C.O.R.N. Newsletter 2005-21

Dates Covered: 
July 11, 2005 - July 19, 2005
Editor: 
Jim Skeeles

Ten Spores is Not Enough

Authors: Anne Dorrance

The big news last week was that 10 spores, which look identical to soybean rust, were identified in a spore trap in Kentucky, 2 spores were found in Tennessee. NO SOYBEAN RUST HAS BEEN IDENTIFIED IN ANY GULF STATES – except a sentinel plot in Baldwin County, Alabama. These spore traps were established throughout the south, courtesy of Syngenta, to assist in monitoring. When spores are found, comparisons are made with the environment that occurred during that week. From the Kentucky reports, environment was not very favorable. Scouting will intensify over the next 2 weeks in Kentucky to determine if any rust was established.

The inoculum in the US is very, very, very low. With these hurricanes, the inoculum is being dispersed over a wide, wide area. Hurricane Ivan, brought spores from northern South America, it took from mid-September to early November before this rust could be detected. It appears at this point, based on inoculum levels in Alabama – that Dennis will carry even lower levels of inoculum. We will be monitoring fields the last week of July and first two weeks of August – to see if Dennis brought us any rust. It takes 5 to 10 days for lesions to form, but 10 days for spores to develop on these leaves. If it continues to stay hot and dry – it may take even longer.

One of the challenges of this past winter – is that we only had information from Brazil to base guidelines and predictions. It is apparent now, that one of the big differences between the two areas is the total inoculum. Brazil especially has very high levels of inoculum from a number of sources. Here in the US, this year (2005), this really has not developed. It’s like starting a camp fire, you need a lot of small sticks and paper to get the fire going, we’ve only got a twig.

Fungicides are not recommended for Ohio for the next few weeks. In fact, with the number of fields with spider mites, adding fungicides may exacerbate another problem.

Understanding Corn Crop Water Needs: Available Soil Moisture

Authors: Peter Thomison

High temperatures and limited rainfall have created stress conditions in many corn fields across Ohio. Average water use by a corn crop during pollination and early grain fill is about 1/3 inch per day. Evapotranspiration rates in corn depend on temperature, humidity, wind, solar radiation and total leaf area of the crop. (Evaporation from the soil surface combined with transpiration from plants is evapotranspiration). When temperature is relatively low and humidity is high as on a calm, cloudy day, the evapotranspiration rate will be low. If temperature is high and humidity low as on a sunny, windy day, the rate will be high.

As drought-like conditions appear to be increasing in parts of the state, especially areas which have received negligible rainfall since early June, questions have arisen as to how much of the corn crop’s water needs can be met by subsoil moisture. After all, excessive rainfall occurred earlier in the spring.

The following is information from the National Corn Handbook - Chapter NCH-20 “Irrigation Scheduling for Corn-Why and How” to help address these questions.

Soil textural characteristics dictate the water holding capacity, intake rate and drainage rate. Soils may have available water capacity of as little as 4 inches or may exceed 8 inches in 4 feet of soil. Table 1 gives the available-water holding capacities of ten different soil types. Soils also differ as to depth adequate for active root development; some have underlying layers of gravel or hard pan that would restrict root growth.

Information in this table might be used to estimate the number of days that moisture stored in the soil could “carry” a corn crop. For example, with a storage capacity of 1.8 in./ft, a fully charged silty clay loam soil might carry corn with a 3 foot rooting depth up to 18 days during silking and early grain fill stages (1.8 in/ft times 3 foot depth= 5.4 inches available water; 5.4 in. divided by 0.3 inch/A/day water requirement = 18 days). Although corn roots can grow as deep as 8 feet, when actively growing, corn obtains 90% of its water requirements from the top 3 feet of the soil profile.

A major factor determining the ability of corn to extract available soil water is soil compaction. In many Ohio corn fields, surface compaction, combined with excessive soil moisture early in the season, late plantings, etc. have resulted in corn root systems restricted to the top few inches of the soil profile. These flat, shallow root systems make the crop especially vulnerable to drought. In addition to soil compaction, when relating the information from Table 1 to various Ohio soil types, keep in mind that other factors may influence water availability. Differences in soil organic matter and texture often occur at different rooting depths, for example the top foot of soil may be a silty clay loam but underlying layers may be clay, sand, or gravel.

Table 1. Available-Water Holding Capacity of Ten Soil Types.*


 Soil type  
Textural characteristics 
Storage capacity


in./ft.
0
Sandy clay
2.0
1
Silty clay loam
1.8
2
Clay loam
1.8
3
Loam
Low (2%) Very fine sandy loam O.M.
Silt loam

2.0
4
Loam
High (3%) Very fine sandy loams O.M.
Silt loam

2.5
5
Fine sandy loam
1.8
6
Sandy loam
1.4
7
Loamy sand
1.1
8
Fine sands
1.0
9
Silty clay
Clay 
1.6





*Source: Chapter NCH-20 “Irrigation Scheduling for Corn - Why and How”

Tassels Emerging in Many Corn Fields

Authors: Peter Thomison

During the past week tassels began appearing in early planted corn fields, especially those with short season hybrids. The flowering stage in corn is the most critical period in the development of a corn plant from the standpoint of grain yield determination. Stress conditions such as drought we’re currently experiencing in many areas of the state have the greatest impact on yield potential during the reproductive stage. The following are some key steps in the corn pollination process.

Pollen shed usually begins two to three days prior to silk emergence and continues for five to eight days with peak shed on the third day. Under very dry conditions, silk emergence may be delayed, and such “asynchronization” of pollen shed and silking may result in poor kernel set and reduced grain yields. On a typical midsummer day, peak pollen shed occurs in the morning between 9:00 and 11:00 a.m. followed by a second round of pollen shed late in the afternoon.

The tassel is usually fully emerged and "stretched out" before any pollen is shed. Pollen shed begins in the middle of the central spike of the tassel and spreads out later over the whole tassel with the lower branches last to shed pollen.

Pollen grains are borne in anthers, each of which contains a large number of pollen grains. The anthers open and the pollen grains pour out in early to mid morning after dew has dried off the tassels. Pollen is light and is often carried considerable distances by the wind. However, most of it settles within 20 to 50 feet.

Pollen shed is not a continuous process. It stops when the tassel is too wet or too dry and begins again when temperature conditions are favorable. Pollen stands little chance of being washed off the silks during a rainstorm as little to none is shed when the tassel is wet. Also, silks are covered with fine, sticky hairs, which serve to catch and anchor pollen grains.

Under favorable conditions, pollen grain remains viable for only 18 to 24 hours. However, the pollen grain starts growth of the pollen tube down the silk channel within minutes of coming in contact with a silk and the pollen tube grows the length of the silk and enters the female flower (ovule) in 12 to 28 hours.

A well-developed ear shoot should have 750 to 1,000 ovules (potential kernels) each producing a silk. The silks from near the base of the ear emerge first and those from the tip appear last. Under good conditions, all silks will emerge and be ready for pollination within 3 to 5 days and this usually provides adequate time for all silks to be pollinated before pollen shed ceases.

Pollen of a given plant rarely fertilizes the silks of the same plant. Under field conditions 97% or more of the kernels produced by each plant are pollinated by other plants in the field. The amount of pollen is rarely a cause of poor kernel set. Each tassel contains from 2 to 5 million pollen grains, which translates to 2,000 to 5,000 pollen grains produced for each silk of the ear shoot. Shortages of pollen are usually only a problem under conditions of extreme heat and drought. As noted above, poor seed set is more often associated with poor timing of pollen shed with silk emergence (silks emerging after pollen shed). Modern hybrids seldom exhibit this problem in Ohio unless they experience extreme drought stress. In 2002 when high temperatures and severe water shortages impacted much of Ohio, barrenness was not unusual in some corn fields. Let’s hope tropical depression Dennis brings some much needed rain so we can avoid further comparisons of this year’s growing season to that of 2002.

Soybean Defoliators

Authors: Bruce Eisley, Ron Hammond

We are receiving reports from around the state of the presence of all the common leaf feeding insects that are usually found at this time of year. First generation bean leaf beetles are beginning to occur. They will normally feed on the youngest, expanding trifoliates which will cause a shot hole appearance on the top of the plant canopy. In addition, some growers chose to take action against possible vectoring of bean pod mottle virus by bean leaf beetle by applying an early season application of an insecticide after emergence or by using a seed treatment. However, for best virus protection a second application is needed now at the beginning of this first generation.

In addition to bean leaf beetles making their appearance, Japanese beetles are also making their presence known, as well as Mexican bean beetles and green cloverworms. And we continue to get reports of large numbers of small grasshoppers throughout the state.

In terms of defoliation, it would be unusual for any of them alone to cause significant defoliation. However, a complex of two or more might cause defoliation levels to rise above threshold levels. Growers are advised to initiate scouting procedures to prevent defoliation from reaching the 15-20% defoliation threshold during the early reproductive growth stages, R1-R5, which then rises to 20-25% during growth stage R6 late in the summer. A list of labeled insecticides for control of all these soybean pests is available at http://corn.osu.edu/library/articles/03insectupdate.html When sampling, check numerous places within the field, avoiding the field edges which often tend to have higher levels than the rest of the field. Remember to continue to watch for soybean aphids and twospotted spider mites, both which are continuing to cause problems, while sampling for these defoliators.

Potato Leafhopper in Alfalfa

Authors: Bruce Eisley, Ron Hammond, Mark Sulc

Although populations of potato leafhopper on alfalfa were somewhat low in June, we are now receiving reports of large numbers occurring on later cuttings. Growers are advised to check their alfalfa for potential problems. The threshold on leafhopper-susceptible alfalfa is as follows: when the average number of leafhoppers in a single sample (10 sweeps) equals or is greater than the height of the alfalfa, treatment should be considered if harvest is more than 7 days away. For example, if the alfalfa is 8 inches tall and the average number of leafhoppers per sample is 8 or higher, treatment is warranted. If the average is 7 or lower, the grower should come back within a few days to see if the population is higher or lower.

The threshold should be lowered when the alfalfa is under stress, especially for new seedings made this year. Consider lowering the threshold to half the normal level for new 2005 seedings that are growing slowly because of drought stress. In those situations, there may NOT be a yield response from insecticide treatment in the current growth cycle, especially if drought conditions persist. However, protecting the plants from leafhopper damage now will protect the stand and its future yield potential.

The thresholds are higher for glandular-haired varieties rated as highly resistant to potato leafhopper (at least 50% resistance ratings). The threshold for established stands of highly resistant alfalfa, based on three years of our research data, is 3X the regular threshold. Thus, using the example above, if the alfalfa is 8 inches tall, the threshold would be 24 leafhoppers per sample. We recommend this threshold for any highly resistant alfalfa that is beyond its first cutting of the seeding year. However, we feel that the regular thresholds should be used for potato leafhopper resistant alfalfa prior to the first cutting of the seeding year, which includes most fields planted this spring. However, beyond that very first cutting, the higher thresholds can be used, especially if the highly resistant alfalfa is growing vigorously and not under stress.

More information can be found in the factsheet “Potato Leafhopper on Alfalfa”, available at http://ohioline.osu.edu/ent-fact/0033.html

Wheat Harvest Report: Good Yields, Excellent Quality, and Little Head Scab

Authors: Patrick Lipps, Pierce Paul

Wheat harvest started in earnest last week, but was delayed by scattered showers late in the week. Growers from throughout the state are reporting good yields (generally 60 to 90 bu/A) and excellent grain quality (most reporting test weights of 60 to 62 lb/bu). Contrary to early presumptions, the hot weather in early June apparently did not greatly affect the yield potential of wheat, probably due to the fact that little leaf disease was present at that time. The combination of high temperatures and disease generally has a more detrimental effect on yield than either stress alone, especially if they occur during the critical stages of grain fill. The milling and baking industries will greatly appreciate the high quality grain being delivered by Ohio wheat producers this year.

In part, this year's high quality of wheat can be attributed to the very low levels of disease in the state, especially Fusarium head blight, also known as head scab. OSU Extension educators have been instrumental in helping develop the disease predictive models we currently use as an early warning system for head scab. Over the past four years OSU agricultural agents have helped collect disease data from growers fields to be used to validate the accuracy of the disease forecasting models. This year, during June, agents from 24 counties surveyed 124 fields for the incidence of head scab. Each field was assessed by counting the number of heads with symptoms of head scab in one-foot sections of row at 10 different locations in the field. The data are reported and compared to Head Scab Risk predictions made earlier in the season when the crop was in the flowering growth stage. This year the Fusarium head scab prediction system estimated the risk of having an epidemic of head scab was from 0 to 3% in individual counties. Therefore, the overall risk of having scab at an economic level was very low due to dry cool conditions during the 7 days prior to flowering. The results of the survey in June confirmed that the forecasted low risk level was an accurate prediction for Ohio. Of the 124 fields surveyed, 84% of the fields had scab incidence below 1%, and 15% of the fields had levels in the 1 to 5% incidence range. The overall average for the state was 0.6% incidence with a range in scab incidence of 0% to 8.8% for individual fields.

This year was the lowest level of head scab in the state over the past four years. The average incidence of scab for the years 2002, 2003 and 2004 was 4.1%, 8.9% and 13.1%, respectively. The range in scab incidence in individual fields for these three years was 0 to 49%, 0 to 73% and 0 to 61%, respectively. Variation in the incidence of scab from year to year was likely due to differences in weather conditions during the flowering period of the crop when wheat is most susceptible to infection. Variation in the incidence of scab from field to field within years is likely due to differences in flowering date, level of Fusarium spores within fields due to differences in crop residue levels, local precipitation patterns and differences in the susceptibility of varieties grown. Hopefully, the development of newer wheat varieties with greater levels of resistance to head scab will minimize losses from this disease in the near future. Growers are encouraged to evaluate the new scab resistant varieties on at least a portion of their wheat acres to see how they perform. Talk to your seed dealer about the availability of scab resistant varieties for planting in 2005.

Archive Issue Contributors: 

State Specialists: Pat Lipps, Anne Dorrance and Pierce Paul (Plant Pathology), Peter Thomison (Corn Production), Mark Loux and Jeff Stachler (Weed Science), Mark Sulc (Forages) and Ron Hammond (Entomology) Extension Educators: Roger Bender (Shelby), Todd Mangen (Mercer), Greg La Barge (Fulton), Howard Siegrist (Licking), Mark Keonig (Sandusky), Dusty Sonnenberg (Henry), Keith Diedrick (Wayne), Gary Wilson (Hancock), Jim Skeeles (Lorain), Harold Watters (Champaign), Alan Sundermeier (Wood), Glen Arnold (Putnam), Mark Koenig (Sandusky), Steve Bartels (Butler), Bruce Clevenger (Defiance) 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.