C.O.R.N. Newsletter 2007-30

Dates Covered: 
September 11, 2007 - September 17, 2007
Editor: 
Andy Kleinschmidt

Wheat Seed Treatment

Authors: Dennis Mills, Pierce Paul

In Ohio seed-borne wheat diseases such as common bunt and loose smut are rarely ever major concerns because growers routinely plant seeds treated with fungicides. Problems with these diseases usually appear in isolated areas where poorly treated, bin run seeds are planted. In addition to common bunt and loose smut, seed treatment is highly recommended to treat winter wheat seed for control of Stagonospora glume blotch (Septoria nodorum) and scab (Fusarium graminearum). The systemic fungicides Dividend XL and Raxil - Thiram, Raxil XT, and Raxil MD have excellent activity against both loose smut, common bunt, and Stagonospora. Depending on the rate used, these products appear to have somewhat different efficacy for control of diseases other than the smuts and bunts. Dividend XL is effective in controlling seed-borne Stagonospora, but it is more effective against seed-borne scab at the higher rate (1.0 fl. oz./cwt) than at the lower rate (0.5 fl. oz./cwt). Products containing Raxil used at the labled rates are effective against seed-borne Stagonospora and have relatively good activity against seed-borne scab.

Most seed treatment products are combinations of fungicides to provide broad spectrum protection for both seed-borne and soil-borne diseases. For those fields where some level of head scab was present, or in seed lots with high Fusarium infection, it is recommended to use a companion fungicide to treat seeds. LSP Flowable Fungicide contains TBZ (thiabendazole) and provides excellent control of seed-borne scab. When head scab has caused damage to a wheat crop for seed production, the seed processor should add TBZ to the treatment slurry to improve control of seed-borne scab. LSP material should be used at the 0.25 fl. oz/cwt when used in combination with another seed treatment product for seed-borne scab control. During wet fall weather conditions, common to Ohio, soil-borne Pythium damping off can cause loss of stands when seed is planted into wet, cold soils. Dividend XL contains Apron XL (mefenoxam) and Raxil XT and Raxil MD contain Allegiance (metalaxyl) to help prevent seed rot and damping off by Pythium. Apron XL, Apron FL and Allegiance only control Pythium damping off, therefore they should never be used alone on wheat.

Product

Active Ingredient

Loose Smut

Common Bunt

Stagonospora nodorum Fusarium Head Scab

Pythium Damping Off

Allegiance

Metalaxyl

N

N

N N

E

Apron XL

Mefenoxam

N

N

N N

E

Dividend XL

Difenoconazol + Mefenoxam

E

E

E G

E

LSP Flowable Fungicide

TBZ

N

G

P G

N

Maxim 4FS

Fludioxonil

N

N

N G

N

Raxil-Thiram

Tebuconazole, Thiram

E

E

E G

F

Raxil MD Tebuconazole, Metalaxyl E E E G E
Raxil XT Tebuconazole, Metalaxyl E E E G E
RTU-Vitavax-Thiram Carboxin, Thiram G G F G F
Vitavax-200 Carboxin, Thiram G G F G F



Efficacy based on labeled rates of active ingredient for each product.
Efficacy scale rating: E=excellent, G=good, F=fair, P=poor, and N=no activity.

Using combinations of fungicides will broaden the effectiveness against several different diseases. Additional information can be obtained from the Ohio Field Crop Disease website at: http://www.oardc.ohio-state.edu/ohiofieldcropdisease/

 

Corn Drydown

Authors: Peter Thomison

Dry, hot weather in recent weeks has accelerated maturation in many corn fields. Corn will normally dry approximately 3/4 to 1% per day during favorable drying weather (sunny and breezy) during the early warmer part of the harvest season from mid September through late September. By early to mid October, dry-down rates will usually drop to 1/2 to 3/4% per day. By late October to early November, field dry down rates will usually drop to 1/4 to 1/2% per day and by mid November, probably 0 to 1/4% per day. By late November, drying rates will be negligible.

Estimating dry down rates can also be considered in terms of growing degree days (GDDs). Generally, it takes 30 GDDs to lower grain moisture each point from 30% down to 25%. Drying from 25 to 20 percent requires about 45 GDDs per point of moisture. In September we average about 10 to15 GDDs per day. In October (as things cool down) the rate drops to 5 10 GDDs per day. However, note that the above estimates are based on generalizations, and it is likely that some hybrids vary from this pattern of drydown.

Some past Ohio research evaluating corn drydown provides insight on effects of weather conditions on grain drying. During a warm, dry fall, grain moisture loss per day ranged from 0.76 to 0.92%. During a cool, wet fall, grain moisture loss per day ranged from 0.32 to 0.35%. Grain moisture losses based on GDDs ranged from 24 to 29 GDDs per percentage point of moisture (i.e., a loss of one percentage point of grain moisture per 24 to 29 GDD) under warm dry fall conditions, whereas under cool wet fall conditions, moisture loss ranged from 20 to 22 GDD. The number of GDDs associated with grain moisture loss was lower under cool, wet conditions than under warm, dry conditions.

Agronomists generally recommend that harvesting corn for dry grain storage should begin at about 24 to 25% grain moisture. Allowing corn to field dry below 20% risks yield losses from stalk lodging, ear rots, and insect feeding damage. Growers this year should be prepared for stalk lodging problems (associated with drought stress) that may slow harvest and contribute to yield losses. Grower need to consider the impact of widespread uneven plant development on corn maturation. Significant variation in grain moisture among plants within a field may be common due to early season uneven corn emergence and development.

Tip Dieback and Zipper Ears in Corn

Authors: Peter Thomison

Drought stress during the 2007 growing season has resulted in a wide range of ear development problems. Of these, unfilled ear tips, i.e. ears of corn with no kernels and/or undeveloped kernels on the last two or more inches of the ear tip, are among the most common. Several factors may cause this problem. The ovules at the tip of the ear are the last to be pollinated, and under certain conditions only a limited amount of pollen may be available to germinate late emerging silks. Pollen shed may be complete before the silks associated with the tip ovules emerge (not uncommon under drought stress). As a result, no kernels form at the ear tip. Severe drought stress may result in slow growth of the silks that prevents them from emerging in time to receive pollen. Uneven plant development within fields may have magnified this problem. Pollen feeding and silk clipping by corn rootworm beetles and Japanese beetles also contribute to pollination problems resulting in poorly filled tips and ears. I’ve observed this insect injury in late planted (late May/early June) corn fields, especially field surrounded by early (late April/early May planted corn). In several fields, the damage has been extensive with many ears showing most cob and only a few scattered kernels.

Incomplete ear fill may also be related to kernel abortion. If plant nutrients (sugars and proteins) are limited during the early stages of kernel development, then kernels at the tip of the ear may abort. Kernels at the tip of the ear are the last to be pollinated and cannot compete as effectively for nutrients as kernels formed earlier. Stress conditions, such as heat and moisture stress, nitrogen deficiency, hail, and foliar disease damage, may cause a shortage of nutrients that lead to kernel abortion. Periods of cloudy weather following pollination, or the mutual shading from very high plant populations can also contribute to kernel abortion. Some agronomists characterize the kernel abortion that occurs at the end of the ear as tip dieback. Kernel abortion may be distinguished from poor pollination of tip kernels by color. Aborted kernels and ovules not fertilized will both appear dried up and shrunken; however aborted kernels often have a slight yellowish color.

Another widely observed ear development problem involves ears with missing kernel rows on the side of the cob away from the stalk that give a zippering look on the ears. The zippering often extends most of the cob’s length. The zippering is due to kernels that are poorly developed and/or ovules that have aborted and/or not pollinated. Affected ears are often associated with corn plants which have experienced drought stress during early grain fill; cobs associated with the zippering are usually smaller than normal and poor tip fill is usually present. Differences in the degree of zippering among hybrids is evident. What’s difficult to explain is why this very distinct "missing row" anomaly occurs on the outside or underside of the ears fairly consistently.

Some of the explanations for zipper ears that I’ve heard include the following: 1) silks attached to the kernels (associated with the missing row) were covered up by other silks and simply did not get pollinated or, more likely, were pollinated late and as a result were more prone to abortion; 2) differential corn rootworm beetle silk clipping and feeding, i.e. beetles are below the ear during daytime hours, preferentially clipping silks of kernels facing downward? 3) differential kernel growth rate on the ear. Under drought stress, silk emergence can be slower than pollen shed. Perhaps silks on the outside or underside of the ear emerge more slowly than those facing the stalk? As a result, they may be pollinated later or emerge after pollen shed is complete. The later pollinated kernels may be outcompeted for limited photosynthates by other kernels which are larger and further along in development, and thus more effective in competing for the limited supply of photosynthates (similar to the problem that occurs with kernel abortion that occurs at the tip of the ear - "tip dieback"). 4) Small, short ear shanks might play role in this problem - if the shanks collapse or pinch (due to drought) perhaps it might impair the vascular tissue conducting nutrients to kernel rows on the outside or underside of the ear.

In studies in which corn plants have been subjected to severe defoliation during the late silk and early blister stages, we’ve observed the resulting ears to show zippering, which suggests that a sudden reduction in photosynthate supply may be a factor. The zippering did not occur when plants were subject to similar defoliation at the milk or dough kernel development stage.
 

Harvest Aids for Corn and Soybeans

Authors: Mark Loux

Large weeds present at the time of crop harvest can slow the rate of crop maturation, slow the rate of harvest, and increase mechanical stress on equipment. There are essentially two ways to reduce the green weed biomass in the field in order to facilitate harvesting: 1) delay harvest until after a hard freeze (less than 25 degrees F for several hours); or 2) apply herbicides to control and dessicate the weeds. Waiting a week or so after the freeze or herbicide application will allow the weeds to dessicate to a greater extent and become more brittle.

Many glyphosate products can be used in a preharvest treatment in corn or soybeans – check labels for specific uses and rates. In corn, apply apply glyphosate at least 7 days before harvest when grain moisture is 35% or less. Corn should be physiologically mature (black layer formed) with maximum kernel fill complete. In soybeans, apply glyphosate after the pods have set and lost all green color, and at least 7 or 14 days before harvest, depending upon the glyphosate product. Most glyphosate product labels recommend avoiding preharvest application to corn or soybeans grown for seed, due to the potential for a reduction in seed germination or vigor.

Gramoxone can be applied as a preharvest treatment in field corn, seed corn, popcorn and soybeans. Apply when corn is mature, or after the black layer has formed at the base of the kernels, and at least 7 days before harvest. In soybeans, apply at least 15 days before harvest when at least 65% of the pods have reached a mature brown color or when seed moisture is 30% or less. Apply Gramoxone Max with crop oil concentrate (1.0 % v/v) or surfactant (0.25 % v/v). Use a spray volume of at least 20 gpa in ground applications.

Other Considerations:
1. The greener the weeds, the more effective the treatments (including a freeze) and greater likelihood of reducing weed seed viability. Herbicides will not necessarily reduce the viability of seeds that have formed by the time of application.

2. In general, herbicides will be most effective when applied under warm, sunny conditions.

3. Glyphosate can control perennial weeds that are in the appropriate growth stage at the time of application.

4. Herbicide treatments and freezing weather will not necessarily force loss of fruit on black nightshade plants.
 

Soybean Rust Update for September

Authors: Anne Dorrance

Drought conditions throughout much of the southern US have kept soybean rust at very low levels this year. The one exception is region of Texas, Oklahoma and Arkansas which has been turning counties steadily positive over the past few weeks. Some of the counties in these states have very low levels of disease – a few pustules on one leaf and others are more extensive – sporadic hotspots in fields.

The spore trapping results from both the Syngenta spore traps and the USDA-ARS screening from rain traps have been interesting. Only 3 positives so far for Ohio, all of these were from areas that were in the drought stricken areas. We have scouted extensively in these areas, which confirms what others have found in previous seasons.

The spores of the soybean rust fungus are not very hardy, they are killed by UV radiation (data from Iowa State University, Penn State University and Florida State University). So far this year, they have moved with the rain storms, but are obviously not viable once they have gotten here to Ohio. There may be some other factors involved in affecting their viability in this long distance transport.

Based on these findings and some spore trapping information, the experimental models are predicting that soybean rust spores have been carried into the more northern states. The inoculum levels are extremely low and thus do not warrant any measures to be taken, even for our double crop soybeans.

The sentinel plot system is still in progress, we will continue to monitor green healthy soybean tissue throughout the fall, mainly at this point to assess these models. The data that we collect will still be good for future years. Please, if you see anything odd in your fields, a pocket that has lost its leaves prematurely – let your county OSU Extension Educator know or send it in directly to me (Dept. of Plant Pathology Selby Hall, OARDC/OSU, 1680 Madison Ave. Wooster, OH 44691).

 

Archive Issue Contributors: 

State Specialists: Ann Dorrance, Pierce Paul and Dennis Mills (Plant Pathology), Peter Thomison (Corn Production), Mark Loux (Weed Science). Extension Educators: Harold Watters (Champaign), Steve Prochaska (Crawford), Glen Arnold (Putnam), Howard Siegrist (Licking), Steve Bartels (Butler), Mark Koenig (Sandusky/Ottawa), Wesley Haun (Logan), Ed Lentz (Seneca), Jonah Johnson (Clark), Mark Koenig (Sandusky/Ottawa), Roger Bender (Shelby), Steve Foster (Darke), and Todd Mangen (Mercer).

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.