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For Week of May 11-17, 1998
C.O.R.N. 98-9
In This Issue:
A) Forage Harvest Alert
B) Sclerotinia Crown and Stem Rot in Alfalfa
C) When to Switch to Earlier Maturing Corn Hybrids
D) When to Spray Fungicides on Wheat
E) Watch For Head Scab on Wheat
F) Aphids and Barley Yellow Dwarf Virus in Wheat
G) Spindle Streak Mosaic Virus of Wheat
H) Slug Potential in No-Till Fields
I) Yield Benefits From Soil Insecticides
J) Timely Concerns for Herbicide Burndown
K) Specialty Corn and Herbicide Safety
L) ‘Select’ Herbicide Approved for Use in Alfalfa
The optimal time to harvest forages is upon us, depending upon the forage species and where you are in the state. Orchardgrass is heading in central and southern Ohio, which means it is already past prime for high producing animals, such as lactating dairy cattle. Tall fescue is also heading out, and ryegrass is in late boot to very early heading stage. Alfalfa is about ready to harvest as well.
The readiness of the forage crop presents a real dilemma for producers who also have row crops to plant. For dairy farmers, high quality forage is essential, and should be given planting priority over row crops, especially if the row crop to be planted is silage corn.
The optimal time of harvest depends on your forage quality goals. Harvest at the boot stage for high quality orchardgrass, ryegrass, tall fescue, and reed canarygrass. Timothy and bromegrass harvest should be delayed to early heading.
Alfalfa neutral detergent fiber (N.D.F.) concentration can be estimated quickly and easily in the field by measuring the height of the tallest stem along with the stage of the most mature stems. These estimates provide an indicator of the readiness of alfalfa for harvest based on forage quality goals. The optimal fiber content of alfalfa for lactating dairy cows is 40% N.D.F. For central Ohio, this year’s alfalfa samples in early bud stage have ranged between 38-41% N.D.F. Last spring at Columbus, alfalfa did not reach 32% N.D.F. until May 20, so alfalfa development this year is well ahead of 1997. In northern Ohio, this year’s alfalfa samples in vegetative stages have ranged 33-39% N.D.F.
Bottom line - Alfalfa development is well ahead of last year. For high quality forage, harvest should begin as soon as possible in central and southern Ohio. To the north, harvest can begin next week to achieve high quality forage.
Sclerotinia crown and stem rot has been observed in several alfalfa fields in Jackson, Hocking, Crawford, and Geauga counties. These fields were seeded in late summer 1997. Damage has ranged from light to severe, with some stands sufficiently damaged to warrant re-seeding. The mild temperatures and ample rainfall during this past winter and this spring have been favorable for development of this disease.
Stand loss caused by sclerotinia frequently goes unrecognized, and the damage is blamed on 'winter injury' or competition from chickweed and other winter annuals. But frequently, these weeds are just colonizing the open spaces left after sclerotinia has killed the plants. But it is likely that most of the 'winter stand loss' in late summer-seeded alfalfa observed this spring is due at least in part to sclerotinia. Red clover is even more susceptible than alfalfa and 'winter stand loss' in red clover is frequently associated with this pathogen.
Sclerotinia crown and stem rot is a fungal disease that is particularly damaging to summer-seeded alfalfa (and red clover). It is more common in stands established using minimum or no-tillage methods, but can occur even with conventional tillage, especially where clover was present in the recent cropping history. The later the stand becomes established in the summer, the more susceptible it is to infection. Spring seedings are essentially unaffected by sclerotinia.
Airborne spores infect plants in the fall, but the plants show absolutely no visible signs of infection until later in the winter and spring when stems and crowns begin to rot. Dead plants often look like they have melted down and appear 'plastered' flat against the soil. A white, web-like growth often appears on dead and dying plants near the soil surface. Small, round black fungal bodies (sclerotia) can frequently be found attached to dead stems and in dead crowns. This pathogen is different than the sclerotinia species that causes white-mold in soybeans.
Absence of fungal growth or sclerotia does not mean that sclerotinia is not the cause of plant death. For example, in a sample taken from a sclerotinia infested field, only 17% of the dead crowns showed evidence of sclerotinia in the field. In the laboratory, isolations showed that at least 64% of these same plants had been killed by sclerotinia.
There are two options for alfalfa re-establishment in a field severely damaged by sclerotinia:
With fields soggy from rains over the past few weeks, it's likely that some corn in Ohio will not be planted during the recommended time period (mid-April through May 10 according to most guides). Yield losses to delayed planting of 1-1.5 bushel/acre/day can be expected after May 10.
Is there a need to switch from full season to shorter season hybrids due to these weather delays? Probably not-in most situations full season hybrids will perform satisfactorily (i.e., will achieve physiological maturity or "black layer" before a killing frost) even when planted as late as May 20, if not later in some regions of the state.
Several years ago I have conducted a field research study with Dr. Bob Nielsen, corn extension agronomist at Purdue University, to evaluate hybrid response to delayed planting dates. Results of this study indicate that hybrids of varying maturity can "adjust" their growth and development in response to a shortened growing season. A hybrid planted in late May will mature at a faster thermal rate (i.e., require fewer heat units) than the same hybrid planted in late April or early May).
However, there are other factors concerning hybrid maturity that need to be considered. Although a full season hybrid may still have a yield advantage over shorter season hybrids planted in late May, it could have significantly higher grain moisture at maturity than earlier maturing hybrids if it dries down slowly. Moreover, there are many short- to mid-season hybrids with excellent yield potential. Therefore if you think you may end up planting in late May consider the dry down characteristics of your various hybrids. Past years associated with delayed harvest due to wet cool weather have revealed major differences among hybrids in drying rates. Some mid- to full-season hybrids have grain moisture levels at harvest similar to those of short-season hybrids because of rapid dry down rates.
Corn planting in the northern third of the state is most likely to be impacted by more wet weather. If planting delays continue beyond May 20, growers in northeast and north central Ohio should consider corn hybrids that require 100 to 150 fewer GDD’s than the adapted full-season hybrids usually planted. These areas of the state historically accumulate fewer GDD’s during the growing season than other regions of Ohio.
Wheat is now heading in southern Ohio and in boot stage in northern Ohio. At these growth stages, the decision to spray or not to spray wheat for diseases will be made by most growers over the next 7-10 days. Fungicides can be used to control powdery mildew and Stagonospora (Septoria) nodorum leaf and glume blotch. Symptoms of these diseases has been discussed in the previous two C.O.R.N. messages. Powdery mildew is affecting only the more susceptible varieties in the state at this time. The weather conditions most favorable for powdery mildew development include scattered light showers once or twice a week, moderately warm-day and night temperatures (55-75 degrees F), and high relative humidity. Thus, we expect powdery mildew to continue to develop on susceptible varieties over the next two weeks. Temperatures in the 80's or very dry weather will limit the development of powdery mildew.
Stagonospora nodorum can be found on lower leaves in most wheat fields at this time. The disease can be seen as small round to oval dark brown spots, sometimes with a yellow halo. As the temperatures begin to rise over the next few weeks we expect this disease to become much more important. Disease development is favored by 2-3 days of rain each week and temperatures in the 70-82 degrees F range. This disease usually starts relatively slow in early May then appears to develop rapidly after head emergence. Most varieties planted in Ohio are susceptible or moderately susceptible to stagonospora leaf blotch.
Do not confuse stagonospora with wheat spindle streak mosaic or with physiological leaf flecking. Wheat spindle streak mosaic causes light green to yellow streaks or dashes on the upper leaves. Physiological leaf flecking can be seen in some varieties as very small, round, yellow flecks scattered over the leaf surface. This leaf flecking is common in some varieties like Freedom and Dynasty. A good color photograph of leaf flecking can be seen in OSU Extension Bulletin 811, ‘Profitable Wheat Management’, page 46.
The yield potential of wheat fields is excellent this year. Fungicide applications generally provide the greatest economic return on high yield potential fields when the upper two leaves of susceptible varieties are in danger of infection. When the upper two leaves of susceptible varieties are infected at the head emergence stage, yield losses are in the range of 15-25%. The presence of both powdery mildew and stagonospora in fields increases the risk of yield losses. Apply a fungicide when the first lesions can be detected on the second leaf (leaf below the flag leaf). The Special Local Need (24c) registration for Tilt fungicide on wheat in Ohio permits applications of Tilt (4.0 oz/A) through full head emergence (Feekes growth stage 10.5), but not within 40 days of harvest. Bayleton (4.0 oz/A) or Bayleton (2 oz/A) combined with Benlate (4 oz/A) or a Mancozeb type product (2.0 lb/A) can be applied up to 35 days before harvest.
Head scab infection occurs during the flowering period of the wheat plant. Flowering occurs 3-5 days after head emergence. Wheat heads are emerging in southern Ohio now and heads will probably emerge in northern Ohio over the next week. Thus, infection by the scab fungus will occur over the next 7-14 days. Infection occurs when there are two to three consecutive days of precipitation during flowering. Fields of wheat planted no-till into corn stubble are most at risk. We will be watching the number of rain events over this time period to determine the potential for scab in this year’s wheat crop.
We have received numerous questions about aphids in wheat, yield losses and the connection with barley yellow dwarf. Barley yellow dwarf is a viral disease of small grains that is transmitted by aphids. Aphids pick up the virus from infected plants in the southern regions of the U.S. and are blown northward in August and September. The aphids land on newly emerged wheat in September and transmit the virus to the young seedlings. Some aphids are transported from the south to the north in the spring as well, but infections that occur in the spring are not nearly as yield limiting as those that occur in the fall. Plants infected in the fall develop the most serious symptoms which include stunting and upright yellow and red leaf tips. Plants infected in the spring are not stunted and only the upper leaves turn yellow/red.
The winged aphids transmit the virus to the young plants within the first 4-6 hours of arriving in the field. Wingless aphids that develop on plants are capable of transmitting the virus after feeding on an infected plant for 12 30 hours. These wingless forms are quite sedentary. Movement from plant to plant by wingless aphids is by wind currents, but spread is limited . To cause extensive yield loss, many winged aphids must arrive and transmit the virus to most of the plants before it gets too cold for their movements. This only occurs in early to mid September in Ohio. Yield losses from barley yellow dwarf has been controlled in Ohio by planting wheat after the Hessian fly-free date in late September or early October. By planting after this date, the temperatures are too cool for aphid migrations into the area and the newly emerged wheat escapes infection. Cool temperatures also limit the activity of aphids within the field. This year we have observed relatively high levels of barley yellow dwarf in research trials at South Charleston and Hoytville where wheat was planted two weeks before the Hessian fly-free date. In the same trials, barley yellow dwarf virus was absent in plots planted on or after the fly-free date.
Insecticide applications have not been successful for control of barley yellow dwarf because the aphids are able to transmit the virus within hours after they arrive in the field. Thus, by the time you see them it is too late. If you consistently plant before the Hessian fly-free date it may be worthwhile to apply an insecticide at planting as a preventative treatment, but there is no need for aphid control to limit barley yellow dwarf infection after this date.
According to entomologists at Ohio State and Purdue universities, aphids cause little damage to wheat, and their control with insecticides is rarely economic. Of the various aphid species that occur on small grains, only the greenbug can cause economic damage by itself. However, the greenbug has not been seen in Ohio for many years.
We have more reports of wheat spindle streak mosaic virus in fields scattered throughout the state. Please see last week's C.O.R.N. newsletter for more information on this viral disease and a list of varieties known to be susceptible. Wheat varieties that have been reported to have this viral disease this year include: Pioneer 2548, Pioneer 2571, Pioneer 25R57, AGRA GR942, Wakefield, Countrymark 558, Podach and Vigoro V934. Extended cool temperatures have favored symptom development which include thumb-nail size yellow streaks or dashes on the upper leaves. Yield losses to this viral disease will depend on the extent of flag leaf yellowing and if the yellowing becomes so severe that the tissue dies. Wheat producers can expect some remission of symptoms with warmer weather, thus minimizing losses. Fungicide applications will have no effect on this disease.
A number of factors are pointing to a season of above normal slug problems. First, it has been a mild winter which favors over-wintering of both adult and egg stages. Second, spring conditions are both mild and wet. Third, planting has been delayed and crops will be more vulnerable to slug populations when the juveniles develop to a point that they can cause maximum injury.
Ron Hammond (O.A.R.D.C. entomologist & slug expert) has been monitoring a number of field sites in Wayne County and reports that slug activity is well above normal this spring. Above normal slug activity was observed last fall in his monitoring program, and slug activity this spring is ahead of schedule based on observations that they have hatched earlier than normal.
In summary, the potential exists for slug problems in no-till corn and soybeans. Problems may or may not actually develop depending on environmental conditions between now and mid-June. If wet conditions continue, slug problems are very likely; if dry conditions prevail, the potential for slug problems may be eliminated.
Last week I discussed the yield results observed in 18 trials of soil insecticides on continuous corn conducted during the past decade. In these trials, continuous corn planted with a soil insecticide averaged 11 bu/A more yield than that of the untreated plots. In this case, the average treated plot refers to the average of plots treated with Counter, Force and Lorsban (CFL standard), which have been routinely included in all 18 trials. Since I did not discuss differences between specific treatments last week, this article will address some of the differences observed between the products used as standards.
In 13 trials conducted at the O.A.R.D.C. Western Branch station, average yields for Counter, Force and Lorsban treatments were 126.4 bu/A, 128.1 bu/A, and 126.1 bu/A compared to an average yield in the untreated checks of
113.9 bu/A. In five trials conducted at the Northwestern Branch station, average yields for Counter, Force and Lorsban treatments were 107.4 bu/A, 108.2 bu/A, and 107.6 bu/A compared to an average yield in the untreated checks of 101.6 bu/A.
In brief, differences in average yields observed over time among the three standards are marginal. It is very interesting to note the equal yields of the Counter and Lorsban treatments, since the two products often exhibit different levels of efficacy against different soil pests. Although Counter generally achieves better rootworm control, Lorsban achieves better stand protection against cutworm. Over time, the benefits of each product tend to generate equivalent results. However, in the case of Force, which achieves rootworm protection equal to Counter and cutworm protection equal to Lorsban, a bushel or two more per acre may be realized over a period of time.
Currently, our yield data records on the new products of Aztec, Fortress and Regent are fewer than that on the CFL standards. However, I tend to believe that yield results of these new products over time will fall within the range of the CFL standards. In summary, the primary decision regarding soil insecticide use is to determine whether treatment is warranted since most products will generate a beneficial return if a significant risk of pest injury exists. Selection of a specific soil insecticide treatment is dependent on a variety of factors ranging from the pest complex to be targeted to the availability of equipment for application
When using burndown herbicides in fields to be tilled, try to apply herbicide a few days before tillage. The time required is longer for perennial weeds than annuals, in order to allow movement of herbicide to roots and other underground structures. Most burndown herbicides can be applied prior to or after tillage, from a crop safety standpoint. However, do not apply 2,4-D in fields that have yet to be tilled. It is also preferable to apply burndown herbicides a day or so prior to planting, rather than after planting, especially when planting with a drill. If this is not possible, best activity may result from delaying herbicide application for several days after planting to allow weeds to recover. We realize this approach has to be balanced against workload and demands on time.
When planting high oil corn, popcorn, inbreds for seed production, and other specialty corns, be sure to check herbicide labels and with the seed supplier regarding herbicide tolerance. These types of corn can be more sensitive to a number of herbicides, including 2,4-D, dicamba, and many ALS inhibitors. Labels for some products prohibit use on certain specialty corns. Labels for other products may allow use on specialty corns, but they often indicate increased risk of injury and refer the user to the seed company for more information. This type of a statement protects the herbicide manufacturer from damage claims, and places the responsibility on the producer and/or applicator to acquire information on crop tolerance prior to application.
Select herbicide (Valent) has received approval for use in alfalfa. Some details:
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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: Anne Dorrance, Pat Lipps & Landon Rhodes (Plant Pathology), Hal Willson (Entomology), Peter Thomison (Corn Production), Mark Loux (Weed Science); District Specialist: Ed Lentz (Agronomy) Extension; Agents: Steve Bartels (Butler), Joe Beiler (Mercer), Roger Bender (Shelby), Clark Hutson (Seneca), Dave Jones (Allen), Greg Labarge (Fulton), Larry Lotz (Fayette), Max Mohler (Lima Branch), Tom Noyes (Wayne), Steve Prochaska (Crawford), Howard Siegrist (Licking), Barry Ward (Marion), Ray Wells (Ross)
Editor: Ed Lentz; Web Editor: Nathan Watermeier
Information presented above and where tradenames are used, they are supplied with the understanding that nodiscrimination 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.
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