C.O.R.N. Newsletter 2007-21

Dates Covered: 
July 9, 2007 - July 17, 2007
Editor: 
Howard Siegrist

Frogeye Leaf Spot - Now Is The Time To Scout

Authors: Anne Dorrance

Frogeye leaf spot is a foliar disease which can impact yields of highly susceptible soybean varieties. Last year, we had some losses on about 500,000 acres when frogeye was present in fields at R2 (flowers on top four nodes of plant) and R3 (bean pod, 3/16in long on one of top four nodes). This year, frogeye leaf spot has already been found in a number of the sentinel plots around the state.

Frogeye leaf spot has a characteristic lesion that forms on new leaves. On the top of the leaf, the lesion is round, the center is gray and it is surrounded by a characteristic cranberry red border. On the bottom of the leaf, in the center of the lesion, a spore mass can be seen. There are other lesions that are minor; brown spot, alternaria leaf spot, downy mildew and bacterial leaf spot have also been found. Brown spot is just like it is called – brown spots that are small, then the leaf begins to turn a dull yellow. Bacterial leaf spot is a dark brown irregular spot (the border has an irregular shape) surrounded by bright yellow. Alternaria has a similar shape to the frogeye lesions, but the spot is tan and on the top of the leaf, the spores form in black rings inside the lesion. Downy mildew has also shown up this past week, and this also impacts the upper canopy. The top of the leaf will have a chlorotic to tan spot, irregular in shape, but on the bottom of the leaf you will be able to see the white fuzzy mycelium. From all of these diseases – we have only been able to document economic losses to frogeye.

Do all fields in Ohio need to be sprayed? NO!! Only those soybean fields that have a susceptible variety planted. More than 90% of the varieties in the Ohio Soybean Performance trial were resistant so there are only a few that we need to keep our eyes on. In our plots, only the susceptible varieties have frogeye.

The best means to manage frogeye leaf spot is with a fungicide. The stobilurins (Headline and Quadris) and thiophanate methyl (Topsin M) have been reported to have the best activity in trials in the southern US. The triazoles have ranked in the fair to poor category but more data is needed. In Ohio last year, Folicur had no effect on Frogeye at 3 locations. This is a tough year to call for when to spray and that is mainly due to this dry weather. The infections so far are old and in the lower canopy. The soybeans are drought stressed in much of the state – so for these fields it may be best to wait until the weather looks like it is going to be more favorable for this disease (rain). If fungicides are sprayed now and the dry weather continues, then we run the risk of eliminating the beneficial fungi which are keeping our mites at bay.

Ohio weather has been quite variable. Areas with beneficial rains and/or morning fog should scout for foliar diseases. If new lesions can be found in the top of the canopy- then the R3 is the time to spray to protect the new leaves that form over the next few weeks.

So You Are Trying Fungicides On Corn And Soybeans, How To Evaluate If They Are Worth The Cost?

Authors: Anne Dorrance, Pierce Paul, Dennis Mills

We posted these guidelines last year so here they are again. For those of you that would like to “experiment” with this plant health aspect, here are a few guidelines that can help evaluate if this is truly providing any benefit.

1. Know what the variety or hybrid is. For both corn and soybeans only the moderately susceptible and highly susceptible hybrids and varieties have demonstrated yield impacts in fungicide applications.

2. Have more than one check strip, wider than your combine, and space them across the field. Three should be plenty but two is not enough.

3. In your comparisons, do not include the parts of the field where you have weed escapes or along tree lines, these areas are going to yield less anyways, so this is false data whether it was treated or not. Make sure other variables such as soil types, soil fertility variations and pest issues are not biasing your field experiment.

4. Approximately 3 weeks after applications, walk some of the strips. For corn, look at the ear leaf – what percent leaf area has got lesions in the treated vs nontreated. Is gray leaf spot, anthracnose, or northern leaf blight present? For soybeans – look at the upper canopy for frogeye or downy mildew and on the lower canopy look at how much brown spot is present.

5. Take averages. When you do harvest your fields, take several strips, both untreated and treated. Then take the average of the untreated strips and compare that average to the treated. Fields are not uniform and with our stand issues and unevenness across many fields this year, there is going to be even more variability.
 

Soybean Aphid Update

Authors: Ron Hammond, Bruce Eisley

As we get further into July, we need to become vigilant for the arrival of winged soybean aphids, followed by their buildup towards economic populations. If the anticipated happens, we will begin to see the arrival of winged aphids from northern locations within the next 2-3 weeks, followed by an increasing unwinged aphid population. At this time, there appears to be few aphids in Ohio soybean fields.

The big question on our mind is will this happen as anticipated. We know that aphids overwintered very well and in high numbers this past winter, which led us to predict economic problems this summer. What was unexpected and brings in a big “if” into the equation were the cold temperatures that the Midwest experienced in early April. If you remember, temperatures in early April were extremely cold for a short period of time. For example, at the Northwest Research Station near Bowling Green, temperatures ranged from a high of 32 degrees to a low of 24 degrees for four days, April 5 through April 8. The question is whether these extremely cold temperatures that followed aphid egg hatch, which occurred on the aphid’s overwintering host, buckthorn, might have (1) caused significant aphid mortality, or (2) burning back of early buckthorn foliage growth, which depleted the host material for the aphids resulting in massive mortality. While aphid populations are more common in areas to our north, we are not hearing of many large aphid populations as of yet.

Because of the possibility for lower populations than were originally expected, we urge growers to closely monitor their soybean fields from now until the end of August. While we want growers to ascertain and treat economic populations, we also do NOT want growers making unnecessary treatments. Remember that the threshold for treating soybean aphids is 250 aphids per plant with a rising population. Keep reading this CORN newsletter for weekly updates on the aphid situation.

Treating For Mites And Aphids

Authors: Ron Hammond, Bruce Eisley

With continued dry weather in parts of Ohio that will enhance two-spotted spider mite populations, and the possibility of soybean aphids beginning to occur in high numbers within a few weeks, the question has come up about treating for either or both insects.

There are only two materials effective against mites, chlopyrifos (Lorsban, Nufos, Yuma) and dimethoate. Of these two, only chlopyrifos is effective against soybean aphids; we do not recommend dimethoate for aphid control. Most of the other insecticides for soybean aphid control are not active against mites, and might actually aggravate the mite situation. Which insecticide(s) should be used is of major concern.

If only mite control is needed, either of those two insecticides should be used. Read the label for the proper way to control mites. However, if both mites and aphid control becomes necessary, we would recommend using a chlopyrifos formulation. If only aphids are present, you have a long list of materials to use.

The question that begs answering is “Can we get long term residual control of soybean aphid if spraying for mites now by tank-mixing with one of the pyrethroids such as Warrior, Baythroid, Mustang Max, Asana, etc? Could this save an additional trip over the field at a later date?” To that question, we do not know! Our concern is that by applying any insecticide(s) at this time, while controlling two-spotted spider mites, would also greatly reduce natural enemies which might help to keep aphid populations from increasing later in the summer. Although a pyrethroid will have longer residual activity and perhaps some repellency towards the aphid, it might be too early to expect that pyrethroid to provide adequate aphid control later in the summer. If needing to treat for mites at this time, we would recommend using one of the above mentioned insecticides, and then closely monitoring the aphid situation. If and only if aphids reach the threshold of 250 aphids per plant, should be treated with an appropriate insecticide. As mentioned in the other article on soybean aphids in this week’s newsletter, there is the possibility that they might not occur in large numbers.

Corn Leaf Feeding By Western Corn Rootworm Adults

Authors: Ron Hammond, Bruce Eisley

We have received numerous calls on adult western corn rootworms feeding on corn leaves. These adults are doing this while waiting for the corn silks to emerge. While leaf feeding is mostly cosmetic damage, the concern will be when the corn silks emerge and the adult rootworms begin their silk clipping. Growers will also see Japanese beetles move into corn fields for additional silk clipping. Please see last week’s CORN newsletter for a discussion of silk clipping feeding and thresholds.

Soybean Defoliators

Authors: Ron Hammond, Bruce Eisley

As soybeans are beginning to enter their reproductive stages, with flowers now being found on most plants, we need to remind growers of the various soybean defoliators that are making their presence known. We are beginning to see Japanese beetles, first generation adult bean leaf beetles and Mexican bean beetles, and green cloverworm larvae. In terms of defoliation, it would be unusual for any of the above mentioned insects alone to cause significant defoliation throughout a field. However, a complex of two or more might cause defoliation levels to rise above threshold levels. Remember you need to sample from numerous locations in the field to get a good idea of what is happening across the entire field.

Growers are advised to initiate scouting procedures to prevent defoliation from reaching the 15-20% defoliation threshold during the reproductive growth stages, R1-R5, which then rises to 20-25% during growth stage R6 late in the summer. When sampling, check numerous places within the field, avoiding the field edges which often tend to have higher levels than the rest of the field.

Western Bean Cutworm

Authors: Bruce Eisley, Ron Hammond

Last year, we wrote about a new corn insect pest that is working its way towards Ohio, the western bean cutworm (see CORN newsletter #30, Sept 12-18. 2006). This insect had been a sporadic dry-land bean and corn pest in Colorado and Nebraska, but since the early 2000’s, the insect and its damage has progressively moved eastward. In 2006, numerous Midwestern states including Ohio placed out pheromone traps to determine its current spread. Three moths were collected from western Ohio in 2006.

Ohio is again participating in this trapping program, and as of the first of July, we have captured the first adult moth near Bowling Green. As the summer progresses, we will continue to check these traps throughout the state. As you check your fields later in the summer, please let us know of any suspected larval infestations. Larvae most likely will be found feeding on the ears, and unlike corn earworm, which are cannibalistic, you will probably find several western bean cutworms feeding on a single ear. For more information on this insect, including images and maps, see the following website: http://www.ent.iastate.edu/trap/westernbeancutworm/ .

Tissue Sampling – A Useful Tool But It Needs Context

Authors: Robert Mullen, Edwin Lentz

Tissue testing is a useful tool for diagnosing nutrient deficiencies, but the information has to be put into context. Environmental conditions can induce nutrient deficiencies even if historical soil testing reveals that the soil has an adequate amount of the nutrient. Several locations have shown some nutrient deficiencies that we do not typically see and the question is – what value does tissue testing have on those areas?

This year potassium deficiency on corn and soybeans has been noted across the state, and magnesium deficient corn has also been verified. To diagnose these problems, tissue sampling was used, but is it that the end of the story? The answer is no. Even though tissue sampling verified the visual deficiency symptoms, tissue sampling does not necessarily reveal the underlying cause. At least some of the fields that have been diagnosed with potassium deficiency have an adequate soil test potassium level. Thus some other soil factor is inducing the deficiency. In most instances the areas that are exhibiting potassium deficiency have soil pH levels well below what we would consider adequate for crop growth. In dry years, soil pH during the growing season can drop rather dramatically and cause root growth problems and various deficiency symptoms can pop-up. These fields most likely had a marginal soil pH level entering the growing season. Tissue sampling alone would not have identified the underlying problem. Tissue and soil sampling together is a much more powerful tool for diagnosing problems.

Magnesium deficiencies have a similar underlying cause – low soil pH. But again, in the absence of soil testing, this diagnosis would have difficult to diagnose. For the fields that are exhibiting magnesium deficiency, the most prudent course of action would be liming with a dolomitic source of limestone this fall.

During years with weather extremes (too dry or too wet) tissue testing should be used with an understanding that the findings are going to be strongly influenced by the growing season and nutrient levels can be quite odd. Soil sampling used in conjunction with tissue testing provides much more information, and may sort whether additional nutrients are required or if you should wait for better growing conditions (weather). Historical soil samples are also extremely useful for providing context.

Drought and Heat Stress Affecting Corn Yield Potential

Authors: Peter Thomison

The combination of high temperatures and inadequate rainfall has created severe stress in many corn fields across Ohio. As dry weather continues, more corn fields are showing signs of moisture stress with leaf rolling evident during midday hours. Cooler weather and the possibility of storms later in the week may ease drought stress, which is important because many corn fields are entering the pollination period, the stage of development most susceptible to drought.

To estimate the impact of dry hot weather on corn yield potential, let's review the effects of moisture deficits on corn growth and development from the late vegetative stages, prior to pollination, to the dent stage of kernel development. Yield losses to moisture stress can be directly related to the number of days that the crop shows stress symptoms during different growth periods. The following summarizes findings of Iowa research by Claassen and Shaw on effects of drought on grain yields in corn. This Iowa data is widely used in estimating the potential impact of water stress on yield potential.

Vegetative Stages: Drought stress during early vegetative growth usually has a negligible impact on grain yield. However, during later vegetative stages, when kernel numbers per ear are determined, plants are more sensitive to stress. According to Claassen and Shaw's findings, four days of stress (i.e. corn wilted for four consecutive days) at the 12th-14th leaf stage has the potential of reducing yields by 5-10 percent. Kernel row numbers on the ear are determined by the 12th collared leaf stage and the potential number of kernels per row is complete about one week before silking.

Tassel Emergence: As the tip of the tassel begins to emerge from the whorl, the upper stalk internodes rapidly elongate and the ears begin to expand. Silks from the base of the ears are also rapidly elongating. Four days of moisture stress at this stage has the potential to reduce yields 10 to 25%.

Silk Emergence to Pollen Shed: At this stage, leaves and tassels are fully emerged and the cobs and silks are growing rapidly. This is the most critical period in terms of moisture use by the plant. Four days of moisture stress at this stage has the potential to reduce yields 40-50%.

Blister Through Dent Stage of Kernel Development: About 12 to 36 days after silking, the cobs, husks and shanks are fully developed and the kernels are increasing in dry weight. Moisture stress will reduce kernel fill from the ear tip down. Four days of drought at the blister stage has the potential of reducing yields 30-40%, and at dough stage, 20 to 30%.

Where dry weather has contributed to corn stands with uneven emergence, and development, yield losses may be higher depending on various factors such as the length of emergence delays and the percentage and distribution of later emerging plants. Where there is considerable variability in plant size, smaller, stunted plants will be at a competitive disadvantage with larger plants for nutrients, water and sunlight.

Key Steps In Corn Pollination

Authors: Peter Thomison

During the past week tassels began appearing in early planted corn fields, especially those with short season hybrids. The flowering stage in corn is the most critical period in the development of a corn plant from the standpoint of grain yield determination. Stress conditions such as the drought we’re currently experiencing in many areas of the state have the greatest impact on yield potential during the reproductive stage. The following are some key steps in the corn pollination process.

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. Under very dry conditions, silk emergence may be delayed, and such “asynchronization” of pollen shed and silking may result in poor kernel set and reduced grain yields. On a typical midsummer day, peak pollen shed occurs in the morning between 9:00 and 11:00 a.m. followed by a second round of pollen shed late in the afternoon.

The tassel is usually fully emerged and "stretched out" before any pollen is shed. Pollen shed begins in 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 rainstorm 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. As noted above, poor seed set is more often associated with poor timing of pollen shed with silk emergence (silks emerging after pollen shed).

Pollen shed for an individual tassel may require up to 8 days with peak pollen shed usually occurring on the 3rd or 4th day. Pollen shed for a field may require up to 2 or more weeks to complete because of field variability affecting plant development.

A factor that could complicate pollination in many corn fields this year is uneven corn growth and development due to dry soil conditions. Scattered, late emerging plants may be at a disadvantage for pollination. Moreover, in certain hybrids, a significant percentage of corn plants are male sterile, so in a field where there’s significant variability in development, late emergers may have less exposure to pollen than earlier developing plants, especially if drought stress delays silking.

Farm Focus Field Day On July 25 To Highlight GPS, Corn Pests, and Tillage

Mark your calendars now to hold Tuesday, July 24 for the 2007 Farm Focus field day at the Marsh Foundation/Farm Focus research farm in Van Wert, Ohio. This years’ field day will include field demonstrations of GPS guidance and auto-steering technologies, as well as one pass tillage system implements. The field day will also include a workshop conducted by OSU entomologists, Ron Hammond and Bruce Eisley, on corn pests with special focus on corn rootworm. This event is hosted by the Farm Focus committee, Ohio State University Extension, and OARDC.

Registration begins at 8:00 a.m. at the site and is open to the public. There is no charge. Certified Crop Advisors will have an opportunity to sign up for continuing education credits of 1.0 hour in the area of Pest Management (PM) and 2.0 hours in the area of Crop Management (CM).

The field day is set up as a series of structured sessions in the morning so that visitors will have the opportunity to attend all the field demonstrations as well as the corn pests workshop. Lunch will be provided at no charge compliments of the financial sponsors for the field day. In the afternoon, there will be time for one on one visits with presenters/demonstrators.

The Marsh Foundation/Farm Focus site is located on the northeast edge on the town of Van Wert at the intersections of US 127 and Route 30 bypass. Just look for the bright orange signs which will take you right to the site. Don’t miss this great opportunity to learn about the latest in GPS technology and tillage equipment.

Archive Issue Contributors: 

Contributors: State Specialists: Anne Dorrance, Pierce Paul, and Dennis Mills (Plant Pathology), Ron Hammond and Bruce Eisley (Entomology), Robert Mullen (Natural Resources), Peter Thomison (Horticulture and Crop Sciences). Extension Educators: Roger Bender (Shelby), Howard Siegrist (Licking), Steve Prochaska (Crawford), Todd Mangen (Mercer), Harold Watters (Champaign), Greg LaBarge (Fulton), Mark Koenig (Sandusky), Ed Lentz (Seneca), Bruce Clevenger (Defiance), Gary Wilson (Hancock), Alan Sundermeier (Wood), Mike Gastier (Huron), Jonah Johnson (Clark), Keith Diedrick (Wayne), and Gary Prill (Van Wert).

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.