http://www.ag.ohio-state.edu/~corn/ |
|
For Week of July 20-26, 1998
C.O.R.N. 98-19
In This Issue:
A) Leaf Cupping and Puckering Symptoms on Soybeans
B) Examine Fields Now for Phytophthora Stem Rot!: Do You Know if Your
Genes are Working?
C) Corn Recovery from Wind Damage
D) Determining the Success or Failure of Corn Pollination
E) Weed Control in Double Crop Soybeans
F) Control of Weeds in Wheat Stubble
G) Explaining the Presence or Absence of First Year Corn Rootworm
H) Summer Seedings of Forages
I) Late Summer Seeded Forage Legumes at Risk to Sclerotinia Crown and
Stem Rot
J) Post-Flood Syndrome: Interesting Research
As in past years, we have received a number of calls about late- season cupping or wrinkling of soybean leaves. These types of symptoms have been observed around the Midwest over the past five years or so in increasing frequency. A number of factors can cause these symptoms, and it can be difficult to pinpoint the exact cause. Herbicides are often blamed, but are not always the culprit.
One of the first herbicides to get blamed in many fields is Banvel, which may have been applied in a nearby corn field. Spray drift or volatilization of Banvel from a neighboring field can occur, and may be the culprit in some fields. However, many of the affected fields seem to be far enough away from treated corn fields, or Banvel was not used in the area, and this possibility can be ruled out. The most typical symptoms from exposure of soybeans to Banvel or Clarity are puckering of the new leaves that are emerging 7 to 10 days after exposure. This may be accompanied by stunting of the plant. Soybeans may show these symptoms on several trifoliates, and then recover completely. Research indicates that soybean yield is not generally reduced when minor symptoms occur, and yield loss is more likely if soybeans are in the reproductive stage at the time of exposure (although still unlikely unless symptoms are severe).
We have heard reports of and observed fields where leaf puckering or cupping was uniform over the entire field. Other fields have shown symptoms only in some areas. In OSU research plots, we have occasionally observed puckering in Roundup Ready soybeans following application of Roundup. Spider mites and leafhopper have been known to cause cupping and wrinkling of soybean leaves. Many of the fields with puckering were previously treated with a postemergence herbicide other than Roundup. Pursuit, Classic, and Pinnacle seem to be the herbicides most often used in fields with the symptoms, but other herbicides have also been used. One working theory about these symptoms: when the postemergence herbicide causes injury to the terminal buds on soybeans, apical dominance is altered, and plant hormones are redistributed within the plant. The result is the appearance of injury that is similar to that from plant growth regulator herbicides (dicamba, 2,4-D). New shoots may occur at nodes below the injured zone, the plant may take on more of a bushy appearance, and leaves may be wrinkled and cupped. However, most of the fields have not exhibited the increased "bushiness" such as might occur if apical dominance was lost.
While herbicides may be responsible for some of puckering, cupping, and wrinkling that has been observed, we suspect that environmental conditions and soybean variety may have a significant role. This is based on the observation of uniform cupping in fields where no postemergence herbicide was used. Some varieties may be more likely to show symptoms that others. We have not been able to come up with a good explanation for this phenomenon. However, the good news is that leaf cupping and wrinkling should not affect yields, and soybeans generally compensate well for other herbicide-related problems given enough time and moisture.
The main disease management strategy for Phytophthora root and stem rot in Ohio is the use of resistant varieties. Overtime, Phytophthora sojae populations will change and specific resistance genes will no longer be as effective. Future Phytophthora disease management decisions will rely on choosing soybean varieties that have specific resistance to the P. sojae population in fields or by selecting varieties with high levels of partial resistance. Knowing which resistance genes are no longer effective in that field will help in selecting varieties for the next soybean rotation.
This is an excellent time to begin surveying fields for soybeans with symptoms of Phytophthora stem rot. Several sites have been reported in Ohio this past week. Severely infected soybean plants appear wilted, turn yellow and die. When the plants are examined closely, the stems turn brown from the soil line up to the 5th or 7th node. When examining fields for symptoms of Phytophthora stem rot, look for plants with a few leaves that are beginning to wilt. Then examine these plants closer for the discoloration at the base of the stem. This is the only soybean pathogen that will colonize soybean plants from below the soil line up through the plant. These symptoms are the result of infections that took place when the soil was saturated earlier this growing season. For farms using precision management, locations with symptoms of Phytophthora stem rot should be added to the maps in order that assessments of yield loss can be made at harvest.
 |
Phytophthora Root and Stem Rot |
Strong winds (as high as 70 mph in some areas) and heavy rains associated with severe thunderstorms Sunday night (July 19) caused varying degrees of lodging in corn fields. Although these lodging problems may be related to rootworm injury in some cases, recent hot, dry weather conditions and soil compaction may have inhibited good nodal root formation and predisposed corn plants to such wind injury. Strong winds can pull corn roots part way out of the soil, a condition referred to as root lodging. If root lodging occurs before mid-grainfill, plants usually recover at least partly by "kneeing up." This results in the characteristic goose-neck bend in the lower stalk with brace roots providing above ground support. Plants exhibiting these symptoms are sometimes referred to as "sled runners." If this stalk bending takes place before pollination, there may be little effect on yields. If it occurs later in the season, there may be some yield decrease due to partial loss of root activity. Hybrids differ in their ability to resist root lodging.
If winds cause plant stalks to snap and break over, then the plants are usually killed outright. During the rapid phase of vegetative growth the stalk tissue can be fairly brittle and stalks can be snapped by wind, especially when environmental conditions are very favorable for rapid stalk elongation. Certain hybrid genetics, herbicides, and other cultural practices can contribute to such stalk brittleness or "greensnap." For more information on green snap, check my previous C.O.R.N. article for the week of June 29 - July 5, (issue #98-16).
There are two techniques commonly used to assess the success or failure of pollination. One involves simply waiting until the developing ovules (kernels) appear as watery blisters (the "blister" stage of kernel development). This usually occurs about 1 1/2 weeks after fertilization of the ovules. However, there is a more rapid means to determine pollination success.
Each potential kernel on the ear has a silk attached to it. Once a pollen grain "lands" on an individual silk, it quickly germinates and produces a pollen tube that grows the length of the silk to fertilize the ovule in 12 to 28 hours. Within 1 to 3 days after a silk is pollinated and fertilization of the ovule is successful, the silk will detach from the developing kernel. Unfertilized ovules will still have attached silks. Silks turn brown and dry up after the fertilization process occurs. By carefully unwrapping the husk leaves from an ear and then gently shaking the ear, the silks from the fertilized ovules will readily drop off. Keep in mind that silks can remain receptive to pollen up to 10 days after emergence.
The proportion of fertilized ovules (future kernels) on an ear can be deduced by the proportion of silks dropping off the ear. Sampling several ears at random throughout a field will provide an indication of the progress of pollination.
Herbicides need to be chosen wisely in double crop soybeans! Research from Penn State University has shown that injury to double crop soybeans can drastically reduce yields. Therefore use herbicides relatively safe to soybeans such as Basagran, Firstrate, Pursuit, Select, Poast Plus, Assure II, Fusion, Roundup Ultra on Roundup Ready soybeans, and Synchrony on STS soybeans. Common and giant ragweed, velvetleaf, lambsquarters and annual grasses are the most likely weeds in double crop soybeans. It is also wise to spray these soybeans as soon as possible, so that they have time to recover from injury. Try to avoid spraying during hot and humid conditions as increased soybean injury will occur. Few weeds are emerging now so an early herbicide application (before canopy closure) can still provide good weed control compared to the same time on single season soybeans. Burcucumber, eastern black nightshade, fall panicum, and large crabgrass will be the weeds most likely to emerge after herbicide application, but few of these weeds will be emerging over the next month. The early applications may allow reduced rates of herbicides to be used if weeds are small.
Herbicides must also be chosen based on rotational restrictions. If corn will be next year's crop then do not apply Synchrony or Classic after August 1st. Be sure to allow the appropriate time period between herbicide applications in soybeans and planting of corn next year for the following herbicides: Flexstar and Reflex 10 months, Firstrate and Raptor 9 months, and Pursuit 8.5 months.
Fall herbicide applications in wheat stubble provide an excellent opportunity to control perennial and many winter annual weeds. Many of these fields will have a new "crop" of summer annual weeds (foxtail, common ragweed, lambsquarters, and smartweed, most commonly) that can be fairly large by the end of July. While there is a temptation to apply Roundup and/or 2,4-D in July or August to prevent seed production by annual weeds, this will actually reduce the control of perennial weeds, which are more difficult to control in the following year's crop. The best timing for perennial and winter annual weed control is generally late September through late October. Herbicide effectiveness at that time will be maximized by applying after several days of warm weather when perennial weeds are fairly large and/or in the bud to flower stage. Where it is desirable to reduce seed production by annual weeds in wheat stubble, the field can be mowed in the summer, but this should be done within the next several weeks. Later mowing may not allow perennial weeds to regrow to an adequate size before fall herbicide application.
Fall applications of Roundup and/or 2,4-D can also control many winter annual weeds, such as horseweed (marestail), mustards, fleabane, and chickweeds, which will result in a much more weed free no-till seedbed the following spring. In a study conducted in 1993 through 1995, we examined the effect of Roundup/2,4-D applications in wheat stubble on annual weed populations the following year. The Roundup/2,4-D (1 quart + 1 pint/A) combination was applied in late July, late August, or late September, and weed populations were measured the following year. The July application controlled the foxtail, common ragweed, and other summer annual weeds most effectively and reduced seed production almost completely. This did tend to reduce annual weed populations the following year, but we saw little impact on control where an effective preemergence corn herbicide was used the next spring. Control of perennial and winter annual weeds was most effective with the late September application, which resulted in a much cleaner seedbed the following spring. Weeds in the 1994-95 study included horseweed (marestail), Philadelphia fleabane, curly dock, and mustards. Control of these weeds the following spring ranged from 83 to 100 percent with the September application, 30 to 64 percent with the August application, and 9 to 45 percent with the July application. In our opinion, control of the winter annual and perennial weeds in wheat stubble is more important than control of summer annual weeds. For this reason, we recommend that Roundup, Touchdown, or combinations of either product with 2,4-D be delayed until later in the fall. Mowing fields by early August will help reduce seed production by summer annual weeds.
During the past week, field surveys have been initiated to determine the presence or absence of first year corn rootworm. At many soybean sites, a set of four Pherocon AM traps have been set to monitor the abundance of western corn rootworm beetles that may be moving into soybean fields from corn fields. In addition, Extension personnel are monitoring a number of sites, where both Pherocon AM and Multigard traps are used in soybeans, Multigard traps are set in adjacent first year field corn, sweep-net samples are taken in soybeans, and visual counts of rootworm beetles are taken in first year field corn.
On Thursday and Friday, we sampled first year corn and soybean fields in Clark, Madison, Delaware and Knox counties. Based on previous survey efforts, these four counties have been assumed to lack the first year corn rootworm problem. Following the past week's observations, I tend to believe that our survey effort will continue to demonstrate a lack of first year corn rootworm in central Ohio. Visual inspection of first year corn fields at each of the four sites resulted in minimal detection of adult western corn rootworms. In fact, only one rootworm beetle was observed in the visual counts (inspection of 20 plants per site) at the four field sites sampled. If any of the first year corn sites had a first year corn rootworm problem, adult rootworm beetles should have been abundant, readily visible, and present on many of the plants inspected. In addition, a half a dozen root systems were sampled at random in each field inspected, and detection of rootworm injury was minimal at all sites. Although our survey efforts have only begun, it will be interesting to see if any significant adult rootworm activity develops over the next six weeks of biweekly corn plant inspections, sweep net collections, and trap catches in corn and soybeans.
In contrast to our observations in central Ohio, Extension personnel in the western Ohio region bordering Indiana are observing clear cut signs of first year corn rootworm activity. Curtis Young has reported that areas (Van Wert, Paulding and Putnam counties) having first year corn rootworm exhibit abundant levels of adult WCR activity, extensive leaf feeding symptoms, and root systems having level 3 injury. However, he also reported that signs of first year rootworm activity in Wood and Hardin counties are minimal.
Visual counts of first year corn last year demonstrated a clear cut difference between areas in which first year corn rootworm were present or absent. From late July through late August, counts taken last year in first year corn where first year corn rootworm is presumed absent averaged about 2.5 beetles per 10 plants. In contrast, counts on first year corn where first year corn rootworm was present averaged 24 beetles per 10 plants. Thus, it should be noted that periodic visual inspection (counts of rootworm beetles on 20 or more plants) of first year corn plantings can provide an indication of the presence or absence of the first year corn rootworm problem.
Finally, it should be noted that some areas in Ohio were recently hit with a heavy storm front which may result in some lodging of corn. Such wind storm injury could be mistaken for signs of rootworm activity. Thus, root systems of lodged corn should be closely inspected to determine the cause of lodging.
Late summer can be an excellent time to establish forage crops. A successful late summer seeding will produce new forage supplies more quickly than waiting until next spring. A summer seeding will usually produce yields the following year that are 90 to 100% of those from a spring seeding made the same year. The most critical factor to successful summer seeding is having adequate moisture. Moisture often determines the difference between success and absolute failure in late summer seeded forages. Aside from adequate moisture, the following 10 steps will improve the chances for successful establishment:
Sclerotinia crown and stem rot is a fungal disease that can be a serious risk in summer seedings of alfalfa and clovers. Red clover is extremely susceptible to this pathogen. The highest risk of infection is in stands that are established using minimum or no-tillage methods, and where clover or alfalfa were present in the recent cropping history.
Airborne spores infect seedling plants in the fall (usually beginning in early to mid-October), but the plants show absolutely no visible signs of infection until later in the winter and spring when stems and crowns begin to rot. Spring seedings are essentially unaffected by this pathogen. This pathogen is different from the warm-weather Sclerotinia infecting soybeans.
Sclerotinia crown and stem rot was observed in many alfalfa fields across Ohio this past spring. These stands had been summer seeded in 1997, and the mild winter conditions favored development of this pathogen. The greatest damage (requiring re-establishment) occurred in stands that were established late -- a result of late planting and/or dry soil conditions which delayed seedling emergence.
Reducing the Risk of Damage:
From: Purdue Pest Management & Crop Production Newsletter (17 July 1998)
R.L. (Bob) Nielsen, Agronomy Dept., Purdue Univ., West Lafayette, IN 47907-1150
Email address: rnielsen@purdue.edu
Frequent and excessive rainfall throughout Indiana during the past several months has resulted in extensive flooding of river bottoms and ponding in low-lying areas of other fields. Some river bottom ground has yet to be planted for the first time this year, some has been replanted two or more times but is still devoid of crop growth because of continued flooding. Many fields throughout the state contain "wet holes" that have ponded numerous times this growing season, killing young corn or soybean, and have not been replanted.
Where such fields or parts of fields have been left fallow due to flooding or ponding, corn growth the following year sometimes displays symptoms of phosphorus deficiency even if adequate levels of soil phosphorus exist. Recently published research from USDA-ARS at the Univ. of Nebraska sheds some light on this phenomenon known as Post Flood Syndrome. The complete citation is : J.R. Ellis, Post Flood Syndrome and Vesicular-Arbuscular Mycorrhizal Fungi, J. Prod. Agric. 11:200-204 (1998).
The fungi referred to in the article's title (acronym: VAM) are plant fungi that routinely colonize roots of crops in a mutually beneficial (symbiotic) relationship. The fungi benefit from the host plants' resources, the crop benefits from the increased nutrient absorption zone offered by the fungal hyphae (the many threads that make up the mycelium of fungi).
The bottom line of the research is that prolonged fallow periods, such as caused by flooding or ponding, significantly reduce the populations of the VAM fungi in the soil. Following the seeding of a subsequent crop, the VAM fungi slowly recolonize and restore their populations. Until the populations of the VAM fungi are sufficient, plant nutrient uptake (especially phosphorus) is restricted and nutrient deficiency symptoms can occur.
The author suggests several alternatives to forestall the development of the Post Flood Syndrome in corn. One is to apply 60-80 lbs per acre of starter phosphorus to the following corn crop, equal to 16 to 21 gallons of 10-34-0 starter fertilizer (easily accomplished by many Indiana corn growers). Another alternative is to seed a host cover crop in the fall to encourage the regeneration of the VAM fungal populations (seeding the areas to corn or soybean now would accomplish the same thing). While VAM fungal inoculants exist, the author suggests that using them would be too expensive for field crops.
Readers can subscribe electronically to this newsletter bysending an e-mail message to:
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: State Specialists: Ann Dorrance (Plant Pathology), Pat Lipps (Plant Pathology), Mark Loux (Weed Science), Landon Rhodes (Plant Path, Forage Crops), Mark Sulc (Forage Crops), Jeff M. Stachler (Weed Science), Peter Thomison (Corn), and Hal Willson (Entomology); Extension Agents: David A. Jones (Allen Co.), Steve Prochaska (Crawford Co.), Dennis Baker (Darke Co.), Greg LaBarge (Fulton Co.), Gary Wilson (Hancock Co.), Howard Siegrist (Licking Co.), Barry Ward (Marion Co.), Ray Wells (Ross Co.), Clark Hutson (Seneca Co.), and Woody Joslin (Shelby Co.).
Editor: David A. Jones - Web Editor: Steve Lichtensteiger
Information presented above and where tradenames are used, they are supplied with the understanding that nodiscrimination 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
| C.O.R.N. | Newsletter | Archive | Search | Questions? | Ohioline | Publications |