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July 2 to July 8, 2001
C.O.R.N. 2001-20
In This Issue:
A) Scout Soybean Fields For Late-Emerging Weeds
B) Corn Pollination: An Overview
C) European Corn Borer
D) Corn Rootworm Larva
E) Potato Leafhopper In Alfalfa
F) Soybean Aphid
G) Questions And Concerns About Karnal Bunt On Wheat
This is just a reminder to continue scouting of soybean fields for late-emerging weeds. Soybeans in many fields are still small or variable in size due to late planting, replanting, or slow early-season growth. As a result, the soybean canopy may not be sufficiently developed to shade out late-emerging weeds. Timely scouting will allow treatment of weeds when they are still small, which can help keep herbicide rates and costs low. Our research has generally shown excellent weed control when a second postemergence application is applied within 3 weeks after the first application, even if weeds are small.
Pollen shed usually begins two to three days prior to silk emergence and continues for five to eight days with peak shed on the third day. On a typical midsummer Day, the shedding of pollen is in the morning between 9:00 and 11:00 a.m.
The tassel is usually fully emerged and "stretched out" before any pollen is shed. Pollen shed begins at the middle of the central spike of the tassel and spreads out later over the whole tassel with the lower branches last to shed pollen.
Pollen grains are borne in anthers, each of which contains a large number of pollen grains. The anthers open and the pollen grains pour out in early to mid morning after dew has dried off the tassels. Pollen is light and is often carried considerable distances by the wind. However, most of it settles within 20 to 50 feet.
Pollen shed is not a continuous process. It stops when the tassel is too wet or too dry and begins again when temperature conditions are favorable. Pollen stands little chance of being washed off the silks during a rain storm as little to none is shed when the tassel is wet. Also, silks are covered with fine, sticky hairs which serve to catch and anchor pollen grains
Under favorable conditions, pollen grain remains viable for only 18 to 24 hours. However, the pollen grain starts growth of the pollen tube down the silk channel within minutes of coming in contact with a silk and the pollen tube grows the length of the silk and enters the female flower (ovule) in 12 to 28 hours.
A well-developed ear shoot should have 750 to 1,000 ovules (potential kernels) each producing a silk. The silks from near the base of the ear emerge first and those from the tip appear last. Under good conditions, all silks will emerge and be ready for pollination within 3 to 5 days and this usually provides adequate time for all silks to be pollinated before pollen shed ceases.
Pollen of a given plant rarely fertilizes the silks of the same plant. Under field conditions 97% or more of the kernels produced by each plant are pollinated by other plants in the field.
The amount of pollen is rarely a cause of poor kernel set. Each tassel contains from 2 to 5 million pollen grains which translates to 2,000 to 5,000 pollen grains produced for each silk of the ear shoot. Shortages of pollen are usually only a problem under conditions of extreme heat and drought. Poor seed set is more often associated with poor timing of pollen shed with silk emergence (silks emerging after pollen shed).
Inspection of whorl stage corn at the Western Branch, OARDC, last Friday found 2% of the plants showing evidence of European corn borer (ECB) feeding damage. Whereas, inspection of corn at the same growth stages at the Northwestern Branch, OARDC, on Friday found 23% of the plants showing ECB feeding damage.
The larvae in both cases were small (2nd instar) and feeding in the whorl with
windowpane feeding evident. The larvae had not burrowed into the leaf mid-ribs
or stalks of the plants. ECB adults were also evident in the corn at both locations.
The percent feeding damage was still below the 50% economic threshold but it
points out that some early-planted corn fields in
Ohio may have economic levels of ECB and need to be checked.
More information about ECB can be found in a fact sheet on the web at the following location: http://www.ag.ohio-state.edu/~ohioline/icm-fact/fc-15.html.
Inspection of early May planted corn in central Ohio last week found late instar corn rootworm larvae feeding on the corn roots. The larvae were feeding on the corn roots chewing them back to the main part of the plant. The next couple of weeks will be a good to time to inspect corn roots for rootworm damage before the plants have an opportunity to rejuvenate the root system.
Both adults and nymphs of the potato leafhopper (PLH)
can be found on 2nd cutting alfalfa in central Ohio. Sweeps in untreated second
cutting alfalfa in central Ohio last week recorded up to 10 adults and nymphs
per sweep on 10-inch alfalfa. The untreated areas were stunted and yellowing
as compared to the treated areas in the field. Treatment thresholds for PLH
is based on
the number of leafhoppers found per 10 sweeps versus the height of the plants.
More information about PLH can be found on the WEB at:
http://www.ag.ohio-state.edu/~ohioline/ent-fact/0033.html
Reports have been received from most northern states that soybean aphids are being found in soybean fields. We have not found the aphid in Ohio at this time. However, because other states have found it, we need to start examining Ohio soybean fields for the aphid's presence. A generally accepted way to do this is to visit the edge of a field, and closely examine about 25-30 plants. You should count the total number of aphids for all plants examined, and then get an average per plant. You should spend no more than 15-20 minutes in a field. Make sure you do not confuse the aphid with small potato leafhopper nymphs because the aphid is about the same size as a small leafhopper nymph. The use of a good hand lens will allow you to tell the difference. There are some reports that fields with aphids will often have numerous lady beetles, which can be used to indicate fields with possible aphid populations. If you find any aphids, please contact Ron Hammond at 330-263-3727 or hammond.5@osu.edu. At this time, most states are taking a wait and see attitude in terms of how large the populations will get, and how they should be dealt with. A larger article will be placed in this newsletter next week on the soybean aphid, along with an up-date on the situation.
Karnal bunt is a disease that can affect all classes of wheat. It is caused by the fungus Tilletia indica. Fungal spores are produced in the spring and are spread to the heads of wheat where they germinate and infect developing kernels. Infected portions of the kernels are replaced by masses of dark, powdery , fishy-smelling, fungal spores (teliospores). Karnal bunt is difficult to detect in the field because only a few kernels per head are affected. It is easiest to detect in harvested grain where diseased kernels have masses of black spores along the crease and at the embryo end of the kernel. Yield losses from Karnal bunt are minor, but grain quality may be affected. The bunt imparts an unpleasant odor to the grain, much like Common Bunt (Stinking smut) which makes it unfit for use in food items. Regardless, studies have shown that infected grain posses no health risk to humans or animals. Karnal bunt was initially reported in 1931 near the city of Karnal, India from which it gets its name.
Karnal bunt has been known to occur in Mexico, but in 1996 it was reported in southern California and Arizona. It was apparently transported to these areas on infested seed. In 2001, Karnal bunt was detected in two areas of Texas. All of these areas are now under quarantine by the USDA. This means that shipment of grain and grain harvesting or cleaning equipment from those areas are closely monitored. Karnal bunt is significant to wheat producers because a number of major wheat buying countries have import restrictions on grain and quarantines could have a severe effect on grain export trade from affected areas.
Why quarantines? Karnal bunt is known to occur in only a few countries, so international quarantines limit spread of the disease. Spread of Karnal bunt is primarily by contaminated seed, but the fungus can live up to 5 years in soil. It is the soil-borne nature of this disease that makes it difficult to control once an area becomes infested. Thus, soil borne spores can germinate to produce air borne spores that infect the wheat heads directly, thus circumventing control provided by seed treatments that work on the fungus present in the seed only.
Since the discovery of Karnal bunt in the US in 1996, the USDA has been working
closely with departments of agriculture in various states. The Ohio Department
of Agriculture has conducted surveys for Karnal bunt and has never detected
the fungus in Ohio. The cooperation of growers, grain handlers, milling industry
and grain processors is needed to keep this problem from reaching Ohio and to
protect our export grain markets.
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: David Jones 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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