C.O.R.N. Newsletter 2005-18

Dates Covered: 
June 22, 2005 - June 28, 2005
Greg La Barge

Soybean Aphid and Twospotted Spider Mite Will Need to be Watched

Authors: Ron Hammond, Bruce Eisley

In the last newsletter, we mentioned that soybean aphids were being found in Ohio, especially in the northwest section of the state. During our travels last week, we also found aphids in low numbers in numerous other areas, including Fairfield County and in Knox County in south central Ohio. We have to assume that aphids are probably in most counties in the northern two-thirds of the state. Growers should plan on beginning sampling within the next two weeks as soybeans reach their reproductive stages.

We continue to get a few calls on spider mites in soybeans. The major concern is that some areas of the state are very dry, and little rain is in the forecast. Along with sampling for aphids, growers should also note the presence of twospotted spider mites and take action if populations begin to significantly increase.

Ohio's Corn Replant Problem Revisited

Authors: Patrick Lipps

Although there have been several articles in this newsletter on the corn replant problem in Ohio, we would like to give you an update on the seedling disease issue associated with the damage that occurred to corn planted before the snowy, cold weather that began on April 20. Our previous articles adequately described the weather conditions that lead up to poor plant emergence and minimal stands in many fields. We have continued our investigation of this situation by requesting OSU County Extension Educators and various crop consultants to send us plant samples from fields with stand establishment problems. We requested these samples to determine what fungal pathogens may be associated with the emergence problems. We received plant samples from 40 different fields representing 13 Counties.

Examination of the seedlings revealed that some had rotted seminal roots, including the radical, and sometimes diseased areas were found on both nodal roots and on the mesocotyls. However, some plants in each sample did not have disease symptoms. We attempted to recover several different types of fungi from diseased tissues to determine what pathogens may be present. We made 452 attempts to recover fungi from all the samples sent from the 40 different fields. Our assays indicated that of the 452 attempts, about 16% produced several different Pythium species and about 13% produced Fusarium graminearum. Both Pythium and Fusarium graminearum are known pathogens of corn seedlings. However, the rest of the attempts either did not recover any fungi at all (45%) or produced several different species known to not cause disease on seedlings (mostly saprophytic fungi). Although we are still trying to identify the various Pythium species we were surprised that Pythium species were not as common as expected. It now appears that Pythium was not the major issue in the majority of fields, but some fields were obviously impacted by Phytium seedling blight. Secondly, Fusaium graminearum appears to be just as important as Pythium in these situations. Our results from 2004 seedling blight problems also indicated Fusarium graminarum was a major fungal participant causing damage to roots and mesocotyls.

One of the lessons we learned from the 'Replant' experience was that it is very difficult to determine the exact cause of the problem when Mother Nature provides some very long term and unusual cold, wet weather. Our data indicates that a number of different problems occurred following the initial cold temperature injury to the corn. Apparently, the cold temperature injury was confounded by soil crusting and saturated soil conditions that also favored several different seedling blight problems in the more poorly drained fields. In most fields the chilling injury to the recently planted seed was the major cause of poor emergence, but in some cases damage from soil fungi added to the emergence problem. There is little evidence that fungicide seed treatment products had any major impact on the emergence problem this year, probably due to the extended period of time it took for some of the least damaged plants to emerge. In talking with growers about fungicide seed treatment effects, few could sort out the impact of the seed treatment from variation in cold injury, soil crusting or soil wetness in their fields. Regardless, when the seedlings take from three to five weeks to emerge, fungicide seed treatments are probably of little help. They are generally designed to protect seeds and seedlings during the first 10 to 14 days after planting. Hopefully we all have learned how to better assess damage to corn seedlings so that we can better make replant decisions in the future.

Wheat Head Scab Levels Very Low In Ohio

Authors: Pierce Paul, Dennis Mills, Patrick Lipps

During the past week OSU County Extension Educators have been helping assess the level of head scab in wheat fields in Ohio. To date we have received about a third of the county reports and all indicate that the level of Fusarium Head Scab is very, very low. The reported incidence of head scab has ranged from 0% to 1.2% in individual fields. This is extremely good news for our wheat producers in the state. In addition, most other diseases are also low throughout the state, but there have been a few fields treated for powdery mildew. Assuming the weather stays relatively dry during harvest, we expect to have high quality wheat harvested in the state this year.

Mealybugs Found on Soybean-Do Not Confuse with Aphids

Authors: Bruce Eisley, Ron Hammond

Last week we posted an article about finding mealybugs on soybeans and in the article referenced two images of the mealybugs. You may not have seen the images because they are stored on a server that was down most of last week so you may not have been able to see the images of the mealybugs. Realizing this problem, we are repeating the article from last week and uploading the images to a different server so that you can view these images.

We found a whitish-yellow insect on the underneath side of soybean leaves while checking soybeans for aphids this week in central Ohio. At first glance it appeared to be aphids, but on closer examination with a good hand lens, we discovered the insect is a mealybug. Mealybugs derive their name from the white, waxy, mealy secretions that cover their bodies. They attach a number of house plants and some plants in the field. They damage plants by sucking the juice from the leaves. They are in the same order as aphids but a different family.

The mealybugs we found, http://www.ag.ohio-state.edu:8000/~corn/agcrops/images/soybean/mealy1.JPG were whitish-yellow in color, very slow moving, flatter than aphids and of course did not have “tailpipes” that are commonly found on aphids. Many of the leaves sampled had multiple mealybugs on them http://www.ag.ohio-state.edu:8000/~corn/agcrops/images/soybean/mealy2.jpg. The soybeans were no-tilled in April and the vegetation burned off. Later planted no-till soybeans in the area also had mealybugs on the leaves but in lesser numbers.

We do not know why the mealybugs were on the leaves or if they are doing any damage to the soybeans. The important thing at this time is to not confuse these mealybugs with aphids and this reinforces the importance of having a good hand lens when in the field and making sure of the proper identification of any insect that is found.

Leave The Fungicide In The Jug

Authors: Anne Dorrance

Reports from Kentucky and Tennessee are all negative for soybean rust at this time. Kentucky has a number of spore traps out and these were all negative. All states will begin intensive scouting for soybean rust at the end of this week. It takes at least 10 days under optimum conditions to go from spore to a lesion that produces more spores. On Tuesday of this week, I will turn all the counties in Ohio to white on the USDA website(http://www.usda.gov/soybeanrust/). This way as these counties are scouted – you will be able to track this activity. The sentinel plots for Ohio will be scouted on Wed, Thurs and Friday this week.

Will Dry Weather Conditions Impact Corn Ear Formation?

Authors: Peter Thomison

Much of the state’s corn crop is experiencing drier than normal conditions. Moisture stress during the early vegetative stages of corn development is uncommon in Ohio. Many areas of the state have received less than an inch of rain in June. Despite relatively cool temperatures last week, it was not unusual to see stress related leaf rolling in many cornfields. Based on observations made at the end of last week, corn development across the state ranged from about V5 (the five leaf collar stage) to V8 or slightly beyond. Ear formation is probably well underway in most fields. As early as the V4/V5 stage, ear shoot initiation is completed and the tassel is initiated on the top of the growing point. During the rapid phase of corn vegetative growth (which generally starts by V7), ear yield components are being determined. Kernel row numbers per ear are generally established by about V12.

Will the recent moisture stress impact ear formation and yield potential? It takes fairly severe stress conditions during the early vegetative growth stages to impact kernel row numbers per ear. Kernel row numbers are usually less affected by environmental conditions than by genetic background. Therefore, in most cornfields, it’s unlikely that kernel row numbers have been impacted significantly by recent dry conditions. However, unlike kernel rows per ear, kernels per row can be strongly influenced by environmental conditions. Determination of kernels per row (ear length) is usually complete about one week before silking (R1) or about the V17 stage. Severe drought stress during the two weeks prior to pollination can reduce kernels per row and lead to a reduction in grain yield.

Nitrogen Sidedressing for Corn

Authors: Edwin Lentz, Robert Mullen

Deciding what to do at sidedress can be a bit of a guessing game. When is the best time to sidedress? How much should be applied? Do I need a tank additive? Should I use the presidedress soil nitrate test (PSNT)? If so, how should I adjust my rate? This article will attempt to answer these questions.

There is not an optimum time for sidedressing per se. The optimum time for a given field in a specific year is a function of a great many things (most notably climate and soil nitrogen availability). Physiologically speaking the corn plant does not take up a great amount of N prior to the V6 to V8 (6 to 8 leafs) growth stages. So theoretically, applications delayed to this point should result in little if any yield loss. This may be true the bulk of the time, but if rainfall does not incorporate the fertilizer after application there may be some N loss (by volatilization if it is a urea or UAN source) which can depress yield. Last year at an experiment on the Western Research Station a significant amount of yield was lost (17 bu/acre) by delaying the application of N to V8 compared to V4. A partial explanation is that weather conditions after the V8 application were extremely dry for the next 20 days. This inhibited the ability of the applied N to make its way to the corn plant. There is other research that shows that applications made up to V16 can recover fully. Shooting for the V3 to V8 window of application time is the most reasonable.

When deciding upon a nitrogen rate, keep one important thing in mind – NITROGEN DOES NOT DRIVE YIELD POTENTIAL. In other words more nitrogen does not necessarily translate into more yield, so be reasonable with your yield goal when deciding upon a rate. A prime example is last year, I doubt that many applied more nitrogen than they had historically (in fact, I suspect less was applied due to high prices), and the state had a record average yield. The soil (primarily the organic fraction) was able to make available enough nitrogen to support some rather lofty yields. In short, keep your rates reasonable and follow the Tri-State Recommendations (http://ohioline.osu.edu/e2567/).

Five years of research conducted at the Northwest Research Station has revealed that including a nitrification inhibitor (with anhydrous ammonia) or a urease inhibitor (with UAN) in the tank has not resulted in increased corn yields. The potential is there for these products to provide a benefit, but to date the yield advantage has not been consistently shown.

PSNT and Rate Adjustments
Should you use the PSNT as a tool for making sidedress decisions? It depends upon whether or not you fall apply manure for corn production. We have not seen a PSNT value higher than 15 ppm (parts per million) for fields that do not fall apply manure (or where alfalfa was grown previously), so it is not recommended for non-manured fields. If you do apply manure it is an excellent tool to evaluate whether or not you need additional nitrogen. PSNT values greater than 25-30 ppm are not likely to respond to additional nitrogen fertilizer, so additional nitrogen is probably not warranted. Unfortunately, the PSNT is not calibrated, so nitrogen rates are tied to specific PSNT values. A general recommendation is not to apply more than 70 lb N/acre for PSNT values less than 25 ppm (if manure is applied in the fall).

Foliar Fertilizers for Soybeans

Authors: Robert Mullen, Edwin Lentz

Foliar fertilizers on the surface appear to be a great way to infuse some nutrients directly into the plant to provide a boost. While the concept is attractive, the research to support the concept is extremely inconsistent and yield benefits associated with application of various foliar formulations are tenuous at best. In Iowa, at 48 locations where foliar nutrients (nitrogen, phosphorus, and potassium) were applied to soybeans only 13 total sites showed any yield response (Mallarino and Ul-Haq, 2000; Fluid Fertilizer Journal, Fall 2000). This means that application of foliar nutrients increased yield only 27% of the time with an average yield increase of only 1 bu/acre (but yield increases as high of 10 bu/acre were noted). It should be pointed out that application of higher rates occasionally depressed yield. All applications were made at V5. The potential benefit most likely does not out-weight the cost of the material and application.

Identifying Nutrient Problems, Tissue Testing Takes the Out the Guess Work

Authors: Edwin Lentz, Robert Mullen

Nutrient deficiencies can be difficult to diagnose while field scouting and are often confounded with other soil/management factors. To properly identify the source of the nutrient deficiency, tissue sampling can be a useful tool to help make management decisions. If a nutrient deficiency is suspected, collect a tissue sample from the affected area and a tissue sample from an unaffected area and submit them to a lab for analysis. This can clearly identify if and which nutrient is potentially limiting plant production. Soil samples should also be collected from the same affected and unaffected area to provide additional evidence as to the cause of the deficiency.

Unfortunately, this is not the end of the story. Even though a nutrient is found to be deficient, it may be the result of other soil/management factors. Properly identifying the cause of a leaf deficiency symptom can be a little like playing a forensic investigator (eliminate all possibilities to identify the most likely culprit). Soil sampling every two or three years can help identify potential causes of nutrient deficiency. Soil pH is one of the most telling chemical properties of the soil. It affects the availability of all the micronutrient metals (iron, zinc, manganese) and is a good indicator of potential problems. High pH soils (greater than 7.0) are more likely to be metal deficient, so these soils should be scouted for potential micronutrient metal deficiencies. There is also a greater potential for micronutrient metal deficiencies in soils low in organic matter. This is a prime example of how soil and tissue testing can be used to diagnose plant problems.

Environmental conditions can also contribute to nutrient deficiencies. The best example is cool, wet conditions early in the growing season. Despite the fact that the soil has a high soil test phosphorus level, leaf purpling can occur, but it is due to poor root development not soil phosphorus deficiency. Early-season purpling of plants can also occur if the soil was fallow the previous year. Soil mycorrhiza (fungi that infects plant roots and aids in immobile nutrient uptake) levels can become quite low and until the levels rebound leaf purpling can occur, but again it is not due to soil phosphorus deficiency.

Management factors also need to be considered while field scouting. Knowing what has occurred and when can help identify whether or not the chlorosis (yellowing) that is observed in a field is a nutrient deficiency, herbicide injury, or fertilizer burn.

When field scouting remember your eyes can tell you a lot about what is going on, but it is often difficult to identify the true culprit. If a micronutrient deficiency is suspected, tissue and soil testing can validate what is suspected. Below is a table of what and how many samples should be collected for tissue analysis of various crops.

Table 1. Crop, timing, what to sample, and the number of samples to collect for tissue analysis.

What to sample
of Samples

V6 and earlier
Entire above ground biomass
Prior to Tasseling
Upper most fully developed leaf
Ear Leaf
Prior to initial flowering
Uppermost fully developed leaf
Small Grains
and forage grasses
Prior to initial bloom
Upper leaves
Prior to initial flowering
Top 6 inches

Archive Issue Contributors: 

State Specialists: Pat Lipps & Anne Dorrance (Plant Pathology), Peter Thomison (Corn Production), Mark Loux (Weed Science), Jeff Stachler (Weed Science), Bruce Eisley (IPM) and Ron Hammond (Entomology) Extension Educators: Roger Bender (Shelby), Todd Mangen (Mercer), Steve Foster (Darke), Greg La Barge (Fulton), Glen Arnold (Putnam), Howard Siegrist (Licking), Mark Keonig (Sandusky), Dusty Sonnenberg (Henry), Keith Diedrick (Wayne), Gary Wilson (Hancock), Tammy Dobbels (Logan), Jim Skeeles (Lorain), Harold Watters (Champaign), Ed Lentz (Seneca) 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.