http://www.ag.ohio-state.edu/~corn/

CORN
Crop Observation and Recommendation Network

July 10 - July 16, 2000
C.O.R.N. 2000-21

In This Issue:

A. Late-season Weed Control in Soybeans
B. Does Tillering Hurt Corn Yields?
C. Leaf Diseases on Corn; Potential Yield Loss Risks
D. Wheat Harvest Proceeding, Lower Test Weights, but Little Vomitoxin (Common Rust & Gray Leaf Spot)
E. First Year Corn Rootworm Monitoring

A) Late-Season Weed Control In Soybeans - Mark Loux CORN Questions

Most of the soybeans have been treated at least once with postemergenceherbicides at this point, except some of the last fields to be planted.Soybean stands remain thin and variable in many fields, and we can expect somenew weed emergence in the thin areas where soybeans fail to shade the ground. It is important to recognize that it may be impossible to obtain excellentweed control in areas where soybeans are thin. In past years, most of us haveobserved the weeds at the end of season in areas of fields where crop standsare thin or nonexistent. However, a second postemergence application withinthe next several weeks may also go a long way toward preventing major weedproblems in these areas. There is a great reduction in the emergence of newweeds as we progress from late June through late July. Postemergenceherbicides should probably be applied by mid July when newly emerged weedsare still small, rather than waiting until weeds are observed growing abovethe soybeans. Keep in mind the recrop restrictions and preharvest intervalswhen applying postemergence herbicides this late.

Large giant ragweed plants can be observed in many fields where weed controlis otherwise good. A late postemergence application may not kill the giantragweed, but will often injure them to the point that they do not greatlyinterfere with harvest. Making herbicide recommendations for control of thesegiant ragweed that have escaped earlier preemergence or postemergence ALSherbicide treatments (FirstRate, Synchrony, Classic, Raptor, etc) can bedifficult, since their survival may indicate resistance to ALS inhibitors. Ifan ALS inhibitor was used in the field previously this year or resistance issuspected, we strongly recommend applying a diphenylether (Cobra, Flexstar)rather than an ALS inhibitor (except in Roundup Ready soybeans whereglyphosate is the obvious choice). Cobra has the most flexibility with regardto late applications, since it has a 45 day preharvest interval and no recroprestrictions. The Flexstar label states that application should be madebefore soybeans bloom. If there is little chance that the ragweed areALS-resistant based on herbicide history, than herbicide choices includeFirstRate, Classic, or Cobra. Of these, FirstRate is the most effective onplants that are not ALS-resistant. However, expectations of FirstRateperformance on large ragweed are sometimes too optimistic, and it does notalways provide amazing control. Where the possibility of ALS resistancecannot be determined, we would still recommend either a diphenylether or amixture of Classic or Firstrate with a diphenylether. When trying to controllarge giant ragweed, we suggest labeled rates and use of adjuvants thatoptimize herbicide performance.

B) Does Tillering Hurt Corn Yields? - Peter Thomison CORN Questions

This year I've seen more tillering in corn than normal, and there have beenenquiries about the impact of tillers on crop growth. When farmers seeextensive tillering in their corn hybrids they often express concern that thetillering will have a detrimental effect of crop performance (tillers will"suck" nutrients from the main plant and thereby reduce yields). As a result,tillers are often referred to a "suckers" .However, research has shown thattillers usually have little influence on grain yields and what effects theydo have are generally beneficial.

Tillers are lateral branches that form at below ground nodes. Although tillerbuds form at each below ground node, the number of tillers that develop isdetermined by plant population and spacing, soil fertility, early seasongrowing conditions, and the genetic background of the hybrid. Many hybridswill take advantage of available soil nutrients and moisture by forming oneor more tillers where stands are thin in the row or at the ends of rows. Tillers are most likely to develop when soil fertility and moisture suppliesare ample during the first few weeks of the growing season. They are usuallyvisible by the 6-leaf stage of development. Hybrids with a strong tilleringtrait may form one or more tillers on every plant even at relatively highpopulations if the environment is favorable early in the growing season.

A number of studies have been conducted to determine relationships between tillers and the main plant. Defoliation experiments in the 1930's revealedthat defoliated plants that had tillers yielded nearly twice as much grain asdefoliated plants that had no tillers. These results suggested that there wasa connection between the tiller and the main plant that allowed sugarsproduced in the tiller leaves to be moved to the ears of the main plants.

More recent studies have found that there is little movement of plant sugarsbetween the main plant and tillers before tasselling. However, after silkingand during grain fill, substantial amounts of plant sugars may move fromearless tillers to ears on the main plant. When there are ears on both thetiller and the main plant, little movement of plant sugars occurs. The mainplant and tillers act independently, each receiving sugars from their ownleaves. The nubbin ears, that tillers may produce, therefore have no impacton the ear development of the main plant as was once thought.

If a particular hybrid shows excellent yield potential and also produces extensive tillering under some growing conditions, it should not be avoided.However, excessive tillering may indicate problems with stand density anddistribution. If tillering is associated with row gaps and less than optimalplant populations, these are the conditions which need to be corrected toensure optimal yields rather than selection of the hybrid.

Tillering can also be caused by diseases such as "crazy top" and Stewart'sbacterial wilt (which are also associated with other symptoms). Suchtillering is a disease symptom and not beneficial to plant performance.Severe weather conditions ( i.e. hail, frost, and flooding injury) thatdestroy or damage the growing point can also result in tiller development andnon-productive plants.

C) . Leaf Diseases On Corn; Potential Yield Loss Risks - Pat Lipps CORN Questions

Common rust has been reported to be present in most corn fields throughout thestate at this time. Rust was first detected on the lower leaves of plants insouthern Ohio in mid June. The rust fungus does not survive the winter inOhio, but is spread long distances in wind from the southern corn growingregions of the US. The earlier than normal development of rust may mean thatcertain fields of susceptible hybrids may have yield losses if the diseasecontinues to spread and cause significant damage to the upper leaves of thecorn plants.

Common corn rust is usually detected at some time during most growing seasonsin Ohio. The disease is favored by high humidity, frequent rain showers andmoderate temperatures (65 - 70 F). Weather conditions during June has favoredthe early appearance of the disease and relatively cool temperatures over thepast two weeks has favored rust spread. Rust can be recognized as smallbrick-red pustules scattered over leaf surfaces. In most fields the lowerleaves appear to be more heavily infected than upper leaves. This is probablydue to early infections occurring in mid June on the oldest leaves and therapid appearance of new leaves during the past two weeks that have not yet hadsufficient time for symptom development. As the plants begin to tassel, andno new leaves are produced, you may see rapid spread of rust to the upperleaves if weather conditions remain favorable of disease spread. Severe rustinfection can kill leaves, reducing the photosynthetic area of the plant andultimately affect yield.

Common rust is much more important on seed corn, sweet corn and pop corn thanin field corn, because of the inherent resistance of field corn to thedisease. However, different hybrids of field corn differ in their level ofresistance. Under favorable conditions for disease development, field corncan have significant yield losses. There have been no disease severitythreshold levels developed for estimating yield losses for common rust, or fortiming of fungicide applications.

Tilt (propiconazole, 4.0 fl.oz./A) is labeled for use on field corn throughsilking for control of common rust. However, applications should beginearly when the disease levels are very low (1% leaf area affected) and anadditional applications made every 14 days until the end of silking. Considering the cost of the fungicide plus application will range from $15-18per acre, and the low price of corn, a fungicide application at this time willnot likely be economical except on very susceptible hybrids in danger ofsevere infection. It may be wise to contact your seed corn dealer todetermine the level of susceptibility of the hybrids you have planted. Thenscout the fields planted to the more susceptible hybrids to inspect the levelof leaf disease present. The level of yield loss will be related to how soonafter tasseling the upper leaves become damaged by leaf rust, the severity ofthe disease on these leaves, and how soon rust causes premature death of theleaves.

The most important lesson from this experience is that susceptible hybridsshould probably not be grown again since there are many hybrids withadequately high levels of resistance. Lastly, fields with severe leaf rustshould be monitored late in the season for stalk rot and lodging problems. ------------------------------Gray leaf spot is active in many continuous corn fields with significantlevels of residue. In fields near Wooster, gray leaf spot can be detected onthe fifth leaf of plants in the V10 growth stage. Wet weather has providedconditions necessary for spore development and spread from corn residues. Lesions on leaves are also producing spores. Gray leaf spot will continue todevelop on plants, especially after tasseling is complete. Hot, humidweather will promote continued spread of the disease. Take time now to beginscouting field for gray leaf spot. Target those fields with a history of thedisease or field that have been in continuous, reduced-tillage corn for thepast two or more years. Corn growers planting hybrids with resistance togray leaf spot can begin to evaluate these hybrids for the level of resistanceby comparing the amount of disease in fields planted with resistant hybridswith fields planted with susceptible hybrids. Resistant hybrids should showsignificantly lower levels of disease, especially this early in the season. This year may be a good year to evaluate the level of resistance in the newerhybrids available from various seed corn companies.

D) Wheat Harvest Proceeding, Lower Test Weights, But Little Vomitoxin (Common Rust & Gray Leaf Spot) - Pat Lipps CORN Questions

The wheat harvest is nearly complete in southern Ohio and is proceeding innorthern Ohio as fields dry from rain showers. Most locations are reportinggood test weights from the earliest harvested fields, but if harvest wasdelayed due to rain showers, test weights have been low. Most growers arereporting test weights in the 56-59 lb/ bu range with some test weights as lowas 51 lb/ bu. The rain during the latter part of the grain filling andmaturation period and moderate to high severity of Stagonospsora glume blotchon the heads have contributed to the lower test weights. As we reportedearlier, head scab was quite low in Ohio this year. Results from analysis ofgrain samples have also confirmed that vomitoxin levels are quite low. Ofthose grain samples that had some vomitoxin, the levels have been 0.5 to 1ppmwith a few rare samples reaching close to 2 ppm. Our wheat milling industryshould not have a problem dealing with this level of vomitoxin We are stillwaiting to hear from growers in the wetter areas of northwest Ohio concerningyields and vomitoxin levels.

E) . First Year Corn Rootworm Monitoring - Hal Willson CORN Questions

During the next six weeks, field monitoring of adult corn rootworm activity infirst year corn and soybean fields should be implemented to determine thepresence or absence of first year corn rootworm (FYCRW) activity. Thedetection of root system injury in first year corn may be due to either (1)the presence of the new biotype of Western corn rootworm (WCR) that exhibitsthe behavior of laying eggs in soybean fields which subsequently hatch thefollowing year in first year corn and cause rootworm injury equivalent to thatnormally experienced in continuous corn, or (2) the occurrence of rootworminjury in first year corn by a population of Northern corn rootworm (NCR)which presumably results from the extended diapause of eggs from a year of egglaying in first year corn to an egg hatch two years later following an annualrotation with an alternative crop such as soybeans. The FYCRW problem thathas been moving into Ohio from Indiana is primarily the new biotype of theWCR. But, in surveys conducted during the past two years, we have alsoobserved significant cases of NCR in first year corn.

The potential for FYCRW can be predicted in part by monitoring adult WCRactivity in soybean fields with yellow sticky traps from mid-July to lateAugust. The most common trap used for monitoring adult WCR activity insoybean fields is the unbaited Pherocon AM yellow sticky trap. In the 1998 and1999 Ohio FYCRW surveys, four traps were stationed in each soybean fieldmonitored and serviced on a biweekly schedule from mid-July through lateAugust. Based on studies from Illinois and Indiana, an average collection oftwo or more WCR adult beetles per trap per day should indicate a potentialFYCRW problem for first year corn at a monitoring site. In the 1999 survey,only one site in Henry County actually exhibited an average WCR catch on thetraps exceeding the level of two beetles per day. Two-thirds of sitesmonitored had an average catch of less than 0.20 WCR per trap per day which isless than 10% of the threshold of two beetles per trap per day. Ohio countiesin which the average adult WCR catch per trap per day exceeded 0.30 includedHenry, Van Wert, Mercer, Darke, Defiance, Paulding and Williams counties.

Although the use of the Pherocon AM yellow sticky traps in soybeans iscurrently recognized as the best way to evaluate the potential for FYCRWinjury in first year corn, it should be noted that other methods are availablefor FYCRW detection and assessment. Adult WCR activity in soybeans may bemeasured by sweep net sampling. Collection of more than one WCR beetle persweep on soybeans is rare in the absence of FYCRW A high visual count ofadult WCR per plant on first year corn is rare in the absence of continuouscorn unless FYCRW is present. Detection and confirmation of rootworm injury ateconomic levels in first year corn should be reported and monitored in futuregrowing seasons to establish whether a FYCRW problem is occurring on an annualbasis and preventive use of a soil insecticide on first year corn plantingsmay be warranted.

In summary, rootworm management on corn, first year or continuous corn, shouldbe based on observations of the prior growing season. Such observations mustbe implemented during the months of July and August when root system injurymay be evident and adult WCR or NCR activity can be monitored. A number ofpreventive treatments (old and new) currently exist for preventing rootworminjury, but such treatments can be rather expensive if not warranted.

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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.

Contributors to C.O.R.N. this week include: State Specialists: Peter Thomison(Corn Production), Hal Willson (Entomology), Ron Hammond (Entomology), MarkLoux (Weed Science), Pat Lipps (Plant Pathology), and Erick DeWolf (PlantPathology); District Specialists: Ed Lentz (Agronomy); Extension Agents: Dave Jones (Allen), Barry Ward (Champaign), Steve Prochaska(Crawford), Gary Wilson (Hancock), Howard Siegrist (Licking), Glen Arnold(Putnam), Clark Hutson (Seneca), and Roger Bender (Shelby).

Editor: David A. Jones Web Editor: Tom Rosati

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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.