CORN
Crop Observation and Recommendation Network

June 23-29, 2003
C.O.R.N. 2003-19

In This Issue:

A) Wheat Head Scab Reports in Ohio
B) Septoria Leaf Blotch and Stagonospora Leaf & Glume Blotch on Wheat
C) Ear Development in Corn
D) Flooded or Rotten - What is Happening to These Soybean Roots?
E) Corn Insect Update
F) Soybean Aphid
G) Current Weed Control Issues

A) Wheat Head Scab Reports in Ohio - Lipps and Mills CORN Questions

As part of our work to validate the wheat head scab risk prediction system in Ohio we have asked extension agents to survey fields in their counties. The agents report the incidence (percentage of heads with symptoms) from 10 locations in each field surveyed. So far we have had reports from nine counties, mostly from the southern part of the state. Six of the nine counties have reported an average incidence of 5% or less indicating that scab is at low levels in these locations. Three counties have reported an average incidence of from 13% to 20%. It is important to remember this data reported by the agents is disease incidence and not severity.

Severity is rated as the percentage of spikelets diseased per head. Severity levels are always less than incidence. For example if you had a sample of 10 wheat heads from one field and all had one to two spikelets with disease, then each head would have about 10% severity. The average severity would be 10%, but the incidence would be 100% since all heads had some disease. The county data indicates that some fields may have levels of scab that will affect yield and possibly be contaminated with significant levels of DON (vomitoxin). As we receive more reports from counties we will have a better picture of the scab situation in the state. It may be a bit too late for growers in southern Ohio to evaluate fields for scab due to the rapid ripening of the crop.

Now is the time for growers in central and northern Ohio to evaluate their fields. Collect 50 to 100 heads at several locations throughout the field and estimate the percentage of disease on each head. Calculate the average severity for each field. Visit our web site: www.oardc.ohio-state.edu/ohiofieldcropdisease for pictures and a severity rating scale to help in disease evaluations.

Fusarium Head Scab

B) Septoria Leaf Blotch and Stagonospora Leaf & Glume Blotch on Wheat - Lipps CORN Questions

If you have visited wheat fields in the southern two thirds of Ohio in the past week you will know that the wheat condition has changed dramatically. Two leaf diseases have been present on wheat this spring and now that the wheat is beginning to mature the effects are quite visible. Septroia tritici leaf blotch was common in many locations in southern and central Ohio from mid May to the present. The cool wet weather caused it to develop to levels we usually do not see in most of Ohio, but in most cases the flag leaves were not affected. Septoria tritici is a cool weather pathogen that is spread by rain splash and usually in Ohio it rarely infects the top two leaves of the plants.

Stagonospora leaf blotch is similar to the Septroia leaf blotch, but it is favored by wet warm weather. All the precipitation we had this spring helped both of these pathogens spread in the fields. Additionally, as plants mature they become more susceptible to Stagonsopora. Thus, the flag leaves are rapidly dying in fields, but in most cases the wheat is in later phases of grain fill (soft dough or later) and flag leaf death will likely have little impact on yield. More importantly at this stage of development is Stagonospora glume blotch. The lesions on the flag leaves are producing spores that are being rain splashed to the heads. If glume blotch occurs by soft dough the fungus may have sufficient time to colonize most of the glumes and invade the seed. This could result in lowered test weights and infected seed. Growers should look at fields to see if glume blotch is present, especially on susceptible varieties. Glume blotch causes brown to tan lesions on the glumes of the head and the tips of the glumes are usually infected first (show the darkening first). Do not confuse this with head scab, which kills and bleaches out the glumes of infected spikelets on the head.

	Septoria tritici leaf blotch and Stagonospora nodorum leaf blotch
	Stagonospora nodorum glume blotch

C) Ear Development in Corn - Thomison CORN Questions

There is considerable variability in corn growth and development across the state which can generally be related to planting date. During the rapid phase of corn vegetative growth, now evident in the early-planted corn, ear yield components are determined. Kernel row numbers per ear are generally established by about V12 (the 12 leaf collar stage). Kernel row number is generally less affected by environmental conditions than by genetic background. Corn hybrids characterized by "girthy" ears exhibit more kernel rows (e.g. 18 or 20 rows) than hybrids with long tapering ears (e.g. 14 or 16 rows). Determination of kernels per row (ear length) is usually complete about 7 to 10 days before silking (R1) or about the V17 stage. Unlike kernel rows per ear, kernels per row can be strongly influenced by environmental conditions, so severe stress such as a drought occurring the two weeks prior to pollination can reduce kernels per row and ear length.

D) Flooded or Rotten - What is Happening to These Soybean Roots? - Dorrance and Beuerlein CORN Questions

Numerous fields in the state have been at or above saturation levels for weeks. Most recently, some areas of the state received over 10" of rain in a 6 to 8 hour period. We have also gotten a number of samples from other wet fields - the roots look poor and the area at the base of the plant is swollen and cracked. This is typical of beans that are sitting in very wet soils. They take up so much water that the stem does get cracks in it, but if you take your knives and scrape that tissue, you will find healthy green stems underneath.

Flooding injury is a bit more difficult - one of the tell tale signs is the "smell" when you walk into the fields. Smells like dying plants. When you dig these plants up - the cortex, the outer layer of the root can be pulled off with your fingers. What is left is the root stele - it looks like a very skinny rat tail. From research funded by Ohio Soybean Council at OSU & USDA Soil Drainage Unit in Columbus by VanToai and associates, flooding of soybeans at the V2 to V3 growth stages for 3 days had a 20% reduction in yield - when fields were flooded for 6 days there was a 93% yield loss. Another factor - 2 of the locations in this study remained covered with silt - which added to the severity of the loss while locations which had a later rain that washed off the silt - were able to recover. How much flooding injury occurs is dependent on the length of time the water remains in the field.

What's next. From work in my lab, we bubbled carbon-dioxide into pots following inoculations with P. sojae, if your resistance genes to Phytophthora are effective and if your plants are growing so the field resistance has kicked in, they will be able to limit Phytophthora root rot development. However, if your resistance genes do not work against the Phytophthora in a field then you will see more plants die to Phytophthora root and stem rot. The key is to cut the plants open and you will begin to see a brown discoloration of the lower stem - this will begin about 7 to 10 days after a heavy rain. Another difference is the pattern. This will occur on scattered plants throughout an area, not every plant will be killed. Where flooding injury has occured - all the plants tend to look uniformly stressed. Warmer temperatures this week will really activate a lot of these pathogens; the stress of warm dry conditions will really show on these plants with poor root systems.

E) Corn Insect Update - Eisley and Hammond CORN Questions

European Corn Borer (ECB) moths continue to fly in central Ohio and some whorl damage is evident. Inspection of V10 stage field corn last week found early "window-pane" feeding damage in the whorl. The number of plants showing ECB feeding was less than 5% and the larva found on the plants were small. Whorl stage corn needs to be inspected this week for ECB eggs and larval feeding damage. Egg masses may be found on the underside of corn foliage if one searches long and hard. If whorl injury (shot holes and window-pane feeding) appear abundant, then about 20 plants should be inspected at 5 locations in the field to determine the proportion of stand exhibiting whorl injury. In the inspection process, a number of whorls should be pulled and opened to determine presence or absence of ECB larvae. When larvae are found, the average number of larvae per plant may be estimated based on the proportion of stand exhibiting whorl injury and the proportion of injured plants actually having larvae present. If the number of larvae found exceeds an average of one or more per plant and the larvae have not yet begun to burrow into the stalks, the infestation may warrant a rescue treatment.

Common Stalk Borer (CSB) larvae were also found damaging field corn in central Ohio last week. The larvae were found on V3 to V4 stage field corn. The whorl of the corn was dead and when it was pulled from the plant the CSB larvae could be found. The larvae were about 1/2 inch in length. The corn was planted in a field that had a lot of giant ragweed.

F) Soybean Aphid - Hammond & Eisley CORN Questions

Soybean aphids were discovered on growth stage V3-V4 soybeans near Wooster, OH, this past week. A few plants were found with small aphid colonies on the upper leaves and stems. These aphids were found by looking for soybeans that had either adult lady beetles on them and were covered with ants (which are "tending" the aphids). That is a way to help determine the presence of aphids. As mentioned in last week's CORN newsletter, this is the time when growers should begin sampling their fields. If you are examining the plant for the presence of single or a few aphids, you need to make sure that the insects are aphids and not small potato leafhopper nymphs. You need to do this with the aid of a hand lens which is how the aphids in Wooster were confirmed. See last week's CORN newsletter for information concerning management of the aphid, and watch future newsletters for up-dates.

G) Current Weed Control Issues - Stachler CORN Questions

1. In fields that have had no weed control yet this spring, most perennial broadleaf weed species and many perennial grass species are currently in the bud to early flower stage. This stage is excellent for long-term control when using glyphosate. To control Canada thistle and quackgrass use at least 0.94 pound acid equivalent (lb ae)/A of glyphosate (28 ounces/A of Roundup WeatherMax or 40 ounces/A of 3.0 lb ae/gallon glyphosate formulations). For most other perennial broadleaf weeds use at least 1.5 lb ae/A of glyphosate (44 ounces/A of Roundup WeatherMax or 64 ounces/A of 3.0 lb ae/gallon glyphosate formulations). If horsenettle, groundcherry species, common and honeyvine milkweed, and a few others are present, consider rates higher than 1.5 lb ae/A.

2. The delay in planting due to the wet weather in SW Ohio, especially where the glyphosate-resistant marestail (horseweed) exists has created extremely large marestail plants (> 2.5 feet tall). In these fields, the best way to control the large marestail is to use two passes of tillage. The next best option is to consider 1.5 lb ae/A of glyphosate or greater and add Classic at 0.75 to 1.0 ounce/A or FirstRate at 0.45 ounce/A before planting. Another option, if you have a good crop insurance plan, is to consider not planting these fields and work them when you get a chance. To ensure good coverage of the weeds, consider 20 to 25 gallons/A of spray volume.

3. To control large (> 2 feet in height) annual weeds where no prior weed control has been done, apply at least 1.5 lb ae/A of glyphosate before planting. To ensure good coverage of the weeds, consider 20 to 25 gallons/A of spray volume. Using 10 to 15 gallons/A may not allow for adequate plant coverage for good control.

4. For Roundup Ready soybean fields with no residual herbicides applied and they were planted before May 20th, the weeds will grow rapidly with these hot temperatures. Make sure you are applying the appropriate glyphosate rate for the weed size and species that are in the fields. The higher the glyphosate rate used, the less likely weeds are to become glyphosate-resistant.

5. In corn fields that still need a postemergence application, check the corn stage before applying herbicides. The hot temperatures this week will cause corn to be more susceptible to dicamba injury. Once corn is greater than 8 inches tall, it can be injured by dicamba. Using the lowest rate possible, drop nozzles, and a surfactant when mixed with other herbicides will help in reducing corn injury from dicamba. Once corn reaches the V6 stage (6 collars), no ALS-inhibiting herbicide should be applied broadcast over the top of corn as significant yield reduction may occur due to the pinched-ear syndrome.

6. Carefully scout any soybean field that had marestail present at the time of the burndown application. Due to the cool and wet weather, marestail continued to germinate until the end of May. Many of these plants are beginning to bolt, so control should be very easy at this time. Use glyphosate at 0.75 lb ae/A on Roundup Ready soybeans, Synchrony STS at 0.5 ounce/A on STS soybeans and add additional Classic to get to 0.75 ounce/A of total Classic, adding MSO and a nitrogen source to the mix, or FirstRate at 0.3 ounce/A on normal soybeans to control these small marestail.

7. According to the atrazine label, fields with applications of atrazine after June 10th should not be rotated to soybeans.

Readers can subscribe electronically to this newsletter by sending an e-mail message to: corn-out-on@postoffice.ag.ohio-state.edu. A successful subscription message will receive by an automatic reply from the listserv. Contact your local Ohio State University Extension Office or e-mail labarge.1@osu.edu if you have problems subscribing.

Past versions of C.O.R.N. can be found on the World Wide Web at: http:/www.ag.ohio-state.edu/~corn/archive/

C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.

Editor: Clark Hutson        Web Editor: Nathan Watermeier

Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.

All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.

TDD # 1 (800) 589-8292 (Ohio only) or (614) 292-1868

  | C.O.R.N. | Newsletter | Archive | Search | Questions? | Ohioline | Publications |