C.O.R.N. Newsletter 2010-17

Dates Covered: 
June 15, 2010 - June 22, 2010
Editor: 
Jon Rausch

Mid-season weed management issues

This continues to be a challenging year for weed management for some Ohio growers, due in part to wet weather and delayed planting, and also slow crop development in some areas.  A few things to think about relative to this:
 - Some fields have yet to be planted, and weeds in these fields may be extremely large by now.  Tillage can be an effective method of controlling these weeds, but it must be thorough enough to completely uproot and remove them.  Failure to do so results in a weed that regrows and is not very sensitive to herbicides.  One option to ensure effective control is to treat the field with glyphosate a day or two prior to tillage.
- Most growers will not want to wait 7 days to plant soybeans once the field becomes dry enough, which precludes the use of 2,4-D to help control large weeds.  Best option in this situation, where glyphosate resistance is not an issue, is probably a high rate of glyphosate - 1.5 to 2.2 lbs of glyphosate acid/A (44 to  66 oz of Roundup products; 48 to 72 oz of Touchdown/Traxion/Durango/Duramax; 64 to 96 oz of generic 3 lb/gal glyphosate products).  The addition of Sharpen can improve control of marestail and other weeds, and should definitely be included where glyphosate-resistant marestail occur.  The addition of a chlorimuron-containing product (Canopy/Cloak, etc) or cloransulam-containing product (FirstRate, etc) can improve control of broadleaf weeds also.  We would advise being cautious about adding any herbicides with contact activity on foliage (metribuzin, Valor, Authority) in situations where weeds are large and there is a need to maximize glyphosate activity.
- In POST situations in Roundup Ready crops where weeds are large, apply the highest glyphosate rate allowed for a POST application (1.5 lbs/A in soybeans; 1.1 lbs/A in corn).  It is also possible, depending upon corn size and growth stage, to add another herbicide with effective activity on broadleaf weeds, such as Status, Hornet, Callisto, etc.  Be sure to follow the most restrictive label when applying mixtures, with regard to corn size and stage. 
- Be aware that large Roundup Ready corn is not necessarily completely immune to glyphosate that is applied at too advanced a growth stage.  Most glyphosate products can be applied broadcast to corn that is up to 30 inches tall (although drop nozzles may be necessary for spray tom reach small weeds).  Glyphosate must applied as a directed spray using drop nozzles when corn is 30 to 48 inches tall, and application past 48 inches is not allowed.

HOW LATE CAN N BE APPLIED? Corn utilizes large quantities of N during the grand growth stage. From the 8 leaf stage through tasseling N uptake is 4 to 8 pounds per day. For most corn hybrids N uptake is complete shortly after pollination. So, most of the N should be applied prior to the 10 leaf stage, with any supplemental applications complete by or shortly after tasseling.  Under conditions of severe N deficiency, some response would be expected to low rates of N (30 to 60 pounds) as late as three weeks after pollination.

WHAT IS THE BEST N SOURCE TO USE? Ammonia or N solutions knifed in are preferred in most situations, especially high residue fields. Granular urea can also be applied over the top in clean tilled situations, but those applications can be risky if rainfall does not come shortly after application.  Urea stabilizers (Agrotain) should be considered in high residue situations.  Granular urea broadcast in standing corn will cause some foliar burn when granules fall into the whorl. While it may appear unsightly, little yield decrease normally occurs if the fertilizer is applied prior to the 10-leaf stage.

HOW MUCH N SHOULD BE APPLIED? If the corn has gotten too tall to sidedress by this point (mid to late June), it has probably not been severely stressed and yield potential is still good. An example would be rotation corn after beans which had some starter or 28% applied with herbicides with good green color. Nitrogen rates should approach what was initially planned at the beginning of the season.  Research at Ohio State shows that decreased rates can do well, but do not decrease rates by more than 10-15%. 

CAN I BROADCAST 28-0-0 SOLUTIONS "OVER THE TOP"? Using broadcast applications of 28% N solution to sidedress N will cause some burn to foliar tissue of corn plants.
 
The severity of injury is determined by the plant's stage of growth, the amount of N used and form of N. If the plant growing point is at or below the soil surface (or when plant has six collared leaves or less), the extent of foliar injury caused by burn will usually be negligible if the N rate is kept below 50 lb/acre. Even with higher N rates at later vegetative growth stages the injury from leaf burn is normally not so severe that it outweighs the potential benefits received from the N addition. The degree of plant burning is less with urea granules than with other N products.

Dribbling 28% solution with drop nozzles as a narrow band on the soil surface is an alternative approach that can help reduce foliar burning. Dribbling 28% is also a more efficient use of N than broadcast surface application because it helps reduce N volatilization.  Urea stabilizers may be considered for this application, but tillage is a major deciding factor on whether or not they are necessary.  High residue corn fields may benefit from urea stabilizers, but low residue fields are less likely to benefit.  At this point of the growing season, and the need for plenty of nitrogen by the crop, we do not recommend nitrification inhibitors.  We also rarely see positive responses to nitrification inhibitors when N is sidedressed.

CAN I APPLY N TO EVERY OTHER ROW? Research in Indiana, Illinois and Iowa has all shown that farmers can knife ammonia or N solutions in every other row middle (60 vs. 30 inch spacing) with no reduction in yield. The only caution is that extra attention must be paid, especially in wet conditions, that no knives plug with soil. A plugged knife in 60 inch spacing gives 4 rows with no N and will seriously reduce yields.

In another related study, we examined the effect of the R3 fungicide application on brown spot. Brown spot is caused by the fungus, Septoria glycines, and is usually found in the lower canopy. We have seen the disease present as early as the unifoliate stage and most often contributes to early defoliation of the lower canopy. All of the fungicides we evaluated were effective in reducing the severity of brown spot on the lower canopy. However, there was very little yield gain, on average only 3 bu/A.  Brown spot is truly a minor disease.

The first find of Soybean rust in the US was on June 10th in Texas on the border with Mexico,  It will take many more weeks for this to be found in other states as the weather has not been favorable in the south for spread and development of the disease. Current predictions for the states in the southern US are they won’t begin to detect it for another 4 weeks at the earliest. For Ohio, we are at very low risk for soybean rust to develop in Ohio during 2010 growing season.

The weather conditions over the past few weeks are very similar to last year.  If it stays cool and wet, then white mold will be the next issue to monitor.  One hot dry week close to flowering, where the top 2 inches of soil dries out, will knock this one out of contention as a problem. 

Molluscicide Bait Evaluation When Treating for Slugs Slug damage

Molluscicide Bait Evaluation When Treating for Slugs

Numerous locations in Ohio are still reporting slug problems, especially on soybeans, and fields being treated with molluscicide baits, mainly Deadline MPs.  We would remind growers that there are a few things to remember when evaluating the bait’s performance.  First, you will NOT see any dead slugs following treatment.  The majority of slugs are juvenile gray gardens slugs, which are small enough that when they die, they will not be seen.   If you want to see how much the slugs were decreased in numbers, you need to go out and examine the plants at dusk to see the relatively fewer slugs that should be on the plants, assuming you had observed large populations prior to treatment.  The best evaluation of bait performance comes in examining the new growth that will occur on the plants.  Newer growth, whether young leaves in the whorls of corn, or expanding trifoliates at the tops of the soybeans, should be relatively clean of slug feeding.  However, both of these take a number of days of good crop growth to show up.  Trying to evaluate control within only a few days following treatment is difficult until new growth begins to appear.  Otherwise, you will be only seeing old growth already having been defoliated.

Based on a statewide survey of wheat fields over the past 10 days, the incidence of head scab ranges from about 3 to 61 %. This means that between 4 and 53 heads out of every 100 heads have some scab. Head scab is a disease caused by the fungus Fusarium graminearum or Gibberella zeae (two names for the same fungus). Scab develops best when wet, humid weather occurs when the wheat is flowering. The conditions have been favorable in a number of locations this spring. The disease causes shriveled and lightweight kernels, reducing grain yield and test weight. In addition, the fungus also produces a toxin called vomitoxin that is harmful to humans and livestock. So, the effects of scab can be devastating, lower yields, lower test weights, and toxin contamination = price discounts or complete grain rejection at elevators. For a comparison between Fusarium Head scab and Stagonospora Glume Blotch the two photos.

 At this stage, nothing can be done to control scab, but several approaches can be used to minimize losses.  As you scout fields and make decisions or recommendations, here are a few DOs and DON'Ts:

 DON'Ts

 1- Do not make a decision before you know exactly how much scab is out there.

Incidence is a very good measure of disease, but when using incidence one needs to be carefully not to overestimate or underestimate how serious the problem really is. A wheat head has on average 15 spikelets. If you look at 10 heads and 3 out of the 10 heads have a single spikelet diseased, then the incidence is 30%. If in another field, 3 out of the 10 heads have 5 spikelets diseased, the incidence is still 30%. However, the severity of scab is quite different between the two fields. Severity has a closer relationship to how much yield will be lost as well as potential levels of DON. Although this is not always a straight correlation.  To get a handle on how severe disease is we begin to look closer at the spikelets within each head and how many of those are scabby.

Now if you consider that each spikelet will give you 3 kernels. Each set of 10 heads will give you 450 kernels (10x15x3). In the first case, 9 (3 heads each with 1 scabby spikelet x 3 kernels/spikelet) of the 450 kernels will be scabby (2% of the kernels). In the second case, 45 (3 heads x 5 scabby spikelets x 3 kernels per spikelet) of the 450 kernels will be scabby (10% of the kernels). The second field with 10% of the kernels scabby will likely have more vomitoxin than the first field with 2% scabby kernels, even though the incidence is 30% in both fields. 

2- Do not make a decision based on how the field looks from a distance.

Scabby heads contrast nicely with green leaves, making the field look more scabby than it really is. In addition, we have quite a bit of glume blotch (another disease of the head) this year, and you will not be able to tell the difference between glume blotch and scab from a distance. Not because you know for a fact that your neighbor has scab you should assume that you have it too. Your variety may be more resistant to scab than your neighbor’s or even if the varieties are the same, you and your neighbor's fields may have flowered at different times.   

3- Do not wait until it is too late to scout fields

This is the most important. Fields are turning. Scabby heads and maturing heads both take on a straw color. So if you wait until next week you may think all the heads are scabby or all are healthy because they will all look bleached and straw-colored. 

4- Do not feed grain from fields with scab to livestock before getting it tested for vomitoxin.

Animals, particularly swine, may have serious health problems if fed grain with high levels of vomitoxin.

 5- Do not use straw from fields with scab for hay without getting it tested for vomitoxin.

Yes, straw from field with high levels if scab also becomes contaminated with vomitoxin and may cause the same problems caused by feeding scabby grain.

 6- Do not handle scabby grains without gloves and masks.

 DOs

 1- Do scout field to make sure that you do indeed have scab, and more importantly, to determine how much scab is there.  

Walk fields and examine heads at multiple (30 or more) locations spread out across the field. At every point, count the number of heads with scab out of every 40 heads you examine. Also observe the heads to determine how much of each head is scabby, that is, how many of the spikelets on the head are diseased. Assuming that each head has about 15 spikelets and each spikelet produces about 3 kernels, use the table below to help you estimate what percentage of your grain may be scabby. For example if you examine 40 heads and 3 are scabby, the incidence will be 7.5%. If the scabby heads have an average of 4 scabby spikelets, then the percent scabby kernels will be about 2%.

The incidence of scab will likely be highest where there is more corn/wheat residue on the ground close to source of inoculum. It will be higher where the moisture takes longer to dry off. If an area was impacted check the fields that are not tilled or have trees surrounding the field preventing air flow.  

Table 1. Estimate of average percent scabby kernels based on scab incidence and the assumption that each head has 15 spikelets and each spikelet produces 3 kernels. These are assumptions based on some research data. The results will vary based on the variety, disease severity and the environmental conditions that will occur over the next few weeks. This table is a preliminary estimate and guide to use for when fields should be not harvested. All fields with scab should be tested for DON levels.

Vomitoxin will likely exceed 2 ppm in the red area when the percent scabby kernel is greater than 5% (red). Vomitoxin levels can be deceptively high in some years at low scab levels (blue), especially if rains for several days between flowering and harvest. 

2- Do turn up the air on the combine to blow out scabby kernels.

These kernels are lighter than non-infected or healthy kernels.

 3- Do harvest areas or fields with the most scab first and keep that grain separate from the rest.

 4- Do get grain tested for vomitoxin before feeding.

 5- Do plow under scabby wheat stubble, if you choose to abandon wheat fields with high levels of scab to plant soybean. Scabby wheat on the soil surface means more spores available to infect corn and cause Gibberella ear rot. Yes, the same fungus causes both diseases. Remember last year?  Let's break this cycle.

 6- Do continue to read the C.O.R.N newsletter for more on head scab and vomitoxin.

Disclaimer: Dr. Paul, Field crops disease team and The Ohio State University will not be held liable for the information presented in this table. This is just meant to be a guide. The association among scab incidence, percent scabby kernels, and vomitoxin contamination will vary with variety and weather conditions and as such may be different from that shown here. DO NOT use this information as a substitute for vomitoxin testing. Sending sample to a certified lab is the best way to tell how contaminated the grain is.

Wheat Scab Update June 14, 2010: Initial Reports Indicate Moderate to High Levels of Scab in Some areas of Ohio Flowering dates

Wheat Scab Update June 14, 2010: Initial Reports Indicate Moderate to High Levels of Scab in Some areas of Ohio

Surveys of wheat fields for head scab began in Ohio during the first two weeks of June. The best time to survey for Wheat Scab is during the soft dough stage of wheat development, before the wheat starts turning straw color. This is approximately 18 to 21 days after flowering. Areas that received frequent rainfall and warm weather during flowering are at the greatest risk for scab development and vomitoxin contamination and should be scouted before the wheat dries down to determine if there is a scab problem.

Frequent rainfall accompanied by warm temperatures prompted the Wheat Scab Risk Assessment Tool (http://www.wheatscab.psu.edu) to show moderate risk of scab development across several areas of the state throughout the flowering period. So far 73 fields in 16 counties have been surveyed and the incidence of scab has been found to be moderate to high in 70% of the surveyed fields. The incidence in fields surveyed ranges from 3 to 61%, and overall county averages range from 6 to 41%. Of the fields surveyed, 30% had incidence less than 10 percent, 30% were between 10 and 25 percent, and 40% of the fields had higher than 25 percent incidence. All of the fields with lower than 10 percent incidence were in the southern, southwestern, or northern part of the state. Regions with less than 10% incidence may have escaped disease by flowering before or after the period of greatest risk. The majority of fields with greater than 25 percent incidence are in the west-central and northwestern parts of the state. Snapshots from the risk tool show moderate to high risk for many of these areas for May 21-25 flowering dates.

However, scab is not restricted to these surveyed fields and counties alone. Planting date and varietal resistance may also affect scab development.   It is important to scout fields for scab symptoms before the crop reaches maturity. Natural head color at maturity or diseases such as glume blotch may cause the head to have a “bleached” or discolored appearance but are not associated with vomitoxin.  A visual inspection of about 40 heads at 30 or more locations per field will give you a good idea of scab incidence on your farm.

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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.