CORN
Crop Observation and Recommendation Network

May 5-12, 2003
C.O.R.N. 2003-12

In This Issue:

A) Early Season Flooding and Ponding Injury in Corn
B) Diagnosing Emergence Problems in Corn
C) Marestail Update
D) Wheat Progress and Update
E) Insect Update
F) Soybean Rust - Update

A) Early Season Flooding and Ponding Injury in Corn - Thomison CORN Questions

Recent heavy rains have generated questions about flooding and ponding injury in corn. The following are some tips to consider when evaluating possible damage from water saturated soil conditions.

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 collar stage (as measured by visible leaf collars) or when the growing point is at or below the soil surface, corn can usually 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.

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).

Disease problems that become greater risks due to flooding and cool temperatures include pythium, corn smut, and crazy top. Despite fungicide seed treatments, pythium root rot contributed to serious stand reductions in many corn fields last year. 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 from corn smut and crazy top is difficult until later in the growing season.

B) Diagnosing emergence problems in corn - Thomison CORN Questions

A number of factors contribute to poor emergence in corn. The following is some information adapted from a newsletter article by Dr. Greg Roth, my counterpart at Penn State, that addresses this topic. Diagnosing emergence problems early helps to identify solutions and develop replanting plans. As was discussed in the C.O.R.N. article of 4-21-03, corn can emerge from one to three weeks after planting depending on temperatures and soil conditions. Here's a list of a few common things to look for if you encounter an emergence problem in corn this spring.

1. No seed present. May be due to planter malfunction or bird or rodent damage. The latter often will leave some evidence such as digging or seed or plant parts on the ground.
2. Coleoptile (shoot) unfurled, leafing out underground. Could be due to premature exposure to light in cloddy soil, planting too deep, compaction or soil crusting, extended exposure to acetanilide herbicides under cool wet conditions, combinations of several of these factors, or may be due to extended cool wet conditions alone.
3. Seed with poorly developed radicle (root) or coleoptile. Coleoptile tip brown or yellow. Could be seed rots or seed with low vigor.
4. Seed swelled but not sprouted. Often poor seed-to-soil contact or shallow planting- seed swelled then dried out. Check seed furrow closure in no-till. Seed may also not be viable.
5. Skips associated with discolored and malformed seedlings. May be herbicide damage. Note depth of planting and herbicides applied compared with injury
   symptoms such as twisted roots, club roots, or purple plants.
6. Seeds hollowed out. Seed corn maggot or wireworm. Look for evidence of the pest to confirm. Note the patterns of poor emergence. At times they are associated with a particular row, spray width, hybrid, field or residue that may provide some additional clues to the cause. Often two or more stress factors interact to reduce emergence where the crop would have emerged well with just one present. Also, note the population and the variability of the seed spacing. This information will be valuable in the future.
   

C) Marestail Update - Stachler and Loux CORN Questions

In central Ohio, marestail (horseweed) has begun to bolt (produce a stem)! It is about 1.5 to 3.0 inches tall at this time. With the marestail bolting, chemical weed control becomes more difficult. The usage of 2,4-D in the burndown is still very important despite more variable control after bolting. Mix 2,4-D with glyphosate or paraquat to improve control.

Glyphosate should now be used at a rate of at least 1.125 pounds acid equivalent/A (28 oz/A WeatherMax and 48 oz/A of all other formulations) and preferably mixed with 2,4-D at 1.0 pt/A. If the addition of 2,4-D is not possible, glyphosate should be applied at greater than 1.125 pounds acid equivalent/A, especially when plants are greater than 6-8 inches tall. Paraquat should now be used at the maximum rate and mixed with 2,4-D for maximum marestail control.

If you cannot afford to wait 7 days before planting and/or you have glyphosate-resistant marestail, then one to two tillage passes should control most of the emerged plants at this time.

D) Wheat Progress and Update - P. Lipps CORN Questions

The warmer day-time temperatures last week caused the wheat to grow quickly. At Wooster, the wheat planted soon after the Hessian Fly Safe date advanced from growth stage 6 (first node visible) to growth stage 8 (flag leaf emergence) in one week. The wheat planted three weeks later is just entered growth stage 6. Growers should be aware that the wheat appears shorter than normal so the wheat is physiologically more mature than the plant height would indicate. Reports from southern Ohio indicate that the wheat is in flag leaf emergence, but the wheat in the northwest portion of the state varies from growth stage 6 to some fields having plants with two nodes visible (growth stage 7). There appears to be a large difference in crop development due to planting date. We have not received any reports of disease problems in the state at this time. The cooler night time temperatures have limited powdery mildew development. Additionally, Septoria leaf blotch has been found infrequently on the lower leaves of plants throughout the state. Freezing temperatures that occurred more than a week ago probably have limited spread of Septoria. The overall level of disease in the state is considered to be quite low, but a change to warmer, wetter weather could change the disease situation greatly.

E) Insect Update - Eisley and Hammond CORN Questions

Corn flea beetles (CFB) are being reported from some areas of southern Ohio. Based on the corn flea beetle index that was reported in newsletter 5 (http://corn.osu.edu/archive/2003/mar/03-05.html#linkd) we did not anticipate much activity from the CFB this year because of the cold winter. However, we were concerned that even though it was cold, a lot of beetles were seen last fall going into the winter and survival may have been better than anticipated. Based on this new information, it will be wise to start scouting corn fields as soon as they emerge for CFB and then apply treatments according to population levels that are found. Rescue treatment is warranted if plants begin to wilt and a potential loss in stand appears likely.

Black cutworm (BCW) moths are being captured in pheromone traps in central Ohio at this time. These moths are migrants and are being blown in from the south by the recent storms moving through Ohio. This is the time of year that moths start moving into Ohio and we use pheromone catches to monitor their movement. There isn't any way to determine whether BCW is going to be a problem in Ohio in corn this year or in what fields it might be a problem. However, we do know that these moths will seek out fields with a lot of weeds, especially winter annuals such as chickweed, to lay their eggs. The eggs are laid in the weeds and the tiny larvae feed on the weeds until the weeds are killed by herbicide or tillage at which time the larvae will move onto the corn planted in the fields. With the wet soil conditions we are experiencing, many of the fields in the state will have more weeds in the next couple of weeks and perhaps will be more attractive to the moths.

Our suggestion to deal with BCW is to scout fields as soon as the corn begins to sprout and emerge from the soil. Rescue treatments can then be applied if necessary. If a field is extremely weedy a preventive insecticide treatment at planting might help to prevent serious cutworm injury. However, there isn't any way to determine which weedy field might have a problem before planting. For additional information about BCW and other early season pests of corn, see OSU FactSheet FC-ENT-12 at http://ohioline.osu.edu/ent-fact/0012.html.

There are a number of insecticides that are currently labeled for use on corn against cutworm. Some can be used as either preventive or rescue treatments, while others are labeled only for rescue treatments. Check the label before using. Insecticides labeled for cutworm on corn include:

Ambush* @ 6.4 to 12.8 fl oz/A
Asana* XL @ 5.8 to 9.6 fl oz/A
Baythroid 2* @ 0.8 to 1.6 fl oz/A
Capture* 2EC @ 2.1 to 6.4 fl oz/A
Lorsban* 4E @ 2 to 3 pints/A
Mustang* @ 1.4 to 3.0 fl oz/A
Mustang MAX* @ 1.4 to 3.0 fl oz/A
Penncap-M* 4 pints/A
Pounce* 3.2EC @ 4 to 8 fl oz/A or 25 WP @ 6.4 to 12.8 oz/A
Sevin (several formulations) @ 2 to 3 lb a.i./A
Warrior* 1CS @ 1.92 to 3.20 fl oz/A

Several of the soil insecticides have activity against BCW. Check the label for activity.
* Use is restricted to certified applicators.

F) Soybean Rust Update - Anne Dorrance CORN Questions

As growers we successfully manage rust diseases on both wheat and corn. You probably see some rust in your home gardens on snap beans, holly hocks etc. But you've never seen rust on soybeans. Actually, in OHIO, we don't get very excited about foliar diseases in soybeans to speak of. They rarely cause enough damage to substantially reduce yields. But as you have read in recent farm magazines there is a rust disease on soybean. At this time this disease does not occur in the US.

There are actually two species of the rust fungus, Phakopsora pachyrhizi and P. meibomiae that cause rust on soybeans. P. meibomiae is not as aggressive as P. pachyrhizi therefore if we do find rust in the US it is important to identify which species is here. As with the rust diseases on wheat and corn, soybean rust is spread by wind currents. P. pachyrhizi has moved through 90% of the soybean production region in Brazil over two growing seasons. Whether this rust reaches the US through hurricanes, up through Central America or bioterrorism are all matter of speculation and forecasting at this point. If this rust moves into the States via natural routes, there are already scouts and systems in place in the Southern states for early detection. If the rust becomes established, we can depend on the same pattern of notification that is already utilized for wheat and corn rust. If this enters the US from the north - then it is up to us to detect it early, have it identified correctly and begin management.

What you should look for: in the early stages of rust lesion formation, the symptoms are going to look very similar to several other foliar diseases, bacterial pustule and possibly Septoria brown spot. If you see leaf spots on soybean varieties that you have never seen leaf spots on before, collect the sample and send it to the Clinic. Monitor fields throughout the summer and collect samples if in doubt. Some of the reports from Brazil, indicate that fields can be defoliated in 1 to 2 weeks time.

What to do if you find rust? In the early stages of rust moving into an area there are some extra precautions that you will want to take. If you find rust in a field please take note. Rust spores adhere very easily to clothing and boots. Put on disposable spray suits, change and wash before moving to the next location. The wind and rain will spread the spores much more efficiently than a human. But when a new pathogen enters into a region extra precautions are often warranted.

A protocol has been established by the USDA. If you suspect soybean rust is present in your fields in Ohio, collect samples. Don't be skimpy - 20 to 30 leaves with distinct lesions should be collected. Place them in a plastic bag and immediately ship overnight or deliver them to:

C. Wayne Ellett Plant & Pest Diagnostic Clinic
110 Kottman Hall
2021 Coffey Road ppdc@osu.edu
Columbus, OH 43210
614-292-5006

How will we manage this disease - long term - by planting soybean varieties with high levels of partial resistance. However, there are no soybean varieties currently available at this time. Short term - Fungicides. There are two products currently labeled to manage soybean rust, Quadris and Bravo Weather Stik. Quadris is a the fungicide azoxystrobin. Fungi can become insensitive to this material if it is overused. There is currently an effort underway to have additional fungicides labeled through Section 18. If these are approved, we will have 3 types of chemistries available to manage this disease. There were a number of fungicide trials in South America this past winter which are comparing different fungicides for management of rust. Since rust is not here and we don't have the data from last winter's trials nor do we know if Section 18 requests will be approved, I think it's a bit premature to make recommendations. But we will keep you posted on any progress or developments.

There is a substantial amount of information on the web regarding Soybean Rust. We have a fact sheet written which can be accessed through our website : http://www.oardc.ohio-state.edu/ohiofieldcropdisease and the USDA web site: www.aphis.usda.gov/ppq/ep/soybean_rust/

Finally, if you have any doubts or questions, contact your local county agent or the nearest state specialist. To manage this disease successfully, detection and identification early will be the keys.

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.

Editor: Steve Prochaska        Web Editor: Nathan Watermeier

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