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May 21 to May 28, 2001
C.O.R.N. 2001-14
In This Issue:
A) Lots of Rain May Change Disease Picture in Wheat; Head
Scab Risk Assessment
B) Early Season Soybean Diseases - What to Watchr
C) "Patching In" Poor Corn Stands and Related Problems
D) Flooding and Ponding Damage to Corn
E) Consider Pre-sidedress Nitrate Test for Manured-fields
F) Postemergence Weed Control in Corn
G) Canada Thistle Control in Corn
H) Scout for Armyworm Now
I) Slugs Beginning to Injury Corn and Soybeans
J) New & Revised Soybean Disease Fact Sheets
During the week of May 14 to 20, many counties received rainfall on 4 to 5 days. Up to this time wheat diseases have been limited to only a few fields. With the rainfall may come new problems. Fungi that have overwintered on crop residues become active when the residues become saturated from rain. These fungi will begin to produce spores that will be available to infect the crop. At this time we are becoming concerned about wheat head scab. The southern and central portion of Ohio has wheat in flower at this time. This is prime time for infection by the Fusarium fungus that causes head scab. Although the weather must be wet during the flowering time of the crop for infection to take place, the fungus needs to be present to cause infections. Areas of the state that were extremely dry up to this time probably have little Fusarium spores present in the air to cause infections.
The weather data that we have looked at from Southern Ohio indicates that there was not enough wetness prior to flowering of the crop to activate the fungus, thus spore levels are likely to be quite low in that portion of the state (below I-70). So even a lot of wetness during flowering will likely not cause severe disease problems in this area. As we look at the weather conditions and the crop growth stage in more northern areas, the scab situation changes. Wheat fields between I-70 and St. Rt. 30 will be in flower this week. The rain last week has produced favorable conditions for the development of spores for this area. Continued rain this week will likely favor infection. The wheat in the northwest corner of the state is just beginning head emergence and will flower about 4 days later. The rain last week and rain predicted for this week will favored spore production. Any rain that occurs at the end of this week or over the weekend will favor infection. Cool weather (below 60F) will limit spore development and infection. All we can say now is that the wheat in northern Ohio is in risk of scab development. As time passes we will have more information available.
May 21, 2001 - Head Scab Risk Assessment:
Southern Ohio: low risk
Central Ohio: moderate risk
North west and north central Ohio: high risk
North east Ohio: low risk
This information will be updated as weather information accumulates over the
next weeks.
Fields which have been in continuous soybean production are more prone to a
build-up of these pathogens. Fields that are in continuous soybeans as well
as poor drainage should be scouted first. Fields with poor stands <100,000
plants per acre may require replanting. A seed treatment should be added to
the seed for these fields. If Phytophthora sojae is diagnosed as the culprit,
changing to a variety with a different resistance package may be necessary.
In these cases, choose a gene package that includes high levels of partial resistance
in addition to Rps genes 1c, 1k or 3a. Varieties with partial resistance alone
needs a seed treatment to protect seedlings until they become actively growing
plants, even on replanted beans. For more detailed information on all of these
soybean seedling diseases see: http://www.oardc.ohio-state.edu/ohiofieldcropdisease/
Corn growers are often confronted with corn stands that
have been reduced in limited areas of a field, or plant stands with uneven emergence.
Questions often arise as to whether to patch-in these poor stands, replant stands
with poor emergence, or to protect late emerging plants during row cultivation.
The following are some guidelines to consider in these situations based on findings
of Illinois and Wisconsin research (Table 1).
When should you patch-in a poor stand?
Growers will sometimes attempt to plant over or "patch in" a poor
stand rather than kill the existing plants and replant at a full population.
However,"patching in" is generally of limited benefit unless the surviving
plant population is less than one half that of the original. The success of
such an approach is even less likely late in the planting season (i.e. after
June 1). Later planted corn cannot compete effectively with the remnants of
the original plant population for sunlight, water, and nutrients. In these late
planting situations, late emerging plants often function more like weeds, and
contribute little to grain yield.
Should you replant stands with uneven emergence?
Should late emerging plants be protected during row cultivation?
Table 1. How Uneven Emergence Affects Grain Yield. Grain yields are shown as percentages of the maximum yield of 187 bu/A obtained with even emergence of a full stand (26,000 plants/A) with early planting. Yields are averages of studies with two corn hybrids in seven environments in Illinois and Wisconsin.
| Planting Time and
Within Row Pattern of Emergence |
Grain Yield As a % of The Maximum | |
| Full Stand | Early* | 100 |
| Uneven Emergence | Medium* | 95 |
| Late* | 88 | |
| Full Stand | 3/4 Early, 1/4 Medium | 94 |
| w/Uneven Emergence | 1/2 Early, 1/2 Medium | 92 |
| Involving Medium Delays | 1/4 Early, 3/4 Medium | 93 |
| Full Stand | 3/4 Early, 1/4 Late | 90 |
| w/Uneven Emergence |
1/2 Early, 1/2 Late | 80 |
| Involving Long Delays | 1/4 Early, 3/4 Late | 78 |
| Reduced Stand |
3/4 Early, 1/4 Missing | 90 |
| w/Missing Plants |
1/2 Early, 1/2 Missing | 70 |
| 1/4 Early, 3/4 Missing | 49 |
*Early = May 1; Medium = 1 1/2 weeks after early planting; Late = 3 weeks after
early planting.
Source: National Corn Handbook Chapter 36, "Effects of Uneven Seedling
Emergence in Corn"
The recent thunderstorms and heavy downpours that hit parts of Ohio during the past week resulted in localized flooding in some corn fields, and ponding in others. The following are some tips to consider when evaluating possible injury from ponding and flooding.
The extent to which flooding injures corn is determined by several factors including: (1) plant stage of development when flooding occurs, (2) duration of flooding and (3) air/soil temperatures. Prior to the 6-leaf stage (measured by visible leaf collars) or when the growing point is near or below the soil surface, corn can survive only 2 to 4 days of flooded conditions. The oxygen supply in the soil is depleted after about 48 hours in a flooded soil. Without oxygen, the plant cannot perform critical life sustaining functions; e.g. nutrient and water uptake is impaired, root growth is inhibited, etc. If temperatures are warm during flooding (greater than 77 degrees F) plants may not survive 24-hours. Cooler temperatures prolong survival. Once the growing point is above the water level the likelihood for survival improves greatly. Corn in southern Ohio that had not yet reached the 6-leaf stage, had great potential for flooding and ponding injury.
Even if flooding doesn't kill plants outright, it may have a long term negative impact on crop performance. Excess moisture during the early vegetative stages retards corn root development. As a result, plants may be subject to greater injury during a dry summer because root systems are not sufficiently developed to access available subsoil water. Flooding and ponding can also result in losses of nitrogen through denitrification and leaching.
If flooding in corn lasts less than 48 hours, crop injury should be limited. To confirm plant survival, check the color of the growing point. It should be white to cream colored, while a darkening and/or softening usually precedes plant death. Also look for new leaf growth 3 to 5 days after water drains from the field. Sometimes the growing point is killed by bacterial infections during and after flooding, but plant growth continues in the form of non-productive tillers (suckers).
Additional disease problems that become greater risks due to flooding and cool
temperatures are corn smut and crazy top. The fungus that causes crazy top depends
on saturated soil conditions to infect corn seedlings. There is limited hybrid
resistance to these diseases and predicting damage is difficult until later
in the growing season.
Producers that have applied manure to their fields should consider the pre-sidedress nitrate test (PSNT) as a nitrogen management tool for corn production. This test estimates the level of nitrate-nitrogen in a field and is adapted to a sidedress program. Samples should be collected 5 to 10 days prior to time of sidedress. The optimum time for sidedress is between the V4 to V6 growth stage (4th and 6th -collared leaf, respectively). The depth of core samples should be 12 inches and removed from 15 to 20 sites across a uniform area no larger than 20 acres. Samples should be dried, or at least, not sent to laboratories in plastic bags. Lab selection should be based on accuracy and timeliness (receiving samples and reporting results). If the results are in excess of 30 ppm, adequate nitrogen should be available for this year's corn crop. If it is less than 15 ppm, the normal nitrogen rate should be applied. Between 15 and 30 ppm, other factors should be considered before reduction of the normal nitrogen rate.
Research from Ohio State University has shown that in most situations, normal nitrogen rates are required at values less than 30 ppm. The test only measures nitrate-nitrogen. If samples are collected too soon, ammonium nitrogen may have not converted to the nitrate form. Some labs include the ammonium value. This value may be used to confirm the conversion (or lack of conversion) of ammonium-nitrogen to nitrate-nitrogen. However, only use the nitrate value for management decisions. Test results may not be useful if heavy rains or several days of soil saturation occur between date of sampling and reporting of results. In summary, the pre-sidedress nitrogen test has been a useful management tool for manured-fields, and occasionally useful for fields with cover crops. In some cases, it may assist in estimating nitrogen losses from early preplant nitrogen.
Maximum Grass Size for Postemergence Corn Herbicides:
at least 90% control:
Distinct, Hornet + Banvel/Clarity, Hornet + Stinger, Roundup
at least 80% control:
Accent Gold, Hornet, Marksman, Hornet + Basis Gold, Basis Gold + Banvel/Clarity,
Lightning + Banvel/Clarity, Laddok, Buctril+atrazine
around 70% control:
Beacon/Spirit + Banvel/Clarity, Accent + Banvel/Clarity, Liberty
Detection of foliar feeding injury by armyworm on 15 to 20% of a stand should be regarded as an indicator of a potential problem, and the field should be rechecked within a few days to determine whether the impact of defoliation is increasing and a rescue treatment may be warranted. In general, a severe infestation will impact almost 100% of a stand and defoliation of the plants will exceed 50%, stand height is being reduced, and some plants being eaten down to ground level. If defoliation remains less than 50% and the new growth exhibits minimal feeding injury, the stand will likely recover with minimal impact on yield. Rescue treatment in corn may be needed if stand infestation is greater than 50% and larvae are not mature.
Since armyworms are foliar feeders, they are relatively easy to control with most foliar treatments. During the day, armyworm larvae will most likely be found seeking shelter in the whorl or possibly in the ground cover. In general, armyworm larvae will feed first on the lower leafs and then progress to the new growth - especially when corn approaches the early whorl stage. For additional information see the armyworm fact sheet at: http://ohioline.ag.ohio-state.edu/icm-fact/fc-14.html
Wheat fields should also be checked at this time for armyworm. Check several
places in the field and determine the number and size of the worms present.
Rescue treatments is recommended in wheat if counts average 6 or more worms
per row foot and larva are in the early stages of development or if head cutting
is occurring. Treatment of armyworm larvae reaching maturity will achieve limited
results because the period of maximum feeding will have passed.
Growers should closely monitor their fields and take action if feeding becomes heavy and crop growth appears stunted. If treatment is warranted, the primary molluscicide available is Deadline MPs, which should be broadcast at 10 lbs per acre. At this time we do not have sufficient data to recommend some of the sprays, such as 28% nitrogen, that some growers are using. We have received many comments over the years on this approach, both positive and negative in terms of efficacy. We would like to receive information on the effectiveness or lack thereof of these applications. If using such sprays, leaving 3-4 check areas at least 50 by 50 ft would be helpful to ascertain whether these sprays work. Please send any information concerning these applications to Ron Hammond: Hammond.5@osu.edu.
If replanting of either corn or soybeans become necessary because of slug injury,
treatment prior to replanting will become a consideration. Slugs will still
be active and larger, and in all likelihood, capable of greater levels of feeding.
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.
Contributors to C.O.R.N. this week include:Editor: Clark Hutson Web Editor: Tom Rosati
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
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