop areas should be treated if you find 20 or more grasshoppers per square yard. In corn and sorghum, a treatment is justified if you find 5-8 grasshoppers per square yard. In soybeans, the threshold is one per sweep and 30% defoliation. Sevin and Warrior are labeled on all three crops and have provided the most consistent control. Dimethoate is also labeled but must be applied when grasshoppers are small to provide good results.



Begin to watch for mites, especially in early planted fields. Low numbers can now be found along field edges. Look for early signs of mite damage which appears as white stippling at the base of the leaves. Treatment should be considered if you find 20-30 mites per leaflet and 10 % of the plants show damage. Dimethoate is the only available systemic insecticide available for mite control in soybeans. If populations explode, Lorsban, Penncap-M or Parathion will provide contact control. Remember, parathion can only be applied by air and Penncap should not be used if bees are foraging in the area.


Irrigating Double-Crop Soybeans -

Richard W. Taylor, Extension Agronomist


Bob Uniatowski, Extension Associate - Field Crops



If you recall from our article on irrigating full-season beans in the June 5, 1997 issue of Weekly Crop Update, full-season beans yield best when irrigation is withheld, except for 4 specific situations, during the vegetative growth phase. For double-crop beans, this does not hold true. The shorter growing season already limits maximum yield potential. This double-crop maximum is quite close to the yield that continuously irrigated full-season beans can make. Therefore, double-crop beans should be irrigated continuously and not allowed to undergo a period of drought stress. This is needed to increase early vegetative growth to support pod set and seed fill.


What target yield should you get? For beans double-cropped after barley, yields in the 50 to 55 bu/A range can be achieved. For beans double-cropped after wheat, yields in the 40-45 bu/A range are possible. The key to top yields is not just wise variety selection; effective irrigation; integrated pest management of weeds, insects and diseases; but also early planting. By July, yield potential declines by as much as 1 to 2 bu/A/day. Also, don’t forget to check fields for soybean cyst nematode pressure. That information will be critical for proper variety selection.


Postemergence in Soybeans -

Mark VanGessel, Extension Weed Specialist.



Preemergence herbicides are not performing as well as we would like in some areas due to lack of rainfall needed to activate them. As a result, postemergence sprays may be needed. When considering your options, keep in mind what you sprayed preemergence. You want to be sure to avoid using the same type of herbicide chemistry with both soil-applied and postemergence sprays in a year like this where the rain has not thoroughly activated the herbicide. The problem is that additional herbicide applied postemergence can move into the soil with a heavy rain and be available to the plant, in addition to the herbicides applied earlier. If Canopy, Scepter, or Broadstrike was applied before or at planting then avoid using Pursuit, Classic, or Pinnacle. This strategy will also reduce the risk of selecting for weed species resistant to these herbicides. If you have to use these postemergence herbicides, the rates should be at low-end of the range or even less to avoid injury. Injury is most likely to be seen as stunted plants due to shortened internodes.


Upcoming Events:


Crop Diagnostic Field Day

July 31, 1997

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

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

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

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

Contact Mabel Hough at



Weed Twilight Tour

July 21, 1997

6:30 p.m. in the grove.

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

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.


Vegetable Crops:

Vegetable Crop Insects -

Joanne Whalen, Extension IPM Specialist.



Crop Pest Hotline.

As corn borer and corn earworm pressure starts to increase, be sure to call the Crop Pest Hotline for the most recent trap catches and spray schedule information. Reports are updated by noon on Tuesday and Friday throughout the summer. In-State: 1-800-345-7544; Out -of - State: 1-302-831-8851.



Continue to watch fields for melon aphids and mites. Both species can be found at threshold levels. In many cases, at least 2 miticide applications spaced 5 days apart will be needed to get control.



In areas where corn borer catches range from 3-5 per night, peppers should be sprayed on a 7-10 day schedule. If Orthene is used, it will also provide pepper maggot control. If a pyrethroid or Lannate is being used, dimethoate should be added to the mix for pepper maggot control.


Snap Beans.

All snap beans should be sprayed by the early pin stage for corn borer control. Processing snap beans should be sprayed with Orthene at the bud and pin stage (1 1/3 lb per acre). Fresh market snap beans should be sprayed with Lannate. At the present time, sprays should be applied on a 7 day schedule. Be sure to call the Crop Pest Hotline for the most recent trap catches and suggested spray intervals. Watch for leafhoppers and thrips in seedling stage beans. The recent hot weather has resulted in explosions of both species. The treatment threshold for thrips is 6 per leaflet and the threshold for leafhoppers is 100 per 20 sweeps prebloom. If both species are present, the threshold for each should be reduced by one-half. Asana, Lannate, or Orthene will provide control of both species.


Sweet Corn.

All fresh market silking sweet corn in Kent and Sussex Counties should be sprayed on a 3-4 day schedule. In New Castle County, sprays should be applied on a 4-5 day schedule. Continue to check the crop pest hotline for the most recent trap catches and suggested spray intervals. Small fall armyworm can now be found in the whorls of later planted corn. No treatment will be needed until 15% of the plants are infested. Remember, insecticides must be directed into the whorls and high gallonage used to get effective control. In recent years, Larvin or Warrior have provided the most consistent control.


Fungicide Application on Vine Crops -

Ed Kee, Extension Vegetable Crops Specialist



Watermelon and Cantaloupe growers often ask whether ground sprayer or aerial applications of fungicides are more effective. Research conducted by North Carolina State, Clemson University in South Carolina, and the University of Florida proved that satisfactory control of fungus diseases was achieved by both application methods. Indeed, here on Delmarva, aerial applications and ground applications have both provided effective control of fungus diseases in watermelons. I have worked with growers for over 20 years that have achieved good disease control in vine crops with aerial applications.

However, it is interesting to note that while satisfactory control was achieved in the research tests by air, the N.C. State and Clemson work indicate that ground applications were more effective. Florida found them to be equally effective. Is the difference between ground and air applications found in the two studies enough to make a significant difference? In most cases, in most years probably not. Occasionally, disease, weather, and other factors may warrant a ground application as the preferred method. However, growers should know that a timely, well-applied fungicide program, whether applied by ground or air, should provide good control.


Vegetable Diseases -

Kate Everts, Extension Plant Pathologist, Universities

of Maryland and Delaware




The EPA has approved a Section 18 specific exemptions for use of several fungicides to control late blight on potatoes in Maryland. (Bob Mulrooney previously reported that they had been approved for use in Delaware). The compounds are Acrobat MZ, Curzate M-8, Manex C-8, and Tattoo C. These products are for control of late blight caused by Phytophthora infestans. Acrobat MZ may be used 5 times per season, at a rate of 2.25 lbs. of product per acre. Curzate M-8 or Manex C-8 may be applied a maximum of 7 times per season at a rate of 1.5 lbs product/acre. Tattoo C may be applied at a rate of 2.3 pints product per acre a maximum of 5 times.



Maryland has also received a Section 18 for use of Tattoo C on tomatoes for control of late blight. Tattoo C may be applied up to five times at a rate of 2.3 pints product per acre. A seven day preharvest interval should be observed.

Snap beans.

A section 3 label for Ronilan DF on snap beans has also been approved by the EPA. This product is used to control white mold in snap beans and has a 10 day preharvest interval. To determine if a spray is needed, scout the field to determine if the soil remains wet for 6 to 10 days before bloom. If these conditions occur, a fungicide application is warranted. Consult the label for use rates and additional information.


Vegetable Crop Diseases-

Bob Mulrooney, Extension Plant Pathologist, University of Delaware




Late Blight Report.


DSV accumulation as of June 23, 1997 are as follows:


Location/ Emergence date


DSV’s June 20


DSV’s June 23



Baldwin - 4/20



10-day, high rate

Jackewicz - 4/26



10-day, high rate

Zimmerman - 4/28



10-day, mid-rate

Baker - 5/7 -5/9



7-day, low rate


The dry, hot weather has not been very favorable for late blight. Continue with protectant sprays of EBCD’s (Dithane, Polyram, Manzate), chlorothalonil (Bravo, Terranil), or copper at the recommended interval and rate. Late blight information hotline number 1-888-831-SPUD

Weather Summary

University of Delaware,


Week of June 20 to June 26, 1997


0.41inches: June 23

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



Ranged from 96 F on June 26 to 83 F on June 20.


Ranged from 74 F on June 26 to 60 F on June 20.

Soil Temperature:

76.2 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.