C.O.R.N. Newsletter 2004-15

Insect Update in Corn

Authors: Bruce Eisley, Ron Hammond

Cutworms - Slight cutworm leaf feeding and cutting was observed this past week in central Ohio. The worms in central Ohio ranged in size from ¾ to 1 inch in length and were cutting primarily above the soil surface. Soil moisture conditions can sometimes dictate where you might expect to find cutting with above ground cutting occurring under moist soil conditions and below ground cutting occurring under dry soil conditions. Even though cutting was observed, the damage was not at the level of 3 to 5% of the plants damaged that would be needed to justify a rescue treatment. Reports are also coming in from eastern Ohio concerning cutworm damage in a lot of fields.

Common Stalk Borer - Common stalk borer (CSB) larvae were found feeding on corn in central Ohio at the end of last week. The larvae were small (< 3/8 inch in length) and they were feeding in the whorls of 2 leaf stage corn. CSB larvae can be difficult to control especially if they are deep in the whorls of corn. However, since these larvae were still small and the plants were small, the worms may be moving to additional plants to feed and a rescue treatment may be beneficial. Check areas in no-till fields that have weeds residues from last year or along field margins for CSB feeding. Treatment is justified if 3 to 5% of the plants show feeding injury from CSB worms are small and not deep in the corn whorls.

European Corn Borers - We began operating pheromone traps for European corn borer (ECB) moths several areas of the state during the last couple of weeks. We captured our first moths in central Ohio last week. Since most of the corn is still very small, ECB will not be attracted to the corn at this time. However, as the corn grows and additional moths emerge early feeding in the whorl from ECB may become evident in some fields. We will continue to operate the ECB pheromone traps and the moth counts will be reported in the newsletter and on the web at: http://entomology.osu.edu/ag/04traps.htm.

Denitrification of Applied Nitrogen and Sidedressing Considerations

Authors: Robert Mullen

With the excessive rainfall causing saturated soil conditions in certain areas and warm temperatures that have been prevalent the past couple of weeks, denitrification losses may be a concern. The problem however is in identifying how much N was actually lost. If the majority of the N budget was applied preplant and rates were relatively high (based on yield potentials well above historic yield levels), N will probably be adequate for maximum yield. If N rates applied preplant were lean, sidedressing may need to be considered. Estimates from a study conducted in Illinois, reveal that approximately 4 to 5% of N applied is lost via denitrification for every day a soil is saturated. Fortunately, the recent rains have been occurring as large, quick events. Thus the infiltration rate of the soil was quickly exceeded resulting in runoff. While this can lead to serious erosion, it actually helps avoid long-term saturated soil conditions. This reduces the amount of N subject to denitrification losses. Soil testing is one way to find out if current nitrate levels are adequate for maximum production or if additional N is necessary. Another method of evaluating whether or not the field needs additional N is the application of a reference strip or reference area. Apply N in a couple areas of the field at rates of 100 lb/A (these areas do not need to be excessively large and make sure you remember where you put them). Evaluate the areas throughout the rest of the growing season. If the areas are apparent visually, you probably need a little additional N, otherwise don’t worry about sidedressing.

Yellow Corn, Should I Be Concerned?

Authors: Peter Thomison, Robert Mullen

Current environmental conditions (high rainfall causing saturated soils) can lead to the appearance of yellow corn. The visual appearance may be interpreted as N deficiency, but this is rarely the case. Excessive water leads to poor respiration of the roots inhibiting nutrient uptake, or accumulation of nitrite (especially true on poorly-drained soils) in the soil which the plant takes up (nitrite is toxic to the plant). This results in the chlorotic appearance which resembles N deficiency. After soils dry out, the appearance returns back to normal. If the chlorotic condition persists after the soil dries, the problem should be investigated further. This short-term condition should not affect yield potential of the crop.

Flooding & Ponding Impacts On Corn Stands

Authors: Peter Thomison

Recent heavy rains have resulted in localized flooding and ponding in corn fields across the state. The following are some tips to consider when evaluating possible damage from water saturated soil conditions.

The extent to which flooding injures corn is determined by several factors including: (1) plant stage of development when flooding occurs, (2) duration of flooding and (3) air/soil temperatures. Prior to the 6-leaf collar stage (as measured by visible leaf collars) or when the growing point is at or below the soil surface, corn can usually survive only 2 to 4 days of flooded conditions. The oxygen supply in the soil is depleted after about 48 hours in a flooded soil. Without oxygen, the plant cannot perform critical life sustaining functions; e.g. nutrient and water uptake is impaired, root growth is inhibited, etc. If temperatures are warm during flooding (greater than 77 degrees F) plants may not survive 24-hours. Cooler temperatures prolong survival. Once the growing point is above the water level the likelihood for survival improves greatly.

Even if flooding doesn't kill plants outright, it may have a long term negative impact on crop performance. Excess moisture during the early vegetative stages retards corn root development. As a result, plants may be subject to greater injury during a dry summer because root systems are not sufficiently developed to access available subsoil water. Flooding and ponding can also result in losses of nitrogen through denitrification and leaching.

If flooding in corn lasts less than 48 hours, crop injury should be limited. To confirm plant survival, check the color of the growing point. It should be white to cream colored, while a darkening and/or softening usually precedes plant death. Also look for new leaf growth 3 to 5 days after water drains from the field. Sometimes the growing point is killed by bacterial infections during and after flooding, but plant growth continues in the form of non-productive tillers (suckers).

Disease problems that become greater risks due to flooding and cool temperatures include pythium, corn smut, and crazy top. Despite fungicide seed treatments, pythium root rot contributed to serious stand reductions in many corn fields last year. The fungus that causes crazy top depends on saturated soil conditions to infect corn seedlings. There is limited hybrid resistance to these diseases and predicting damage from corn smut and crazy top is difficult until later in the growing season.

Hail Injury To Corn Should Be Limited

Authors: Peter Thomison

Some of the severe thunderstorms over the weekend were accompanied by hail, which may have caused damage to crops. In corn, the impact of hail damage is largely dependent on the crop's stage of development. Hail affects yield primarily by reducing stands and defoliating plants. Most of the hail damage results from defoliation. Generally, the corn plant is little affected by hail prior to the 6 to 7 leaf stage because the growing point is at or below the soil surface and in the leaf whorl. However, once the growing point is elevated above the soil surface due to internode elongation, the plant grows rapidly and becomes increasingly vulnerable to hail damage with the tassel stage/pollen shedding stage (VT) being the most critical period.

Severe hail damage prior to the 6 to 7-leaf stage can also result in “twisted” or “tied” leaf whorls as injured plants recover and new leaves try to unroll; however, most plants will grow out of this problem and tied whorls seldom cause major yield loss.

Leaf damage by hail usually looks much worse than it really is, especially during the early stages of vegetative growth. Shredded leaves still have some capacity to contribute to plant growth. Plants not killed outright by hail usually show new growth within 3 to 5 days after injury occurs (i.e. if damage occurs prior to tasseling). For this reason, estimates of hail damage should be delayed several days to allow for this period of re-growth.

The hail insurance adjustor's growth staging system counts leaves beyond the last visible collar to the uppermost leaf that is 40-50% exposed whose tip points downward - usually this results in a leaf stage that is numerically 2 leaves greater than the "leaf collar method" (e.g. a V6 plant according to the leaf collar method would probably correspond to a 8-leaf plant according to the hail adjustor's method).

How do we estimate the potential yield loss from recent hail storms? Corn growth stages in Ohio vary considerably depending on location, planting date, etc. Most corn in Ohio has not progressed beyond the V6 stage (as of 5-24-04). Based on estimates of the National Crop Insurance Association (see table below), at the 8-leaf stage (or about V6) if 50% of the leaf tissue is destroyed by hail, a corn plant loses only 3% of its yield potential; if 100% defoliation occurs, a corn plant loses 11% of its yield potential. Since the growing point of most corn is at or below the soil surface, we can probably expect limited yield loss from recent hail damage.

Percent Yield Loss In Corn Based On Growth Stage & Defoliation
(adapted from NCIA corn loss instructions, rev.1984)

	Percent Leaf Defoliation
GrowthStage*	25%	50%	75%	100%
7-leaf	0	2	5	9
8-leaf	0	3	6	11
9-leaf	1	4	7	13
10-leaf	1	6	9	16
*as determined using the hail adjustors leafstaging method

For more detailed information on evaluating hail injury in corn, consult "Assessing Hail Damage to Corn" National Corn Handbook Chapter 1 (NCH-1)." Available on-line at http://www.agcom.purdue.edu/AgCom/Pubs/NCH/NCH-1.html.

 

Seedcorn Maggots in Corn & Soybeans

Authors: Bruce Eisley, Ron Hammond

There is talk about unplanted fields being tilled in the next week to destroy some weeks before corn and soybeans are planted. The seedcorn maggot adults are attracted to the decaying organic matter to lay their eggs and this could result in possible injury to the corn or soybean seed. An insecticide seed treatment might be considered if tillage is going to be performed before corn and soybeans are planted in a tilled field.

Slug Activity

Authors: Ron Hammond, Bruce Eisley

We are beginning to receive reports of slug activity throughout areas of the state with a history of slug problems. Growers are advised to begin monitoring for slug injury if they have not done so already. Late May and early June is the time period when slug damage to corn and soybeans increases to levels that often need treatment. If injury is severe and new growth has significant feeding injury, molluscicide bait should be applied. If the crop is growing well and new plant growth is relative free of slug feeding, growers might wait 3-4 days to see if the growth continues to outgrow slug injury. Currently, the most common bait is Deadline MPs at a recommended rate of 10 lbs per acre.

Soybean Aphid Update

Authors: Ron Hammond, Bruce Eisley

We are currently keeping a watch on the situation with the soybean aphid in more northern states where the aphid is thought to overwinter. As we get into June and early July, we hope to be able to predict the potential for the soybean aphid to achieve population densities similar to last year in Ohio. Based on observations from last fall, we are not yet sure if the soybean aphid will reach economic levels in our state. We will have a lengthy discussion in this C.O.R.N. newsletter in early June with an update on what is happening with the soybean aphid and what growers should do. At this time, we recommend that growers NOT make plans on treating based on unsubstantiated rumors. As always, treatment should be based only on thresholds being reached after populations begin to increase. Treatments should NOT be done for preventive purposes. Because we are not sure what will happen this summer, we recommend growers continue to read this newsletter to keep informed.

Fungicide Applications On Wheat: Timing Restrictions

Authors: Patrick Lipps, Dennis Mills

Fungicides have been applied to a number of wheat fields in Ohio this year in an attempt to control powdery mildew development on susceptible varieties. The relatively cool and damp weather forecasted for this week indicates that in those fields that were not sprayed, powdery mildew will likely continue to develop on the upper leaves. With the heavy and frequent rains over the past week we expect to see Stagonospora leaf blotch move to the upper leaves of the plants within the next week or so as well. Fungicides applied for powdery mildew control will also help control Stagonospora leaf blotch.

The wheat crop has now advanced to, or past, the flowering growth stage in most of the state so most fungicides can no longer be applied according to label restrictions. Only fields in the more northern Counties of Ohio are not yet in flower. Several fungicides (Tilt, PropiMax, Stratego) have 24c registrations in Ohio to be applied up to flowering (when anthers are exposed on the heads). Quadris has a Federal label that limits application to before flowering, but Headline fungicide can be applied through the flowering period (until anthers are shed). Quilt fungicide can only be applied up to full flag leaf emergence based on its Federal label.

Wheat Scab Risk High In North Central Ohio

Authors: Patrick Lipps, Dennis Mills

We have been monitoring the risk of head scab in Ohio using the Wheat Fusarium Head Blight Prediction Center (http://www.wheatscab.psu.edu/). Wheat is most at risk to infection when plants are in flower. Epidemics of head scab occur when there is an extended period of high humidity (above 90%) and temperature between 48 and 85 degrees F. Additionally, having old corn residue in the field increases the risk of scab greatly. Wheat in southern Ohio was in flower from about May 13 to May 19. The wheat in Central Ohio was in flower from May 17 through May 22 and in nothern Ohio most fields are now flowering (from May 21 through May 25) with some later headed fields to flower later this week.

Based on these flowering dates the Wheat Fusarium Head Blight Prediction Center has presented the following risk predictions for various locations in Ohio for wheat fields plated after soybeans, if the wheat was planted after corn, then the risk would likely be higher.

Head Scab Risk based on weather data from May 13 to May 24:
South Central: Low risk
South West: Moderate risk
West Central: Moderate risk
Central: Moderate risk
East Central: Moderate risk
North West: Low risk
North Central: High risk
North East: Moderate risk

The highest risk of head scab is for fields in flower during the May 20 to May 23 time period, mostly in North Central Ohio. This includes Hardin, Hancock, Marion, Wyandot, Crawford, Seneca, Sandusky, Huron, Richland, Morrow, Knox, Ashland, Wayne, Medina, Stark, Delaware and Licking Counties. Moderate risk of disease occurs throughout the northern part of the state except for counties north and west of Findlay.

The severity of disease is highly dependent on flowering date, wheat variety, residue management practices used on the farm and weather conditions in each field. These predictions indicate there is risk of disease at certain locations in Ohio, but the forecast does not predict the severity of the disease. It would be safe to say that at this time head scab will likely be an issue in Ohio this year. We will be providing an update on the development of the disease in the next few C.O.R.N. newsletters.

Armyworms in Wheat

Authors: Dennis Mills, Ron Hammond

Small common armyworm larvae (CAW) were found in wheat in central Ohio last week. The larvae were small (< ¾ inch in length) and feeding on the leaves and awns of the plants. Small numbers were found in the field but it does indicate that it is time to begin checking wheat for CAW larvae. You will normally find CAW larvae hiding near the base of the plants during the day and feeding on the leaves at night. Treatment for CAW is dependent on a number of factors including stage of plant growth and size and condition of the larvae. If larvae are large (1-1/4 to 1-1/2 inch in length) most feeding has occurred and the larvae are no longer a threat to the wheat. Data from other states indicate that once the wheat is in the dough stage armyworm leaf feeding does not do any additional economic damage. When scouting for CAW, check the condition of the larvae because there are several parasites and diseases that can kill the larvae.

Potato Leafhopper in Alfalfa

Authors: Ron Hammond, Bruce Eisley

PLH adults have been in the state for the past several weeks having migrated from the south but we do not worry about PLH damage to first cutting because the insect shows up too late in first cutting to do any damage. However, we need to begin checking second cutting alfalfa for PLH as soon as new regrowth can be swept. Checks should be made with a standard sweep net by taking several 10 sweep samples randomly across the field and counting the number of PLH adults and nymphs found in each 10 sweep sample. Treatment for PLH is based on the number of adults and nymphs found when sweeping versus the height of the alfalfa. For example, treatment is warranted if the number of adults and nymphs in 10 sweeps equals or exceeds the height of the alfalfa in inches (8 adults & nymphs in 10 sweeps in 6 inch alfalfa would justify treatment for PLH). The following table gives numbers for treatment. Additional information about PLH can be found in a fact sheet on the web at: http://ohioline.osu.edu/ent-fact/0033.html

Action thresholds for control of potato leafhopper.

	AlfalfaTolerance For Stress
StandHeight	Low	Normal	High
Inches	Action Threshold  of  PLH  per 10 Sweeps
6	3	6	9
8	4	8	12
10	5	10	15
12	6	12	18
14	7	14	21
16	8	16	24
18	9	18	27
20+	10	20	30
Low:Alfalfa under environment stress and very suceptible to PLH injury. High: Alfalfa exhibitingvigorous growth and capable of tolerating someinjury.
Chemicals Labeled  On  Alfalfa  for  PLH  Control
Chemical	Rate	PreHarvest Limitation in Days
Ambush	3.2-6.4Fl oz/A	0-14
Baythroid2	.8-1.6fl oz/A	7
Dimate4E	.5-1pint/A	10
Furadan4F	1-2pints/A	7-21
Guthion50WP	.5-1lb/A	14-16
Imidan70 W	1-11/3 lb/A	7
Lorsban4E	1-2pints/A	7-28
Malathion57EC	1.5-2.25pints/A	0-5
MustangMAX	2.24-4oz/A	3
Pounce3.2 EC	4-8fl oz/A	0-14
Sevin80 S	1.25lb/A	7
Warrior	2.56-3.84floz/A	1-7

 

A Few Weed Management Items

Authors: Mark Loux

Wet weather throughout the state has hampered the implementation of weed management plans in some fields. In no-till fields that have not yet been planted, be sure to increase glyphosate rates to improve control of large weeds. A glyphosate rate of at least 1.5 lbs acid equivalent/A (44 oz of Weathermax or 64 oz of most other glyphosate products) is probably justified in most fields based on weed size.

In fields that will still be tilled, it may be extremely difficult to completely control large weeds with any type of spring tillage tool. Tillage often knocks down large weeds which then recover and become difficult to control with herbicides. Consider application of glyphosate prior to tillage, in order to ensure effective control. Wait at least one day between glyphosate application and tillage for annual weeds, and 2 to 3 days for perennial weeds.

Corn and weeds are growing extremely rapidly in some fields that will receive postemergence herbicide applications when weather allows. Scout these fields prior to application to make sure that the corn is not too large for the intended herbicide treatment. For maximum crop safety, treatments containing dicamba should be applied when corn is less than 8 to 10 inches tall and before the 5-leaf stage. Labels of ALS-inhibiting herbicides vary with regard to maximum corn size and growth stage. Check labels and the OSU/Indiana Weed Control Guide for more information (Table 7 on page 75 is a quick reference for this information). Also, be sure nozzles are high enough to ensure the proper spray pattern and avoid application of excess herbicide directly into the corn whorl.

The good news about planting delays – some perennial weeds such as Canada thistle are advanced enough in growth stage to allow effective control of roots and shoots with glyphosate or other appropriate herbicides. Canada thistle is in the bud to bloom stage in some areas, which enables systemic herbicides to move into roots as well as shoots.

A brief article on cressleaf groundsel appeared in a previous issue of C.O.R.N., and we continue to receive questions on this weed. A fact sheet on cressleaf groundsel is posted in the “Of Interest” section of the OSU weed management website, agcrops.osu.edu/weeds.

What Is That Tall, Stinking, White - Flowering Weed?

The tall weed with carrot-like leaves, a purple-spotted and hollow stem, with white flowers in a compound umbel that has started flowering is poison hemlock, NOT wild carrot. This plant is poisonous to most livestock and humans when consumed in high enough quantities. Poison hemlock has a biennial life cycle, meaning it germinated last spring. As with any annual species, stopping seed production of a biennial is also critical to a successful control program.

The best time to control this species is in the fall after mid-October. At that time a combination of glyphosate at 0.75 pounds acid equivalent (lbs ae)/A and 2,4-D ester at 1.0 pint/A is usually sufficient to control poison hemlock. A combination of 2,4-D ester at 1.0 pint/A and Canopy XL at 4.0 oz/A or greater may also control it. At this time of the year glyphosate at 1.5 lb ae/A plus 2,4-D ester at 1.0 pint/A must be applied for maximum control in any burndown program. This treatment may not completely control all plants, but should drastically reduce seed production and competition. Soybean planting must be delayed for 7 days after application of 2,4-D ester at 1.0 pint/A. This burndown application must go on as soon as possible for maximum control.