Vol. 5 No. 3 April 17, 1997

Weekly Crop Update
University of Delaware Cooperative Extension

Volume 5, Number 3

April 17, 1997

Field Crops:

Field Crop Insects -

Joanne Whalen, Extension IPM Specialist.

Field Corn. A combination of the wet weather last fall, warmer temperatures in late winter and the recent cool, wet conditions will increase the potential for slug damage in no-till corn this spring. Eggs laid last fall hatched due the warmer temperatures in February and March so young slugs can now be found under the surface residue. If 3-5 slugs are found per square foot, the potential for economic injury is present. The use of tillage to reduce food and shelter for the slugs and trash wheels on corn planters to encourage quicker corn emergence can reduce injury by up to 50 percent. If cool, wet conditions persist, the above cultural practices will need to be combined with timely applications of liquid N or metaldehyde bait.

Small Grains. Cereal leaf beetle egg hatch has been observed on the lower Eastern Shore so we should see hatch in most areas of the state this week. As temperatures turn warm, be sure to watch for larvae on the entire stem. As experienced in past seasons, defoliation can quickly increase from 10-20 to 50% and controls are often applied too late. As indicated in the last newsletter, the threshold of 0.5 larvae per stem will result in economic control if fields are scouted and treatments are applied in a timely manner. If fields are approaching the threshold level., fields may need to be scouted twice a week to properly time insecticide applications. In high management fields where significant egg laying has occurred. the lower threshold of 25 eggs and/or larvae per 100 tillers may be considered, especially if a fungicide application is needed. However, no insecticide treatments should be applied before 50% of the eggs have hatched. Although aphids can be found in a number of fields, populations are generally low and below the threshold of 150 aphids per linear foot of row. In addition, lady beetles can be found feeding on aphids in most fields.

Considerations For Burndown Herbicides - Mark VanGessel, Extension Weed Specialist.

The timing between herbicide application and disturbing the weeds with tillage or planting can influence the effectiveness of the burndown herbicides. A weed that is not actively growing due to cool or cloudy weather or due to mechanical disturbance will not respond as rapidly to the herbicide application as an actively growing weed. Weed size will also influence the time required to kill the weeds (smaller weeds will die more rapidly). With that said, considerations for timing between Gramoxone Extra application and tillage or planting for an annual weed species is 1 to 5 days; and Roundup is 3 to 10 days. Currently, we should be looking at the longer intervals to give the herbicides a chance to work. The shorter time span is appropriate for mid­May to later plantings. In the case of stressed plants or perennials you should also consider the longer intervals. A final note on intervals between burndown herbicide application and planting, if considering 2,4­D or Banvel, refer to the labels for the interval restrictions.

Burndown herbicide application after planting can have mixed results. Coulters and seeding units can disturb those weeds in the row and will slow their growth, making them less susceptible. Planting under dry conditions can cause many of the plants to be covered with dust that will bind with Gramoxone Extra and Roundup and can result in poorer control. Unless you have had experience with applying burndown after planting and feel comfortable doing so, I recommend spraying your burndown herbicides before planting.

pH Problems in Small Grains - Derby Walker, Extension Ag Agent.

As in previous cropping seasons, we are seeing pH related problems in small grains this year. Manganese deficiency is the result of high pH. Low pH ties up phosphorus and magnesium. Depending on soil texture, barley will have manganese problems at pH=s ranging from 6.0 to 6.5. Wheat is more tolerant, and we normally don=t see problems until we exceed pH=s of 6.3 to 7.0. The lighter the soil the less tolerant small grains are of too low and too high ph=s. Low pH problems begin around 5.4 and lower. Most of our growers are experiencing problems with pH=s that are too high, because of the low buffer capacity of our sandy soils.

Reminders Before Spraying - Derby Walker, Extension Ag Agent.

Just a quick reminder to double check the label before mixing your chemicals to be sure the products you are mixing are compatible. Each year I usually get a grower call that involves a mixing problem where the materials used were incompatible. Also recheck your sprayer calibration during the season, after all parts do wear and break. Be sure to clean your sprayer when switching from one crop to another or changing over from herbicides to insecticide and fungicides. A thorough cleaning of your sprayer would include washing and rinsing the tank, lines, screens, pump screen and inductors. The rinse water should be sprayed out on a legal crop. In the rush to get crops in the ground, each year we see problems with chemicals left in the tank. The biggest problems are seen when moving from corn and soybean herbicide to spraying vegetable crops. Failure to do a thorough cleanup, can be very expensive when you injure or destroy a vegetable crop.

Field Crop Diseases - Bob Mulrooney, Extension Plant Pathologist.

Wheat. Powdery mildew can be found at this time on the lower leaves and stems. Wheat that is beyond jointing, which should be most of it by now, should be checked weekly for disease development. Spray decisions should be delayed until flag leaf emergence, then evaluate the amount of mildew. A powdery mildew spray guide can be found in the 1997 Pest Management Recommendations for Field Crops, Bulletin 237.

Soybeans. Soybean seed treatments are good insurance when planting soybeans under adverse conditions. Planting into cool, wet soils especially in no­till and dry soils are situations when seed treatments can be beneficial. Normally seed with germination of 85% or better does not need seed treatment unless planted under adverse conditions. Seed germinating at 75% to 85% should be treated. Seed treatments containing captan, thiram, or carboxin are suggested. Many hopper­box treatments are also available.

Alfalfa. Sclerotinia crown and stem rot has been seen on fall-seeded alfalfa seedlings this spring. White cottony fungal growth can be seen on the crowns and stems during wet weather. The best diagnostic feature is the presence of hard, black, fungal structures called sclerotia on the infected stems. These sclerotia are often embedded on the infected stems and often covered by the white fungal growth. Fall seedings are particularly susceptible and the wet, warm winter has been very favorable for disease development and plant mortality. Stand reductions can be severe under these conditions.

To Cerone or Not?

Richard Taylor, Extension Agronomist.

Cerone is a plant growth regulator that can be used to shorten the distance between nodes (where each leaf attaches to the main stem) in small grains. By shortening internode length, plant height is reduced. This helps reduce the likelihood of lodging during grain-fill and ripening. Cerone must be applied at the correct growth stage or yield losses can occur. Late application, especially to barley, can significantly reduce yields. Also, keep in mind that Cerone cannot increase yield potential, but only minimize yield loss due to lodging problems.

Cerone is very beneficial in reducing yield losses from lodging in fields that traditionally have problems with lodging. Cerone should be applied only on fields that are frequently scouted to determine growth stage (see below).

Situations that favor consideration of Cerone are as follows:

Cerone should not be used in the following situations:

In summary, if your situation indicates a need for Cerone, read and understand the label. Because of the economic consequences of misapplication, understand the limitations and restrictions on when it can be applied.

Vegetable Crops:

Vegetable Crop Insects - Joanne Whalen, Extension IPM Specialist.

Cole Crops. Early detection of diamondback larvae is critical to achieve effective control, especially with the Bt insecticides. As soon as temperatures increase, we should begin to see an increase in egg laying. Fields should be scouted twice a week to detect the presence of small larvae mining within the leaf tissue. Remember, the Bt insecticides will work best in warmer temperatures, Monitor at 1 qt per acre is still providing good control of small diamondback larvae. At this time, imported cabbageworm eggs and small larvae can be found at low levels. The treatment threshold is 5% infested plants. If no diamondback are present and the temperatures remain cool, a pyrethroid will provide the most cost effective control.

Sweet Corn. As soon as sweet corn emerges from the ground, begin scouting for flea beetle and cutworm activity. Pheromone traps in Delaware and Maryland indicate that black-cutworm moth activity is higher this season. In addition, warmer winter temperatures have increased the potential for flea beetle activity. Although Furadan and Counter can provide good control, spring weather conditions may affect the performance of these materials. The treatment threshold for flea beetle is 5% infested plants. At the one to two-leaf stage, the threshold for cutworms is 3% cut plants. If flea beetles and cutworms are both a problem, Asana, Ambush, Pounce or Warrior will control both insects, Sevin will also provide flea beetle control.

Vegetable Crop Diseases- Kate Everts, Extension Plant Pathologist, Universities of Maryland and Delaware.

Watermelon. Fusarium wilt on watermelon can severely reduce yields of varieties which do not have resistance. This disease is especially severe on sandy soils that have a low to moderate pH. Losses have been reduced in recent years because resistance has been widely incorporated into many varieties. Unfortunately, none of the widely grown seedless varieties are resistant to this disease. In the absence of resistance, the only effective control is to use a long crop rotation (6-8 years) or plant on new ground. Some growers on the Delmarva have strip fumigated the ground under plastic and planted watermelon in a two or three rotation. This is a very risky practice, especially where seedless varieties are grown. The ground between the fumigated strips is a reservoir of inoculum, and the fumigated area has reduced populations of soil micro flora. The pathogenic fusaria can reinfest the fumigated area quickly and with little competition. In addition, the watermelon roots will eventually grow to the edge of the fumigated strip, and some will grow beyond. These roots will come in contact with the inoculum and become infected. The pathogen can invade the vascular system and become systemic in the plant. Give careful consideration to this disease in planning the location of watermelon fields.

Watermelon Transplant Disease Control. It is important to observe transplant seedlings for the presence of seed borne diseases. The most serious of these diseases is bacterial fruit blotch. Fruit blotch symptoms first appear as water soaked areas on infected cotyledons. These lesions eventually turn brown. Lesions developing on the young leaves are small and dark brown, and may spread along the veins. These leaf lesions are surrounded by a light green or yellowish (chlorotic) ring. If you observe transplants with these symptoms, destroy them and isolate any adjacent flats. Observe the adjacent flats for the development of symptoms.

The greenhouse will be less favorable for disease spread if plants are irrigated in the morning. Also, irrigate with as little splashing as possible. To reduce humidity, it may help to vent the greenhouse briefly in the evening, bringing in less humid outdoor air.

Pea Planting Proceeds Smoothly - Ed Kee, Extension Vegetable Specialist

Good weather in April has allowed pea planting to move along at a good pace. For the most part compaction has been avoided, which gives the peas a good chance to maximize their growth.

Weather Summary

University of Delaware, Georgetown

Week of April 12 to April 17.


0.43 inches: April 13

0.01 inches: April 14

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



Ranged from 69EF on April 14 to 56EF on April 15.


Ranged from 55EF on April 13 to 32EF on April 15.

Soil Temperature:

45.7EF. Average for the week.


Very little frost damage has been evident. Where it did occur, the damage was negligible. More damage was seen in some fields from wind and sandblasting. Even with stand losses approaching 50% in some areas of the field, it is difficult to justify disking them up because usually the whole field is not affected.

As the peas reach the 3-6" growth stage, scout the fields for grass and weed break throughs. Basagran, which controls broadleaf weeds works better on small weeds. While not as critical, the same is true for the post-emergent grass control herbicides.

As the spring goes along, be alert for pea aphids.

Compiled & Edited By:

Tracy Wootten

Extension Associate - Vegetable Crops

Cooperative Extension Education in Agriculture and Home Econonmics, 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.