Weekly Crop Update

University of Delaware Cooperative Extension

Volume 5, Issue 11

June 12, 1997


Field Crops:

Field Crop Insects -

Joanne Whalen, Extension IPM Specialist.




With the recent warm weather, be sure to watch fields for leafhopper activity. Adults have been in the area and laying eggs for at least 3 weeks. Although adults are the predominant life stage, we should begin to see nymphs by next week. Remember, nymphs can cause damage quickly, especially during hot, dry weather. Once fields become yellow and stunted, significant damage has already occurred and the best control strategy would be an early harvest. In 4-6 inch alfalfa, the threshold is 50 per 100 sweeps, in 7-11 inch alfalfa the threshold is 100 per 100 sweeps, and in alfalfa greater than 12 inches in height the threshold is 150 per 100 sweeps. Ambush, Baythroid, Dimethoate, or Pounce have all provided good control.


Field Corn.

Although soil insect problems can still be found in later planted corn, in many cases the damage can be attributed to birds. The damage pattern can appear similar to soil insects i.e. long strips with missing plants and plants that appear cut off at the soil surface. In addition, damage has occurred in fields not usually prone to bird damage. However, you should also be able to find an occasional plant that has been pulled out of the ground and /or a depressed area where the bird has "rooted" the seed out of the ground. Since the birds are remaining active in replanted fields, a seed treatment containing lindane or a mesurol hopper box treatment are recommended if replanting is still an option. You may also want to consider a Bt hybrid, since late planted corn will be subject to attack from corn borer, corn earworm and fall armyworm. Armyworm activity is starting to increase in no-till corn (especially if a pyrethroid was not tank-mixed with the herbicide) and along field margins as small grains dry down. A rescue treatment should only be applied if you find 25% of the plants infested and worms are less than inch long. A pyrethroid will provide good control.


Small Grains.

We are still getting reports of fields with threshold levels of armyworms ranging in size from 2 to 1 inch long. If you have not scouted your fields, be sure to do so at least once during the next week. In wheat, the treatment threshold is 2 per foot of row.



With the recent warm weather, grasshoppers are becoming active, especially along field margins. Be sure to check full season no-till soybeans for grasshopper feeding activity. A treatment should be considered before grasshoppers move into a field. Once grasshoppers are in a field, the treatment threshold prebloom is one grasshopper per sweep and 30% plant defoliation. Asana, Sevin and Warrior have provided the most consistent control.


Field Crops Diseases -

Bob Mulrooney, Extension Plant Pathologist, University of Delaware.




We have had a report of Leptosphaerulina leafspot or lepto leafspot for short. It is often called pepper spot too. This fungus disease affects young leaves primarily, but also attacks petioles and older leaves. It often attacks the young leaves that grow back after the first cutting if moisture is available and the weather is cool. At first , numerous small, reddish- brown to black flecks (pepper spots) appear on both leaf surfaces and ont he petioles. As the flecks enlarge they acquire a tan center with an irregular brown border surrounded by a yellow halo. If severe, early cutting to minimize leaf loss is suggested. Be sure to rotate away from alfalfa and other susceptible legumes when it is time to replace the stand.



Prospects are good for a pretty disease-free crop. I have seen no scab to date and glume blotch should be minimal. Septoria (Stagnospora) leafspot can be fould but in geneneral is of minor importance. With no rain to spash spores around, I look for little to no glume blotch. A few rust pustules can be found butit is too late for concern. In the southern part of the state wheat is starting to turn, and barley harvest has begun.


Herbicide Drift -

Mark VanGessel, Extension Weed Specialist.



Calls have started about drift complaints. Herbicides are developed to kill plants which means weeds but can kill or severely injure crop plants. Injury can occur at very small doses of some herbicides. Use common sense when spraying, particularly with non-selective herbicides such as Roundup and Gramaxone and near sensitive plants. Drift control agents, slower speeds, larger nozzle sizes, and lower pressure will all help reduce drift, but they do not eliminate drift.


Hot Days And Banvel or 2,4-D Do Not Mix -

Mark VanGessel, Extension WeedSpecialist.



Most of the corn is taller than 8 inches which requires 2,4-D to be applied with dropped nozzles. Banvel can be applied to corn greater than 8" but rates must be below 2 pint/A. Applications of 2,4-D to corn taller than 8 inches and higher rates of Banvel can cause twisted and distorted corn growth. Also, brace roots do not develop normally which can result in lodged corn. Finally, neither herbicide should be applied when temperatures are greater than 80 to 85 degrees because of the increasedrisk of volatility and movement to susceptible plants.


Crop Diagnostic Field Day Planned -

Mark VanGessel, Extension Weed Specialist.



A field day designed to improve diagnostic skills has been planned for July 31, at 8:00 am at the Research and Education Center in Georgetown, DE. Participants can earn 3 continuing credits for the certified crop advisor program. Registration is limited to the first 60 people who register. Cost is $25 and pre-registration is required. If you have not received a flier within the past week and you are interested in registering, contact Mabel Hough at 302/856-7303.


Grain Marketing Highlights -

Carl German, Extension Crops Marketing Specialist, University of Delaware.



This statement is based upon the release of USDA's June 12th crop report. The numbers in this report are said to favor the new crop over the old crop. Since there were no significant changes in the old crop numbers the over riding factor is for longs to liquidate old crop positions. This is likely to have a short term negative impact on commodity prices, for both the old and new crop. It is imperative to seriously consider the current price level that is obtainable for pricing 1997 production, particularly for soybeans. USDA now projects the season average price for new crop corn, wheat and soybeans to be $2.25-$2.65; $3.45-$4.05; and $5.60-$7.00, respectively. For further assistance contact Carl German at 302-831-1317.


Sidedressing Your Corn -

Richard W. Taylor, Extension Agronomist.



Many corn fields have reached the stage of growth when it is time to apply sidedressed nitrogen (N). An "in season" approach to N management for corn can be effective in improving N efficiency , particularly where poultry manure is used. This approach combines new soil and plant N tests with factors such as actual plant populations, climate, and insect and disease pressures to identify a realistic expected yield goal early in the growing season and to select an appropriate rate of N to sidedress or fertigate.


When manure is used, a pre-sidedress soil nitrate test (PSNT) can successfully identify situations when available soil N is totally adequate for the crop or give an estimate of fertilizer N needed at sidedress time. You can have PSNT's run at the Kent and Sussex County Conservation Districts or samples can be sent to the University of Delaware Soil Test Laboratory. Check with your County Extension Ag Agent for sampling instructions and bags. There is a $6 per sample fee at the UD Soil Test Laboratory.


Plant tissue testing also can be used to estimate corn N needs. For early whole plants, the sufficiency range for N is 3.5 to 5.0 percent N. When fertigating, a second tissue sample can be taken at silking. Ear leaves at silking should contain 2.7 to 4.0 percent N for optimum yields.


For further information, request the fact sheet entitled "Advances in Nitrogen Management for Corn in Delaware from your County Extension Ag Agent.


Potato Leafhopper In Second Cutting Alfalfa -

(Extracted from Penn State Field Crop News)

R. W. Taylor, Extension Agronomist.



Cool dry spring conditions and a lack of storms developing over the Gulf of Mexico and sweeping up into the northeastern U. S. have delayed the northward movement of leafhoppers. Potato leafhopper is a migratory insect that overwinters in the Gulf States and needs the right conditions to move up into the northeast. By watching weather patterns for the first storm sweeping north out of the Gulf of Mexico in the spring, you can estimate when leafhopper adults are likely to first appear in alfalfa fields.


Some alfalfa now is approaching the stage for a second cutting. Once regrowth reaches a height of two to four inches, you should begin to scout fields for this pest especially after weather conditions permit the movement of coastal storms into our region. Guides are available describing scouting methods and threshold levels for treatments. Be sure to refer to these guides or use IPM scouts to ensure high quality alfalfa.


Vegetable Crops:

Vegetable Crop Insects -

Joanne Whalen, Extension IPM Specialist.




We are starting to see an increase in thrips activity as a result of the warmer weather. A treatment should be applied when 20% of the plants are infested with thrips. Dimethoate, Metasystox-R and Warrior have provided the best control.



Corn borer moth counts have dropped significantly and the percentage of infested terminals is very low. In many cases, a second application of Furadan will not be needed unless we see an increase in activity. Potato leafhopper adults can be found in fields where Admire was not used at planting. Since adults have been found for at least 10 days, a treatment should be applied if you find 0.5 to 1 adult per sweep or 5 nymphs per 50 leaves. Provado, Furadan or a pyrethroid will provide control.

Vegetable Crop Diseases-

Bob Mulrooney, Extension Plant Pathologist, University of Delaware





Late Blight Report.

Weather conditions have not been favorable for DSV accumulation.

DSV accumulation as of June 12, 1997 are as follows:



Location/ emergence date



DSV=s June 9



DSV=s June 12





Baldwin - 4/20






10-day, low rate


Jackewicz - 4/26






10-day, low rate


Zimmerman - 4/28






10-day, low rate


Baker - 5/7 -5/9




33/ 25


10-day, low rate


Right now we can use some rain. Disease pressure is low. Nothing prettier than a field of potatoes in full bloom.


Late blight information hotline number 1-888-831-SPUD



Complied & Edited By:

Tracy Wootten

Extension Associate - Vegetable Crops




Weather Summary

University of Delaware,



Week of June 6 to June 12, 1997



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



Ranged from 87EF on June 11 to 59EF on June 8.


Ranged from 57EF on June 11 to 42EF on June 8 & 9.


Soil Temperature:

63EF. 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.