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June 9-15, 2003
C.O.R.N. 2003-17
In This Issue:
A) Poor Soybean Root Health Leads to Yellow Beans
B) Postemergence Considerations for Soybeans
C) Residual Weed Control from Fall Herbicide Applications
D) Grain Sorghum as a Replacement for Late Planted Corn
E) Switch to Bt Corn Borer Hybrids
F) Questions on Corn Seedling Blights
G) Wheat Head Scab Risk Still Low for Most of State
H) Cephalosporium Stripe in Wheat
Soybean roots are suffering throughout the state this year, mainly due to lower than normal soil temperatures and high levels of soil moisture. Cool temperatures and saturated soils promote growth and infection of soybeans from Pythium and inhibit growth and infection of roots by Bradyrhizobium, the nitrogen producing bacteria in soybean inoculation products.
Pythium is a seed and root rotter. This is another soil born pathogen that is related to Phytophthora. They produce swimming spores and infect seeds and small fine roots of soybeans, corn and wheat. The symptoms are very similar to Phytophthora, brown, soft roots. In general, these are wimps as far as other soybean pathogens. They weaken root systems, but if the soybeans can begin to produce a larger root system and the soils become drier. The plants may recover. If the Pythium has gotten the seed, it's a goner, but stands of 80,000 to 100,000 can produce 100% of the yield through soybeans ability to compensate for missing neighbors.
Bradyrhizobium or Rhizobium is not a pathogen, but are living bacteria that
were added to seed at planting to produce the nodules on the roots. These bacteria
need to infect the soybeans roots in the top 2 inches of soil to be the most
efficient in capturing Nitrogen from the air. This is the symbiotic relationship
that fixes Nitrogen from the air and make it available to the soybean plants
in the form of nitrate. The inoculants that were placed on the seed will die
if the soil stays saturated for several days or becomes dry. In addition, these
bacteria are not able to form their association with soybean roots very well
if soil temperatures are below 60 F. The reduced volume of roots in most fields
and slow root growth is also limiting the establishment of nodules this spring.
Because of these factors, the formation of nodules may be reduced in some fields
such that extra nitrogen could be beneficial. This decision is NOT made until
early flowering (first week of July). Adding nitrogen prior to this date will
inhibit the natural nodule establishment process and actually reduce yield.
If soybeans are still pale yellow at the early flower stage, side-dress applications
of 70 pounds of nitrogen per acre may increase yield by up to 8 bushels per
acre. In the meantime, dig a few plants to look for the nodules on the roots.
Bright pink on the inside indicates a happy nodule on hopefully a healthy root
system.
- We have received questions about dandelion and Canada thistle control in non-Roundup Ready soybeans. Best option for dandelion is a high rate of Classic, but expect suppression rather than control in most cases. Flexstar, Basagran, and Galaxy are the most effective options for Canada thistle, but Classic, Synchrony, or FirstRate can also suppress thistle. Combinations of a contact and systemic herbicide may be most effective for immediate thistle knockdown and prevention of regrowth later in the season.
- Another question we received deals with the variation in size among soybean plants in fields, and the small soybean size relative to weed size. The concern is that herbicides may cause unacceptable injury to very small soybean plants. Our response: 1) follow labels with regard to minimum soybean size, and 2) be sure to spray when the weeds are small enough to be effectively controlled even if soybeans are small. Soybeans generally outgrow injury from labeled rates of herbicides. This is a better approach than waiting until weeds are larger and having to use what is sometimes called a "jungle mix" of herbicides just to achieve control.
- Postemergence control of lambsquarters with glyphosate has occasionally been difficult over the past two years, based on the comments of some dealers and producers. We have attributed some of this to lambsquarters' tendency to become generally more tolerant of postemergence herbicides with time when growing under cool, wet conditions. Based on greenhouse research, we have also identified several lambsquarters populations that may have increased tolerance to glyphosate. We are currently attempting to confirm this in field research. Be sure to adjust glyphosate rate for lambsquarters size - consider using 33 oz of WeatherMax or 48 ounces of other glyphosate products when lambsquarters is more than 6 inches tall.
- OSU field research in 2001-02 showed that the time of day of herbicide application could influence herbicide effectiveness. We applied glyphosate, FirstRate, and Flexstar from 6 am to midnight at 3-hour intervals to determine the effect of time on broadleaf weed control (previous research had shown that grass control is not influenced by time of day). While the results varied somewhat with location, year, and herbicide, we observed a general trend for herbicide activity to be reduced at 6 am, 9 pm, and midnight. This was evident for giant ragweed control in all studies where it was present. Herbicide activity was generally unaffected by time between the hours of 9 am and 6 pm. Although application of herbicides in early morning and late evening (and middle of the night with guidance equipment!) can reduce drift problems, we suggest applying between the hours of approximately 8 am and 8 pm to maintain herbicide effectiveness.
Results of 2002-03 OSU research
A number of herbicide treatments can be effective for control of emerged winter annual weeds and dandelions when applied in the fall. Another potential benefit of fall applications is residual control of annual weeds that emerge in the spring or early summer. We have been asked often about the residual control that can be expected from various treatments, but our database has not supported many concrete conclusions. We knew in general the relative persistence of soybean herbicides, and could surmise that when persistence and effectiveness of key weeds are considered, products containing chlorimuron, cloransulam, or imazaquin are likely to provide more effective residual control than less persistent herbicides such as Sencor, Python, or Valor. We had consistent populations of lambsquarters and giant ragweed in several studies where herbicides were applied last fall at Western Branch OARDC, and we were able to get a better idea of residual control of these weeds. We also observed a substantial amount of spring-emerging chickweed, and evaluated herbicide effectiveness on this weed as well. Herbicides were applied November 8, 2002, and control data shown below are from a May 28 rating. Treatments containing just glyphosate or glyphosate plus 2,4-D are not included here because they did not provide any residual control, although they effectively controlled weeds that had emerged by early November. Chickweed control ratings reflect both fall and spring control, so treatments that controlled emerged chickweed in the fall but provided no residual control still ended up controlling about 50% of the total chickweed that was present in May. All treatments included 2,4-D ester and/or glyphosate unless otherwise noted, so these results reflect differences in residual control only. Key: CC = common chickweed; LQ = lambsquarters; GRW = giant ragweed (very high population).
Sencor (8 or 10 oz): CC - 63%; LQ - 3%; GRW - 0%
CanopyXL (2.5, 3.5, or 4.5 oz): CC - 98%; LQ - 100%; GRW - 78%
Python (1 oz): CC - 70%; LQ - 20%; GRW - 13%
FirstRate (0.3 oz): CC - 55%; LQ - 50%; GRW - 63%
Backdraft (5 pts): CC - 58%; LQ - 23%; GRW - 23%
Valor (2 oz): CC - 58%; LQ - 70%; GRW - 6%
Boundary (1.75 pt - without 2,4-D or glyphosate): CC - 37%; LQ - 0%; GRW - 0%
Simazine (1 lb ai): CC - 77%: LQ - 11%; GRW - 16%
Basis (0.33 oz): CC - 77%; LQ - 17%; GRW - 20%
These results are from just one season of research, but nonetheless provide
a good idea of the relative degree of residual control from various herbicides.
We would normally expect there to be more of a response to rate with Canopy
XL - control was similar with rates ranging from 2.5 to 4.5 oz in this study.
The chlorimuron component of Canopy XL or SP tends to be most effective for
residual control under the type of weather and soil conditions we have experienced
this past month or so - frequent rain and continuously wet soils. Over the past
10 or more years, we have observed less response to chlorimuron rate with spring
applications also under this type of weather pattern, especially on tough weeds
such as giant ragweed.
Since, wet weather has caused major delays in planting (or replanting) corn in parts of Ohio, there are questions about substitutes for corn. Grain sorghum may be a possible alternative to corn for some Ohio farmers. Grain sorghum has some virtues such as greater drought tolerance than corn. Dr. Bob Nielsen, my counterpart at Purdue, has established the following web site on grain sorghum management which should also be useful for Ohio growers. This site contains links to a number of other web pages with information on grain sorghum production. http://www.agry.purdue.edu/ext/sorghum/index.html
Substituting grain sorghum for late planted corn is not a practice I normally recommend since we can still plant corn hybrids that should safely mature prior to a fall frost (32 degrees F) until mid June in much of Ohio. In northern Ohio, this may require planting hybrids which are earlier maturing (i.e. in the 95-100 day relative hybrid maturity range) than we normally use. However, in southern Ohio, adapted hybrid maturities (110-112 days) can still be planted through mid-June. For more information on selecting hybrids for late planting, consult Extension Bulletin AY-312-W "Delayed Planting and Hybrid Maturity Decisions" available on-line at: http://ohioline.osu.edu/b902/b902.pdf
The following are some comments Bob Nielsen shared with me several years ago concerning late planted grain sorghum as a replacement for corn, and I think most are still pertinent this year.
1) The use of grain sorghum in northern Indiana (and probably northern Ohio) is limited since the last safe planting date for corn is nearly the same as for sorghum.
2) The supply of sorghum seed for short season hybrids may be limited.
3) Sorghum can attract birds very easily, especially anywhere close to towns or cities and the subsequent damage to the grain can reduce yields very dramatically.
4) For folks who have never grown sorghum before, learning how during a crisis
situation can easily lead to disastrous results.
Corn planted from now on in Ohio might benefit by the
planting a Bt corn borer (BTCB) hybrid. We would normally expect 2nd brood European
corn borer to be more damaging on this later planted corn. OSU entomologists
have compared BTCB hybrids with their isolines for borer damage and yield when
planted on time (late April & early May) and corn planted later (late May
& early June). In 2001 and 2002 when corn was planted in late May at the
Western Branch of OARDC, the average yield with the BTCB hybrids was 7.5 bushels
per acre greater than their isolines. A field in Licking County in 2002 was
not planted until 11 June and resulted in an average yield benefit 11.5 bushels
per acre with the BTCB hybrids. So you might want to check with your seed supplier
and see if a BTCB hybrid might be the way to go at this time. Results from the
corn borer trials in 2001 and 2002 can be found on the web at: http://entomology.osu.edu/ag/reports.htm
The level of seedlings blights in corn this year has prompted many questions about the fungi that cause these diseases and how they operate in the field. The two most common groups of fungi attacking corn in wet cool conditions are Pythium and Fusarium. Pythium is a water mold that requires saturated soil conditions for a given period of time (usually 24 to 48 hours) to initiate disease. Pythium is present in the soil as resting structures usually associated with decomposing residue. There are several different Pythium species that attack corn seeds and roots and each operates a bit differently. Once the Pythium fungus infects the young corn seedling the fungus quickly colonizes the plant tissue (corn seed, seminal roots, nodal roots or mesocotyl), develops resting spores (oospores), and then goes into a state of relative dormancy. When another soil saturation period occurs the fungus again becomes active, attacks plants, colonizes corn roots and goes dormant. Thus, several cycles of corn infection can occur given saturated soil conditions. The corn plant limits Pythium damage by growing and producing new nodal roots.
Fusarium is generally also associated with crop residues. Fusarium can be easily obtained from year old corn, wheat and soybean residues. However, there are several species of Fusarium on these residues. Fusarium is usually spread by splashing rain at the soil surface or when roots of seedlings come in contact with infested crop residues. The fungus grows on most all tissues of seedlings, especially tissues that have been previously damaged by insects, wind, hard rain or hail and by other fungal pathogens (i.e. Pythium). Once Fusarium gets into plant tissue it continues to grow and produce spores until all the tissue is consumed, until the weather conditions become unfavorable for its growth, or until the plant restricts its development by growing more vigorously and producing new roots.
Seed treatments are designed to limit the development of Pythium and Fusarium on the seed and young seedling roots (seminal roots) to protect the plant during the first 10 to 14 days after planting. After this time the plant is on its own. When the weather becomes wet and cool plant development slows and the fungi have an opportunity to 'feed' on the plant. Thus, as soil temperatures increase over this week and the corn begins to grow new roots, surviving plants in affected fields should begin to recover and grow more vigorously. If disease has advanced to the crown region of the plant and many nodal roots are infected, plants will likely not recover. Continue to evaluate plants over the next week or so to make sure fields have adequate stands.
In spite of the wet weather the risk of head scab in wheat remains low for most of the state due to the cool weather that has occurred during the flowering period of most of the crop. The only exception to this is wheat in the south west corner of the state (south of Darke County and west of Pickaway and Ross Counties) where the risk of scab was moderately high for fields that were in flower early (May 11 to May 17). Growers may observe wheat with symptoms of head scab in this area this week. The cool weather after this critical period should limit the extent of disease severity in this area. In central Ohio the risk predictions is very low and some fields may show a few scattered affected heads by late this week or early next week. This level of disease will likely have little or no affect on yield. Wheat in northwest Ohio has just started flowering last week and we are still collecting weather data for scab risk predictions in this area. Regardless, the scab risk level for northwest Ohio has been very low so far and based on weather forecasts we expect it remain relatively low.
We have been receiving samples of wheat from various locations in the state that have been affected by Cephalosporium stripe. The disease is very localized in certain fields but samples have come from northwest, central and eastern Ohio. Symptoms of the disease are very distinct. The leaves have broad yellow stripes that run the length of the leaf. In the yellow stripe is a thin dark-brown streak that is the leaf vein that the fungus has colonized. The yellow streaks show up in stark contrast to the green areas of the leaf.
The leaf symptoms are first detected on the lower leaves and as the disease progresses the upper leaves begin to show symptoms. The fungus infects by way of the roots during early spring. Freezing and thawing causes damage to the plant roots and the fungus enters the damaged roots and travels into the plant via the vascular system. Plants affected by Cephalosporium stripe will die prematurely producing little to no grain. Thus, if a major percentage of the plants in the field are affected severe yield losses can occur. The fungus lives in old wheat residue and on weed grasses. The only control for the disease is to destroy the wheat residues by burying them and rotate to another crop for at least three years. Good grass weed control is also essential. For more information on this disease access our fact sheet at http://ohioline.osu.edu/ac-fact/0009.html.
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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.
State Specialists: Mark Loux, Jeff Stachler and Tony Dobbels (Weed Science), Jim Beuerlein (Soybeans and Small Grain Production), Pat Lipps, Anne Dorrance and Dennis Mills (Plant Pathology), Peter Thomison (Corn Production), Bruce Eisley (IPM) and Ron Hammond (Entomology); District Specialists: Ed Lentz (Agronomy); Extension Agents: Barry Ward (Champaign), Steve Foster (Darke), Gary Wilson (Hancock), Dusty Sonnenberg (Henry), Howard Siegrist (Licking), Harold Watters (Miami), Jim Lopshire (Paulding), Glen Arnold (Putnam), Ray Wells (Ross), Clark Hutson (Seneca) and Roger Bender (Shelby).Editor: Clark Hutson Web Editor: Nathan Watermeier
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