C.O.R.N. Newsletter 2006-23

Dates Covered: 
July 24, 2006 - August 1, 2006
Jim Skeeles

How to Assess Corn Fields for Disease

Authors: Pierce Paul, Dennis Mills, Anne Dorrance

Numerous reports of spraying fungicides on corn have come in. This is surprising in many cases because many of the corn fields look the cleanest they have been in a long time. Common rust in particular is at very, very low levels from Kentucky up through Ohio. Foliar diseases can cause significant yield losses if infection occurs between two weeks before to three to four weeks after tasseling. The earlier the disease starts in the field the greater the yield loss. It is important to keep the upper leaves and husks of the plant as disease-free as possible during the grain filling growth stage. Large yield losses only occur when foliar diseases attack the leaves above the ear leaf before mid-dent growth stage. Sweet corn growers routinely use fungicides, and seed producers growing susceptible inbreds have shown large yield increases by controlling leaf diseases with fungicides.

Fungicides are registered for the control of northern corn leaf blight, southern corn leaf blight, northern leaf spot, common rust and gray leaf spot. Current fungicide recommendations for corn are at http://www.oardc.ohio-state.edu/ohiofieldcropdisease/corn/glsfungicides.htm

To review some guidelines on spraying corn again, all of the following must be considered: the developmental stage of the grain, the amount of disease in the field, potential yield, prevailing weather and the price of grain. The corn price this week is $2.00 to $2.12 per bushel. Many areas of the state had stand establishment problems early in the season, so record yields are not expected in those areas.

To assess the amount of disease in the field, some rough guidelines are:
1)Spray applications should begin before the fungus attacks the ear leaf.
2)Greatest yield responses from fungicides occur when plants become diseased prior to tasseling and the plants are protected through the first four weeks of grain fill.
3)Generally, fungicides should be applied when diseases like gray leaf spot or northern leaf blight are detected on half the leaves below the ear leaf.
4)Most of the corn leaf diseases develop lesions from the base of the plant upward, so if you have clean leaves – then there is no need to spray.

In addition, several fields we looked at last week, do have some lesions, but they are small and most likely are resistant to gray leaf spot. The shape, size and color of gray leaf spot (GLS) lesions may vary from one hybrid to another, depending of their susceptibility to the disease. On highly susceptible hybrids, typical lesions are tan to brown in color, rectangular in shape, and flanked by the veins of the leaf. When fully expanded, individual lesions may be 3 to 4 inches long and 1/16 to 1/8 inch wide, depending on the distance between veins. Lesions usually develop and expand quickly on susceptible hybrids (under warm, humid conditions) and if several infections occur near each other on the same leaf, a broader lesion will result. On highly resistant hybrids, GLS produces small, fleck-like or chlorotic (yellowish) lesions that are irregular in shape. These lesions may eventually enlarge, but generally never reach the size and shape of the lesions seen on highly susceptible hybrids. Depending on the hybrid, chlorotic lesions may or may not become necrotic over time.

Unless hybrid resistance is known beforehand, it is difficult to separate susceptible hybrid from resistant hybrids based on early lesions. Both susceptible and resistant lesion types of GLS, early lesions of northern corn leaf blight, and eye spot first appear as small chlorotic specks on the lower leaves. Over time the susceptible lesions will expand while the resistant lesions will remain small. Microscopic examination to look at the spores produced on these different lesions can sometimes help separate early lesions of gray leaf spot, northern corn leaf blight, and eye spot.

Flooding Injury and Phytophthora Still Developing

Authors: Anne Dorrance

Soils are saturated again in many areas of the state. In many areas, less than an inch of rain can bring the soils back to saturation levels. When soils are saturated for 3 or more days, carbon dioxide builds up and suffocates and kills the roots. The Rhizobium nodules are white and pussy when you split them open. Healthy nodules are pink. Some of the plants are now quite large in size, when the heat hit this last week, it was too much. The plants had no roots and with the heat they curled, turned brown and died. If the temperatures had stayed cool or with a little bit more rain, they would have had time to grow new roots and recover to some extent.

Phytophthora stem rot is also present in fields with susceptible varieties. This is a good year to see if your Rps genes are still effective as well as partial resistance levels. If you are finding stem rot in your fields – the best thing to do is to go back and read what the variety was advertised as. I have found through several years of study, stem rot does not develop in fields with partial resistance level scores of 5 or less. This is using the performance trial scale of 1 to 9 – where 9 is dead. Typically the best score that we give varieties is a 3.0 to a 3.5. Each company has its own scoring system, so take the time to read the fine print. For plants where Phytophthora is the primary issue, not flooding, the Rhizobium nodules will still be healthy.

Will Root Lodged Corn Recover?

Authors: Peter Thomison

Strong winds and heavy rains associated with severe thunderstorms can lodge or knock corn plants over, especially if the nodal root system is not fully developed or has been damaged by rootworm. In some fields, excessive moisture from earlier rains and/or soil compaction have inhibited good nodal root formation and predisposed plants to such wind injury. Strong winds can pull corn roots part way out of the soil; a condition referred to as root lodging. The problem is more pronounced when soil are saturated by heavy rains accompanying winds. If root lodging occurs before mid-grain fill, plants usually recover at least partly by "kneeing up." This results in the characteristic gooseneck bend in the lower stalk with brace roots providing above ground support. If this stalk bending takes place before pollination, there may be little effect on yield. When lodging occurs later in the season some yield decrease due to partial loss of root activity and reduced light interception may occur. If root lodging occurs shortly before or during pollen shed and pollination, it may interfere with effective fertilization thereby reducing kernel set. Hybrids differ significantly in their ability to resist root lodging.

In a University of Wisconsin study, root lodging was simulated by saturating soil with water and manually pushing corn plants over at the base, perpendicular to row direction. Wind damage was simulated at various vegetative stages through silking (V10 to R1). Compared to hand harvested grain yields of control plants, grain yield decreased by 2 to 6%, 5 to 15% and 13 to 31% when the lodging occurred at early (V10-V12), medium (V13-V15) and late (V17-R1) stages. Keep in mind that if another factor such as rootworm or compaction was present, yield losses would probably be greater that those observed in this study.

Orthene 97 Labeled on Soybeans

Authors: Ron Hammond, Bruce Eisley

We have been informed by Valent USA Corporation that Orthene 97 is labeled on soybeans for bean leaf beetle, Mexican bean beetle, green cloverworm, potato leafhopper, grasshopper and soybean aphid. Do not apply Orthene 97 within 14 days of harvest. The current information on the web at http://ohioline.osu.edu/b545/pdf/b545.pdf lists Orthene only labeled for soybean aphid on soybeans. Please update your bulletin with this new information.

Tissue Sampling – A Useful Tool to Identify Nutrient Problems

Authors: Robert Mullen

Soil testing is an extremely useful tool to determine potential crop nutrient issues specifically pH, phosphorus, and potassium problems. Unfortunately, there can still be areas of crop production fields that show nutrient issues possibly related to poor drainage, exposed subsurface soil, different soil textures, varying nutrient levels, etc. The best way to identify what is going on in those areas is to collect a tissue sample during the growing season so that you can diagnose deficiencies and set a course of action to avoid the problem in the future.

When collecting tissue samples, gather the appropriate 15 to 20 leaves from the problem area (see table below) that will be submitted to an analytical lab. To help confirm the problem it is also a good idea to collect plant tissue from an adjacent area of the field does not show the suspected nutrient deficiencies. This allows for a direct comparison and can go a long way to identifying the problem. As supporting evidence, collect a soil sample from each area as well. The more evidence that is collected, the more confident you can be that you are diagnosing the problem correctly. Micronutrient deficiencies, for example, are typically associated with high pH soils or low organic matter levels. Thus only collecting a tissue sample may identify the cause of the visual symptoms, but the underlying cause is only revealed by soil sampling.

Crop Sample prior to or during Plant part Number of samples
Corn Initial silk Ear leaf 15
Soybeans Initial flowering Upper fully developed leaf 15
Alfalfa or forage legumes Initial flowering Top 6 inches 15

Another point to consider when collecting tissue samples to help identify crop nutrient problems, weather is a major contributing factor, and it should be considered when determining a course of action to avoid the problem in the future. Poorly drained soils can cause an abundance of nutrient issues due to the stunted root growth and restricted ability of plants to take up nutrients. So poor areas that show up during an extremely wet growing season may not be the result of a nutrient deficiency (although the problem is revealed to be nutrient related due to tissue test). Similarly, during dry years some nutrient deficiency problems are more prevalent than others (manganese deficiency being a prime example).

One final thought on nutrient tissue testing and nutrient deficiencies, during years with weather extremes (too dry or too wet) tissue testing should be used with an understanding that the findings are going to be strongly influenced by the growing season and nutrient levels can be quite odd. For example, if you collect a tissue sample in an extremely wet year and iron levels come back a little high, iron toxicity is not likely causing poor crop growth. The problem is occurring as a result of the saturated soil conditions and improved iron availability to the crop roots.

CCA College at the Farm Science Review

Authors: Harold Watters

Again this year we want to encourage you to attend the Farm Science Review and to do more than just see large equipment and talk with seed sales representatives. Plan now to participate in a program targeting Certified Crop Advisors (CCA) and field agronomists.

The Ohio State University Agronomic Crops Team in cooperation with Purdue University will be presenting the Certified Crop Adviser program at the Review. Last years attendees noted the interaction with state specialists and hands-on activities as highlights of their Farm Science Review visit. The unique perspective of two state specialists on a topic brings real insight to understanding crop progress and problems. Suggestions from last years attendees were incorporated to make this an even more useful program.

The program:
Resistant Weeds - A creeping problem for Ohio and Indiana
- Jeff Stachler, OSU Extension Weed Scientist
- Bill Johnson, Purdue Extension Weed Scientist

Understanding Corn N rates for Ohio and Indiana
- Robert Mullen, OSU Extension Soil Fertility Specialist
- Jim Camberato, Purdue Extension Fertility Specialist

Soybean Management for Pod Fill
- Jim Beuerlein, OSU Extension Soybean & Small Grain Specialist

Water Holding Capacity for Manure Applications
- Jon Rausch, OSU Extension Ag Engineering
- Frank Gibbs, USDA, NRCS
- Brad Joern, Purdue Extension Agronomist

The program will start at 8AM on Thursday of the Farm Science Review, September 21st, and end at noon followed by a lunch. During lunch you will have time for interaction with other CCAs and a chance to ask questions of the Ohio State University and Purdue University specialists. Following lunch we’ll provide a shuttle to the Field Demonstrations or a trip back to the Exhibitor grounds.

Registration is open now:
For the CCA continuing education credits, meal, a parking pass, a ticket to the Farm Science Review and access to some of Indiana and Ohio’s best state specialists, we will charge $70. Registration is limited to 120, please register now through September 8th; no registrations will be taken after that date.

To register on-line click on the “CCA College” box in the upper right hand corner of the OSU Extension Agronomic Crops Team Website - https://agcrops.osu.edu/ We can also mail or fax a registration form if you wish; for more information call Harold Watters at the Champaign County Extension office 937 484-1526 or email watters.35@osu.edu

Kernel Development Stages

Authors: Peter Thomison

Kernel development has started in many corn fields that were planted in April. Following pollination, kernel development (or grain fill) is the most critical period in the development of the corn plant for the determination of grain yield. Kernel development proceeds through a number of stages which have been characterized by such terms as blister, milk, roasting ear, soft dough, dent, etc. Since these descriptive terms can sometimes be difficult to interpret, alternative systems have been proposed. A staging system widely used by agronomists and crop consultants divides kernel development into six stages, designated numerically as R1, R2, through R6. The table below lists kernel developmental stages in sequence and provides a brief description of each phase.

Kernel Development Stages in Corn

Stage* Description Avg. No. of Days/ Stage Approx. Days from Silking
Silking (R1) Fresh green silks, no visible blisters 4 --
Pre-blister Silks brown, not necessarily dry visible kernel pimples, contain little clear fluid 4 8
Blister (R2) Visible blisters w/abundant fluid 4 12
Early Milk Mostly white kernels w/milky-white fluid, some yellow kernels 4 16
Milk (R3) Mostly yellow kernels w/milky-white fluid, no solids yet (“Roasting Ear” stage) 20 4
Late Milk- Early Dough Solids beginning to form, kernel pasty texture (barely edible) 4 24
Soft Dough (R4) Pasty or semi-solid (not edible), no visible denting 5 28
Late Dough-Early Dent Few kernels beginning to dent, especially near butt of ear 5 33
Dent (R5) Majority of kernels dented or denting 8 38
Late Dent Essentially all kernels dented, milk line may just be visible 17 52
Black Layer (R6) Maximum kernel dry weight, kernel moisture 27-32% 10 62

*R-stages 1 through 6; specific number of days associated with each stage may vary from season
to season, from location to location, and from hybrid to hybrid.

Keep in mind that the values for average number of days per stage and approximate days from silking in the table above are based on timely corn planting (e.g. early May). When corn is planted later (i.e. late May or early June), it generally requires fewer heat units to achieve R6, physiological maturity or “black layer”, and this may affect the number of days per stage and days from silking.

Stress conditions such as drought, high temperatures, nutrient deficiency, disease or insect injury, excessive shade, hail damage, and overpopulation during grain fill may cause complete abortion of kernels toward the ear tip (“tip dieback”). Ear tip kernel abortion occurs when the youngest kernels resulting from the most recent pollination are cut off from nutrient flow because the supply is insufficient to fill all the kernels that have been set. Such kernel abortion is most likely to occur during the first two weeks after pollination (during R2, the blister stage). These same stress factors may also reduce kernel size and weight. Premature plant death resulting from diseases (such as stalk rots) or frost cuts off starch accumulation and results in small, light-weight (low test weight) kernels.

Archive Issue Contributors: 

State Specialists: Ron Hammond and Bruce Eisley (Entomology), Anne Dorrance, Pierce Paul and Dennis Mills (Plant Pathology), Peter Thomison (Corn Production) and Robert Mullen (Soil Fertility) County Educators: Roger Bender (Shelby), Harold Watters (Champaign), Kieth Diedrick (Wayne), Bruce Clevenger (Defiance) and Jim Skeeles (Lorain)

Crop Observation and Recommendation Network

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.