C.O.R.N. Newsletter 2004-16

Dates Covered: 
June 1, 2004 - June 8, 2004
Editor: 
Greg La Barge

The Stand Was There …. Then It Was Gone

Authors: Anne Dorrance

I stopped by several fields last week, where post-emergence damping-off was in full swing. This type of stand loss typically occurs 1 to 2 weeks after the fields are saturated. The primary culprits in Ohio are Phytophthora sojae and Pythium spp. We need to culture these out to be sure which pathogen is the cause of the stand loss.

Seedlings will emerge, these soil borne pathogens will infect the plants at the hypocotyls or roots and slowly colonize the plants. The end result are single to few plants in isolated patches throughout a field to large areas of dying seedlings.

Seed treatments can limit these losses to some extent. If the seed has been in the ground for 3 weeks or more and the plants begin to die – this is probably the limit of the seed treatment. The problem with soybean seed treatment is that the seed coat is often pushed up through the soil and does not stay in the ground like a corn seed. Thus, the seed treatment does not stay in the soil –

Soil drainage – the amount of time the soil remains saturated is also crucial for the development of these pathogens, they need free water to produce their swimming spores. Once the water is gone – so are the spores. One field I visited, the tile lines in heavy soil were greater than 50 feet apart – It is not uncommon to have tile lines 15 to 25 feet apart in soils with clay content greater than 30%.

How to tell if your field is poorly drained – how many days does it take for you to be able to walk through the field without getting big piles of mud on your boots – more than 2? It’s time to reassess the drainage.
Resistance package – This is especially important for Phytophthora sojae. The populations of P. sojae in Ohio are adapting to the Rps genes that are currently deployed in commercial soybean varieties. We are becoming more and more dependent on the partial resistance part of the resistance package in many of these varieties. If an Rps gene is effective – it works from the time of the seed through the life of the plant. If an Rps gene is not effective, damping-off will occur. For later season development - Phytophthora stem rot will only develop on varieties with partial resistance scores greater than 5 when the Rps genes are not effective (1= no root rot; 9= all dead).

For replanting – be sure to choose varieties with high levels of partial resistance combined with a seed treatment. The pathogen populations have been primed and are ready for the next “victims” to be put in the ground ….provided it keeps raining.

Wheat Head Scab Showing Up In Southern Ohio.

Authors: Patrick Lipps, Dennis Mills

Wheat head scab symptoms started to show up in southern Ohio by the end of last week. Head scab can be recognized as individual spikelets of the head die and turn straw colored. Sometimes one spikelet will be affected per head, but more commonly a number of adjacent spikelets will be killed. Thus, each head may have a different percentage of spikelets affected. We observed fields in Pickaway, Fayette and Clinton County with from 1 to 15% of the heads with scab. The percentage of heads with head scab will likely increase somewhat during this week in these fields. Wheat in southern and central Ohio were in flower during a period of wet rainy weather from May 18 to May 24. Wheat producers should begin to scout fields for head scab, especially in fields south of I-70. We will likely see symptoms of head scab begin to show up in central Ohio soon, so begin to look at these fields by the end of this week. As we get more information on the level of head scab present in Ohio during the next week or so we will provide more information about some of the problems associated with this significant wheat disease. Photos of scab affected heads and a fact sheet on the disease can be obtained on the Ohio Field Crop Disease web site at http://www.oardc.ohio-state.edu/ohiofieldcropdisease/.

Insect Update

Authors: Bruce Eisley, Ron Hammond

Cutworms in Corn – Cutworms continue to feed on corn at the Western Agricultural Research Station. The worms range in size from ½ to 1 inch in length and the corn is in stage V2 to V5. Cutting is occurring both above and below ground. The worms are not able to completely cut the plant off below ground resulting in a dead plant that is still standing.

Common Stalk Borer (CSB) in Corn – CSB continues to feed on V4 stage corn in plots at the Western Agricultural Research Station near South Charleston. The larvae are still small (< .5 inches) and feeding deep in the corn whorl. Control would be difficult under these conditions.

European Corn Borer (ECB) – Corn borer moths continue to fly in Ohio and even though a lot of the corn is small there may be fields that are attractive to the adults to lay their eggs. Based on degree days, we would expect to find eggs in southern Ohio this week. Taller corn in neighborhoods, which is probably the first planted corn, should be scouted for ECB larval feeding.

Slugs – Economic injury is causing concern for both corn and soybean growers. Numerous fields are currently being treated for slugs in many parts of the state. Fields, especially soybean fields, that have recently been or will be planted and emerging over the next few weeks will be especially vulnerable to slug injury. Those fields should be watched closely. If large populations of slugs are present at planting, growers might consider an at-planting time treatment if they have had problems with plant stands in the past.

Armyworms – A Few Here And There

Authors: Bruce Eisley, Ron Hammond

We have had several reports of armyworm (CAW) in corn. In all cases the corn was planted no-till into rye and the larvae were just starting to feed on the corn. Even though the larvae are still small, one field had already been sprayed because over 50% of the plants were showing feeding damage. In the other fields, only a few plants were being damaged but the fields were being watched closely for additional damage. If > 25% of the plants show injury from CAW and larvae are still present and healthy, treatment for CAW may be justified. If less than 25% of the plants showing feed, then check the field again in a couple of days to make sure the field damage does not increase.

We have only had a few reports of CAW in wheat and only a slight amount of feeding being reported. We have thought, since the CAW outbreak of CAW in 2001, that leaf feeding after the wheat is in the dough stage does not affect yields even if all of the foliage is removed. In fact, there is a report from Arkansas that suggests that defoliation of wheat plants at any stage does not affect yield.

The other part of the CAW threshold on wheat that we normally mention deals with head cutting by the larvae. Even though head cutting is mentioned in all thresholds for CAW, it does not seem to occur very often. We do not know what triggers head cutting and we don’t know what % of head cutting is necessary to justify treatment.

So our recommendation for CAW in wheat in 2004 is:

1. If leaf feeding is occurring and the wheat is in the dough stage or later you might want to hold off spraying.

2. If head clipping is occurring, obtain an estimate of the % of head being cut and treat accordingly. If only a few heads being clipped, check back in a day to see if head clipping is increasing.

3. Whether leaf feeding or head clipping, make sure the larvae are small (larvae >1-1/4” will not do much additional feeding) and healthy. There are parasites http://entomology.osu.edu/ag/cawl2.htm (see picture of white eggs on the backs of the worms) and diseases that can kill the larvae or make them so sick that they do not feed any longer.

Staging Vegetative Growth In Corn And Estimating Leaf Development Based On Heat Unit Accumulation

Authors: Peter Thomison

When estimating yield losses in corn due to hail, frost, and other types of plant injury, it is necessary to establish the stage of plant growth at the time damage occurred. It's also important to know the stage of development in order to use postemergence herbicides effectively with minimum crop damage.

Several systems are currently used to stage vegetative growth in corn. The "leaf collar" system is probably the method most widely used by Extension and seed company agronomists in the Corn Belt. With this method, each leaf stage is defined according to the uppermost leaf whose leaf collar is visible. The first part of the collar that is visible is the back, which appears as a discolored line between the leaf blade and the leaf sheath. The oval shaped first leaf is a reference point for counting upward to the top visible leaf collar. This oval shaped leaf is counted as the number 1 leaf when staging. If a plant has 4 visible leaf collars, then it is defined as being at V4. Normally a plant at the V4 stage will have parts of the 5th and 6th leaves visible, but only four leaves with distinct collars. A field is defined as being at a given growth stage when at least 50% of the plants show collars.

Another widely used staging method is the "hail adjustor's horizontal leaf method" developed by the crop insurance industry. Rather than using the uppermost leaf collar, hail adjustors identify the uppermost leaf that is 40 to 50% exposed and whose tip points below the horizontal. Typically a given "horizontal leaf" growth stage based on the hail adjustor's method will be 1 to 2 leaf stages greater than the collar method. From growth stage V1 through about V5 there is typically one additional leaf (above that leaf with the last visible collar) whose leaf tip is pointing below the horizontal. Beyond growth stage V5, two or more additional leaves with 'droopy' leaf tips will be evident above the leaf with the last visible collar (so a V6 plant according to the leaf collar method will typically be a 8-leaf plant according to the hail adjustor's horizontal leaf method). One problem with the horizontal leaf method is that it is often difficult to identify the uppermost horizontal leaves in fields that have recently experienced severe leaf damage. Hail adjustors get around this problem because they usually assess hail damage 5 to 10 days after the storm, by which time 1 or more leaves have emerged from the whorl.

Corn leaf stage is a more reliable indicator of corn development than plant height. This is especially true in a cool, wet spring when corn is growing more slowly from a height standpoint. Differences in tillage and soil type may have a pronounced effect on plant height but relatively little effect on the stage of vegetative development. For example, corn may be taller in a conventionally-tilled field than in a neighboring no-till field planted on the same date, yet plants in both fields may be at nearly the same stage when counting leaf collars.

At about V6 stage, or 8-leaf stage of the hail adjustor's method, increasing stalk and nodal growth combine to tear the smallest lower leaves from the plant. This results in degeneration and eventual loss of lower leaves. Hail damage, insect feeding, and fertilizer/herbicide burning promote this process.There may also be occasions when the lower leaves are hard to identify prior to V6 stage. When extensive early season leaf damage has occurred, identification of the first rounded leaf and subsequent leaf collars may be difficult. This year, several factors, including strong winds, persistent rains, and periodic flooding/ponding, have contributed to loss of leaves used in staging corn growth.

Dr. Bob Nielsen at Purdue has described a method for predicting leaf stage development using accumulated heat unit or growing degree day (GDD) information. Given an understanding of corn leaf stage development and heat unit calculation, a grower can predict what leaf stage of development a particular field is at given its planting date and temperatures since planting. It is useful if you know when the crop emerged, but if not you can estimate that event also. Corn emergence typically requires 100 to 150 GDDs.
The Purdue research indicates that corn leaf developmental rates can be characterized by two phases. From emergence to V10 (ten visible leaf collars), leaf emergence occurs approximately every 85 GDDs. From V10 to tasseling, leaf emergence occurs more rapidly at approximately one leaf every 50 GDDs. Previously, about 60-65 GDDs were associated with the appearance of new leaf collars during vegetative growth.

Example: A field was planted on April 28, but you do not know exactly when it emerged. Since planting, approximately 785 GDDs have accumulated. If you assume that the crop emerged in about 125 GDDs, then the estimated leaf stage for the crop would be between V7 and V8. We arrived at this estimate by first subtracting 125 from 785 to account for emergence, then dividing the result (660) by 85 to equal 7.8.

Dr. Nielsen warns that these predictions of leaf stage development are only estimates. One of the factors that most influences the accuracy of these estimates is the existence of other growth-limiting stresses and conditions (nutrient deficiencies, compaction, etc.). Despite these potential drawbacks, this heat unit method may be useful in timing when plants will reach an approximate stage of growth.

Hail Damage In Corn Often Results In Tied Leaf Whorls

Authors: Peter Thomison

Last week another round of hail storms battered corn fields in various parts of the state including SW Ohio. For information on assessing hail injury, check the recent C.O.R.N. newsletter article,“Hail Injury To Corn Should Be Limited” from May 25, 2004 - June 2, 2004 available online at http://corn.osu.edu/index.php?setissueID=39#E.

Severe hail damage prior to the 6 to 7-leaf stage (about V5-6 according to the leaf collar staging system) often results in “twisted” or “tied” leaf whorls as injured plants recover and new leaves try to unroll. Corn plants usually grow out of this problem and tied whorls seldom cause major yield loss. In 1999, we evaluated effects of tied whorls and stalk bruising caused by hail damage in early June on corn performance at four on-farm sites by comparing plots within the fields with major and minor hail damage. During the three- to four-week period following the hail storm, the number of plants exhibiting tied whorls decreased sharply. Plots that received major damage from hail exhibited 36 to 61% tied whorls on June 12, which decreased to 0 to 9% by July 6. Silking was delayed approximately 1 to 1.5 weeks in the plots with major damage vs. plots with minor damage. Severe stalk bruising did not increase lodging; lodging was negligible across farm sites, averaging less than 2%. Kernel moisture at harvest was generally higher in plots with major damage (in three of the four fields), and yields were lower compared to the plots with minor damage. More information on this evaluation is in the 2000 Agronomic Crops Team On-Farm Research Projects Summary available on-line at http://ohioline.osu.edu/sc179/sc179_16.html

Late Planted Crops And Soil Fertility Levels

Authors: Robert Mullen

With rain keeping many producers out of the field this year, many fields have been planted later than usual. Often questions arise as to the nutrient demand of these late planted crops – do I need as much N? Do I need as much P and K?

Even though the crop is planted late, P and K should be managed as if the crop was planted on time (this goes for both soybean and corn). The nature of P and K is such that yield potential of the crop does not dictate the amount of these two immobile nutrients needed. They simply need to be at levels adequate for maximum production (use Tri-State Fertilizer recommendations to determine rate). This rule also goes for other immobile macronutrients.

Nitrogen rates, on the other hand, should be adjusted. Yield potential of late-planted corn (either typical maturing hybrids or earlier maturing hybrids) is decreased reducing the N demand of the crop (Table 1). Remember, the amount of N needed by a crop is directly related to the yield level of the crop (unlike the immobile nutrients P & K). Thus adjust yield potential and N rates accordingly. If tempted to apply a lot of N, remember late-planted corn typically does not respond well to high N rates, and can be susceptible to lodging.

Table 1. Expected corn grain yields for various planting dates.

Planting Date
Percent Yield
May 1-7
100
May 8-14
100
May 21-28
86
May 29-June 4
79
June 5-10
70

Adopted from Peter Thomison CORN article May 13-20, 2002.

Archive Issue Contributors: 

State Specialists: Pat Lipps & Anne Dorrance, Dennis Mills (Plant Pathology), Peter Thomison (Corn Production), Mark Loux (Weed Science), Jeff Stachler (Weed Science), Bruce Eisley (IPM), Robert Mullen (Soil Fertility) and Ron Hammond (Entomology); Extension Agents: Roger Bender (Shelby), Ray Wells (Ross), Barry Ward (Champaign), Steve Foster (Darke), Todd Mangen (Mercer), Gary Wilson (Hancock), Greg La Barge (Fulton), Howard Siegrist (Licking), Glen Arnold (Putnam), Harold Watters (Miami), Dusty Sonneberg (Henry) and Steve Prochaska (Crawford).

About the C.O.R.N. Newsletter

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.