Weekly Crop Update

University of Delaware Cooperative Extension

Volume 5 , Issue 12

June 19, 1997


Field Crops:

Field Crop Insects -

Joanne Whalen, Extension IPM Specialist.



Field Corn.

True armyworms and cutworms continue to be found in both conventional and no-till fields. If worms are greater than 3/4 inch long, most of the damage has been done and controls will no longer be cost effective. Low levels of European corn borer larvae can now be found in the earliest planted corn fields. In most cases, larvae are small and infestation levels range from 5-10% infested plants. The best time to control corn borers is before larvae move deep into the whorls or at the pre-tassel stage when larvae are exposed to the spray. No controls will be needed unless 50% of the plants are infested in irrigated corn or 75% of plants are infested in dry-land corn.



The most common leaf feeding damage found in early planted soybeans is from the bean leaf beetle. Damage appears as small circular holes in the leaves. No controls should be needed unless you find 2 beetles per foot of row and the stand is being reduced. A pyrethroid or Sevin will provide control. Grasshopper activity continues to increase, especially in no-till situations. Early control of nymphs will generally result in the most effective control. Treating non-crop areas may also help to prevent whole field treatments at a later date. As a general guideline, non-crop areas should be treated if you find 20 or more grasshoppers per square yard. Once grasshoppers move into the main field, the treatment threshold is one per sweep and 30% defoliation. Asana, Sevin or Warrior have provided the most consistent control.


Grain Marketing Highlights -

Carl German, Extension Crops Marketing Specialist



A private commodity forecaster has lowered the soybean carryover estimate for both the 1996/97 crop and for 1997/98. However, the market is not necessarily convinced that the lower estimates will pan out. Nevertheless, this information, when factored into the market this week should provide a welcome boost to soybean prices. The extent of the boost will largely depend upon 1997 crop development. Although planted early, the development of the corn and soybean crop appears to be lagging, maybe two to three weeks, particularly for corn. For corn, this means that pollination will not be reached until the hottest part of the summer. Rains will need to be timely. Due to this key point, further corn sales might be best placed on hold. For soybeans, a strategy offered is to price enough beans to cover costs at current levels, with the intent to price substantially more if higher prices materialize. Option pricing may play a role. For further assisstance contact Carl German @ 302-831-1317.


Off-Target Movement of Command -

Mark VanGessel, Extension Weed Specialist.



Command is popular for double-cropped soybeans because of its activity on volunteer small grains and annual grasses. It is also one of the more volatile herbicides we have available. Command has the unique property of turning plants white, particularly roses, mulberry, and pears. Off-target movement is very evident because of the whitening affect. Symptoms are usually not lethal but are quite evident. This is one herbicide that one should error on the side of caution when using due to its potential to move off-target. The ME (micro encapsulated) formulation requires 1200 foot buffer between towns and housing developments, commercial fruit and nut production, and commercial greenhouse and nurseries. The label also cautions that prior to application, adjacent properties must be checked and spraying within 300 feet of desirable plants must be avoided. Furthermore, the label suggests that the boom height be 24 inches or less and use coarse sprays. The EC formulation requires a 1500 foot buffer for developments and commercial planting and desirable plantings.


Canada Thistle Control in Wheat -

Mark VanGessel, Extension Weed Specialist.



The time to control Canada thistle in wheat is prior to harvest. Roundup is the product of choice because it is the most effective and allows greatest flexibility in crop rotation. Waiting until after harvest will result in less control due to less leaf area and severely injured plants. Label specifies an application of 1.0 quart/A after the wheat is in the hard dough stage (30% moisture). Wait 7 days prior to cutting the wheat.


Some Reasons For Uneven Corn -

Richard Taylor, Extension Agronomist



During long, cold springs such as we experienced this year, many factors can cause localized soil temperature variations. These variations can significantly affect the germination and emergence rate of corn causing large differences in plant size even for plants very near each other. These factors include changes in microclimate, topography (small frost pockets that collect colder air in the morning), variations in soil moisture, changes in soil organic matter content, variations in crop residues or cover crop thickness, soil color differences, and shade from field edge vegetation. Cold soils stretch our corn germination over several weeks leading to uneven emergence as influenced by the effect of these factors on soil temperature.


Is this the only reason for uneven corn? No, other factors can lead to uneveness. Wind blown soil particles can injure young plants and slow growth. So can insect feeding, hail and frost damage, and nutrition problems (an important factor since root growth in cold soils is less vigorous).


Will the uneveness last until harvest? No, in most cases after the nitrogen (N) is sidedressed, corn’s rapid growth will even out the differences we now see. Usually by the time corn is head high, none of the height differences we now see will be visable unless there is a more serious underlying cause. Yield impact will be minimal except in the latter case.


This year, I have noticed some of the uneven corn is showing serious nutritional deficiencies. These are mostly micronutrients related due to high or low pH vlaues. Unless this underlying problem is corrected or the micronutrients is added, these affected plants will not catch up and yield will be reduced, especially if pH is high. Other symptoms have been due to N or Sulfur (S) deficiencies which disappear once N is sidedressed. The N boost solves the N deficiency problem and also stimulates rapid root growth into deeper soil levels allowing the crop to use the S trapped by clay particles in the underlying soil layers.


Field Crop Diseases -

Bob Mulrooney, Extension Plant Pathologist



Field Crops.

Take-all was seen in New Castle county this week and caused lodging in some areas and uneven growth in others. Typically take-all produces stunted plants that never lodge when it occurs in the lighter soils of Kent and Sussex counties. The diagnostic black streaks on the stem at the soil surface and the lack of roots were present.


Vegetable Crops:

Vegetable Crop Insects -

Joanne Whalen, Extension IPM Specialist.




Melon aphid populations and leaf curling has started to increase as a result of the warmer weather. With the predicted heat for this weekend, we could also see an explosion of mite populations. As a general guideline, the melon aphid threshold is 20% infested runners and the mite threshold is 10-15% infested crown leaves. Lannate is the only available material for melon aphid control in melons. Dimethoate or Kelthane should be used for mite control.



Aphid populations still remain very low. However, the warmer weather this weekend could result in aphid explosions where Admire was not used at planting. Since green peach aphids are generally found on the lower leaves, be sure to sample 5 lower leaves in 5-10 locations throughout a field. At bloom, the threshold is 4 per week. This threshold increases to 10 per leaf at 2 weeks from harvest. The threshold for melon aphids is one-half the green peach aphid threshold. If green peach aphid is the predominant species, Provado or Monitor should be used. If melon aphid is the predominant species, Provado or Lannate should be used.


Snap Beans.

Early planted processing snap beans in bloom should receive an application of Orthene for European corn borer control. A second application will be needed at the pin stage if moth catches remain above 5 per night in your area. Thrips populations are also starting to increase. The treatment threshold is 5-6 thrips per leaflet. Asana, Lannate or Orthene will provide control.


Sweet Corn.

Be sure to scout all fields for European corn borer whorl infestations. A treatment will be needed if 15% of the plants are infested with live larvae. Ambush, Baythroid, Pounce, Penncap or Warrior will provide good control. Penncap should not be used if bees are foraging in the area. Any early silking sweet corn should be sprayed on a 5-6 day schedule. Sprays should be applied when the ear shanks are first visible.


Diagnosing and Settling Pesticide Injury Situations -

Ed Kee, Extension Vegetable Specialist



Recently I have diagnosing several herbicide injury complaints and drift problems and mediated understandings between neighbors and commercial applicators. Over the years, I have settled many of these situations totaling millions of dollars. Perhaps these situations were unavoidable, given the pressure to get things done and the seemingly uncessing winds that blew this spring. When these situations occur, it is important to reach an understanding of what happened and why. First, ask all the standard "troubleshooting" questions. What was sprayed and when was it sprayed? What were the weather conditions when it was sprayed and in the time since the applications were made. Field histories must be understood to avoid misdiagnosing injury symptoms. An understanding of how the materials work and what the associated symptoms look like is essential to figuring out what happened and why.


On the human side, common-sense, courtesy, and patience help solve problems as much as the technical diagnosis. Often, the injured crop will recover. There may be a delay in harvest, especially with some vegetable crops. Yields may be reduced. All parties must come to an understanding and agree to evaluating economic loss fairly and accurately. If the technical information is put to use to access the situation fairly, I have never been in a situation where all sides are not satisfied with the outcome and settlement. Yes, all sides regret it happened, but all sides come together to settle the issue fairly.


Vegetable Diseases -

Kate Everts, Extension Plant Pathologist, Universities of Maryland and Delaware



Bacterial speck and spot on tomato. Several fields on the shore are showing symptoms of bacterial speck. If your tomatoes are showing foliar symptoms make sure you correctly identify the disease so that you can apply a chemical which will treat the problem. Bacterial speck and spot need to be treated with a product containing copper (Bravo C/M, fixed copper, or ManKocide). Many other diseases which cause foliar symptoms are caused by fungi and can be treated with other fungicides. Speck lesions are small, round and dark brown to black. Usually a distinct halo is present surrounding the diseased tissue. The spots may coalesce and kill a large area of tissue. To reduce the spread of the disease, do not work the field when it is wet. This disease does not overwinter well in soil. Reduce the chance of outbreaks in future years by treating stakes with bleach if they are re-used, and buying only disease free transplants.


Vegetable Crop Diseases -

Bob Mulrooney, Extension Plant Pathologist



Common rust has been seen in field corn this week which should alert sweet corn growers to be on the lookout for rust. Heavy infections prior to the whorl stage can affect yield. Some varieties are susceptible so scout your fields regularly. If rust needs to be controlled apply Tilt, 4 oz/A and repeat 7 -14 days later, or mancozeb, 1.5 lb/A on a 7-day schedule. In most years fungicides are not needed.



Late Blight Update


June 16, 1997


DSV Accumulation as of June 16, 1997 are as follows:



Location/Emergence Date


June 12


June 16



Baldwin - 4/20



10- day, mid rate

Jackewicz - 4/26



10-day, mid rate

Zimmerman - 4/28



7-day, low rate

Baker - 5/7, - 5/9



5-day, low rate


Conditions have not been favorable for DSV accumulation at Baldwin’s or Jackewicz, conditions were more favorable at Zimmerman’, and Baker’s. Spray accordingly. The increase in rates is due to the increase in canopy size.




Compiled & Edited By:

Tracy Wootten

Extension Associate - Vegetable Crops



Weather Summary

University of Delaware,


Week of June 13 to June 19.




0.15 inches: June 15

1.12 inches: June 19

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




Ranged from 90 F on June 19 to 72 F on June 16.


Ranged from 68 F on June 19 to 45 F on June 16.


Soil Temperature:

69.9 F. 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.