C.O.R.N. Newsletter 2006-05

Predicting Flea Beetle Activity and Stewart’s Disease for the 2006 Corn Crop

Authors: Ron Hammond, Pierce Paul, Dennis Mills, Bruce Eisley

Stewart's bacterial leaf blight has not been of concern in most areas of Ohio over the past few years due to the colder winters that reduced the populations of the corn flea beetle. The bacterium causing this disease is carried and spread by the adult beetle. In the spring, as corn emerges from the ground, flea beetles feed on the young plants and spread the bacterium which in turn causes seedling wilt and leaf blight. The occurrence of Stewart's bacterial disease is totally dependent on the level of flea beetle survival over the winter. For many years the winter temperatures have been used to predict the risk of Stewart's disease because higher populations of the flea beetle survive during mild winters than during cold winters. The 'flea beetle index' is calculated as the sum of the average temperatures (Fahrenheit) of December, January and February. Index values less than 90 indicate negligible disease threat, 90-95 indicate low to moderate levels, 95-100 indicate moderate to severe and values over 100 predict severe disease levels.

We checked the average temperature for December, January and February at several locations in Ohio to determine the risk level according to the 'flea beetle index' for 2006. The locations and the corresponding indexes were: Hoytville, 91; Ashtabula, 95; Wooster, 96; South Charleston, 96; Columbus, 102; Oxford, 103; Jackson, 106; and Piketon, 107. These numbers indicate that the risk of Stewart's bacterial leaf blight is low to moderate in northern and west central Ohio with a severe disease risk in south central Ohio.

Although the 'flea beetle index' has been a relatively good predictor over the years, we still recommend that growers scout their corn fields for the presence of flea beetles, especially if they know they have planted a hybrid that is susceptible to Stewart's disease. For those growers wishing to take preventive action against flea beetle, commercially applied insecticide seed treatments Cruiser and Poncho, or the grower applied products Concur and Latitude, are labeled for flea beetles.

Flea beetle adults become active in the spring when the soil temperatures reach 65 F. Adults are most active on sunny, warm, windless days. They hide in cracks in the soil during windy, cool or cloudy days. After feeding and mating, adult females lay eggs at the base of the corn plants. Larvae feed on corn roots and are full grown in about two weeks. There are at least two generations per year in Ohio. The beetle over-winters as an adult in the soil near corn fields. It prefers bluegrass sod, but may be found in fence rows, roadsides and woods. If the adult fed on diseased corn in the late summer or fall, it may carry the bacterium that causes Stewart's disease of corn in its gut over the winter. In the spring as the corn emerges, the flea beetles feed on the young plants and spread the bacterium which in turn causes seedling wilt and leaf blight. You can see pictures of flea beetle injury and Stewart’s bacterial blight, and get additional information on Stewart's disease of corn, on the Ohio Field Crop Disease web site at http://www.oardc.ohio-state.edu/ohiofieldcropdisease/corn/stewarts.htm. Additional information on the flea beetle can be obtained from OSU Extension Fact Sheet CV-1000-94.

Preventive Insect Control on Field Crops

Authors: Ron Hammond, Bruce Eisley

Growers are faced with numerous decisions in the spring, including whether to use preventive insecticide treatments against many insect pests on various crops. Although we do not recommend taking preventive measures on a widespread basis, there are certain conditions where we do recommend preventive management tactics. The decision to use preventive treatments should be based on a history of problems in a field, sampling for the insects, or cropping practices being used.

As has always been the case, we would still recommend a preventive treatment be considered for corn rootworm larvae management when growing continuous corn. Although rotating away from corn is the first choice to manage rootworms, we realize many growers choose to plant corn following corn. In those cases, we would recommend using either a soil insecticide, a commercially applied seed treatment of either Cruiser CRW or Poncho 1250 at their high rates, or a transgenic corn hybrid with resistance to rootworm larvae. All three methods should give acceptable control. We do not recommend one over the other, and feel the decision is best left to the grower based on his/her own farming practices and economics. For those corn growers in western and northwestern Ohio where the western corn rootworm variant is a concern, we recommend only treating first year corn if the previous crop, most often soybean, had been sampled either with yellow stick traps and the threshold of >5 beetles/trap/day was achieved, or sweep-net sampling revealed large populations of adult beetles. In those situations, any of the three control tactics could be used.

For European corn borer, we have transgenic corn hybrids that give outstanding control of borer populations. The decision to use them should be based on a number of things, including having had a history of borer problems in the past, and a history of treating for them in previous years. This problem usually only occurs in isolated fields for most growers. We would point out that the corn borer problem has not increased throughout Ohio, so widespread use of these transgenic hybrids is probably not necessary. But for those growers wanting to better manage this pest, transgenic hybrids are an outstanding alternative. Where we do see perhaps a greater need for these hybrids are for late planted corn, corn that is planted in late May or early June. In those situations, we often see greater problems from European corn borer, and usually recommend growers consider these transgenics.

In the first article in this CORN newsletter, we mentioned the greater potential for flea beetle problems because of the milder winter, and the likelihood of a greater incidence of Stewart’s bacterial wilt. Growers planting corn hybrids that are more susceptible to Stewart’s wilt might consider one of the commercially applied seed treatments, Cruiser and Poncho, at either rate. Both are labeled for corn flea beetle control and might help keep populations and subsequent Stewart’s wilt down.

The use of seed treatments, either commercially applied or grower applied, is a preventive treatment that is also recommended against many insects including certain soil pests such as seedcorn maggot, wireworms, and white grubs, and other pests including black cutworm. Although we do not recommend seed treatments for “plant health” purposes, we do see a need for them where any of these insect pests have historically caused concern. The problem is that for most of these pests, we do not have good predictive abilities as to where and when they will be a problem. Growers should examine the history of their field as to whether they have experienced previous problems. If the answer is yes, seed treatments will help to alleviate future injury. If the answer is no, and growers have not had problems in the past, we do not think problems will begin occurring in the future. However, there is one pest where we do have predictive abilities, and that is with seedcorn maggots. As we have written in the past, when growers incorporate green organic matter in the soil in the spring, including old alfalfa fields, cover crops, or even heavy weed growth, the chances of enhanced numbers of seedcorn maggots and subsequent plant stand reductions is high. Thus, we do recommend seed treatments for both corn and soybeans when planting in fields where green organic matter is being plowed or tilled into the soil.

Black cutworm is another insect that we are concerned about each year in corn. This insect is difficult to predict because it is a migrant from the south and early spring temperatures may play a role in whether it will be a problem or not. We still think the best management for cutworm can be obtained by scouting fields regularly beginning at early emergence because unlike some of the other insects we have discussed, such as seedcorn maggot, wireworms or grubs, we do have a number of insecticides that can be applied as rescue treatments for cutworm. However, if a field has a history of cutworm damage and/or the field has a high population of winter annuals such as chickweed, then a preventive treatment could be applied at planting by using either a soil insecticide, a seed treatment insecticide, the inclusion of an insecticide with the pre-emergent herbicide or the use of Herculex corn hybrid since these have cutworm activity.

Finally, there is much discussion of seed treatments on soybeans for soybean aphid and bean leaf beetle control. As discussed in previous CORN newsletters, we do NOT recommend them for soybean aphid control. It is too difficult to predict which fields might have aphid problems, and more importantly, our data from last year indicates seed treatments do not offer management of them. We will continue to recommend scouting, using the 250 aphids/plant threshold, and spraying a foliar insecticide. Perhaps most important, we are predicting that we will NOT have widespread soybean aphid problems in 2006. We will address this issue in a later CORN newsletter.

The situation with bean leaf beetle on soybean is different. For general control of overwintering bean leaf beetle, we do not recommend seed treatments. Although these treatments will control the beetles in early summer, this population of beetles does not cause that much leaf injury unless they are in extremely high numbers, and more importantly, those populations are too difficult to predict. Whether controlling the overwintering population will limit later population growth in mid to late summer is unclear. As with the aphids, we would rather see growers scout their fields for bean leaf beetle in mid to late summer and use a foliar insecticide so beetle densities do reach threshold levels.

However, the recommendations for bean leaf beetle control is different for seed growers, food grade soybeans, and other situations where seed quality is an issue at the end of the summer. This greater concern is related to the beetle’s ability to vector bean pod mottle virus. For growers who decide to control overwintering bean leaf beetles to limit virus transmission, we would normally recommend an early season foliar spray after plant emergence when beetles are entering the field, followed by a second spray in July against the first generation. Because seed treatments will control the overwinter beetles and reduce feeding injury, we would suggest that seed treatments be used in a few fields to see if they can replace the early season foliar spray. Although we do not have all the answers as to whether this will reduce virus transmission, we do recommend that growers who follow the two-spray regime give seed treatments a try in a few fields.

Efficacy of Fungicide Seed Treatments for Corn & Soybeans in Ohio

Authors: Dennis Mills, Pierce Paul, Anne Dorrance

Given all the current discussion about the new insecticide seed treatments for corn and soybeans there was much discussion around the State about fungicide seed treatments. Seed treatments in general are being marketed or offered as packages with seed purchases and many times growers are unsure as whether a certain product is an insecticide or fungicide and what it offers to that grower. Insecticides will not provide good control of seed and soil borne fungi. However, seed treatment fungicides are useful tools to manage seed and soil borne pathogens. Seed treatment fungicides are highly recommended in Ohio on seeds of corn, and soybeans. However, a specific fungicide will not control all of the pathogens that may be present.

The primary soil borne pathogens in Ohio include the water molds, Pythium and Phytophthora sojae as well as Fusarium spp., and Rhizoctonia. All have at one time or another caused a grower to replant fields of corn or soybeans. Environmental conditions that favor these pathogens include cool, wet conditions that delay emergence. Phytophthora is much more aggressive when soil temperatures are above 60F, and soils are saturated for greater than 24 hours.

For soybeans, seed borne pathogens include Phompsis and this year their may be some downy mildew. Downy mildew is another water mold, so metalaxyl or mefenoxam should be used to treat seed that have crusting, white flakes on the seed. Phomopsis makes the seed appear chalky, and if the seed lot germination is below 70 percent, then it should not be used. In some cases, the germination of Phomopsis seed lots will improve over the winter as some of the fungus dies off of some seed.

For corn, seed and seedling diseases are generally more of a concern in early-planted corn. While early planting may increase yield, early-planted seeds and seedlings are exposed to cold soil conditions and are more susceptible to fungal infection. However, fungicide-treated seeds generally germinate and grow well even when planted in cold soils. Damaged seed are subject to fungal infection regardless of when they are planted and should be removed from the seed lot or protected with fungicides.

For more detailed information concerning seed and seedling diseases affecting field crops in Ohio Refer to Extension Bulletin 639 Seed Treatment for Agronomic Crops. The following tables list the effectiveness or efficacy of fungicide seed treatments for corn and soybeans. These tables are based on field trials where these fungicides have been evaluated under very high disease conditions. The listed products may be applied in various combinations within limits of the label. Using combinations of fungicides will broaden the effectiveness against several different diseases. Additional information can be obtained from the Ohio Field Crop Disease web site at http://www.oardc.ohio-state.edu/ohiofieldcropdisease/.

TABLE 1) Relative efficacy of commercial fungicide seed treatments for control of certain diseases of CORN in Ohio

Trade name	Active ingredient	Soilborne			Seedborne
		Fusarium	Rhizoctonia	Pythiumdamping off	Fusarium
Allegiance	Metalaxyl	N	N	E	N
Apron XL	Mefonoxam	N	N	E	N
Captan	Captan	G	P	F	G
Dynasty	Azoxystrobin	ND	ND	ND	G
Maxim	Fludioxonil	G	G	N	G
Trilex	Trifloxystrobin	ND	ND	ND	G

Efficacy based on labeled rates of active ingredient for each material. Efficacy rating scale: E=excellent, G=good, F=fair, P=poor, N=no activity, ND=no data.

TABLE 2) Relative efficacy of commercial fungicide seed treatments for control of certain diseases of SOYBEANS in Ohio.

Trade name	Active ingredient	Phomopsisseed rot	Phytophthoradamping off	Pythiumdamping off	Rhizoctoniaseedling blight
Allegiance	Metalaxyl	N	E	E	N
Apron XL	Mefenoxam	N	E	E	N
Maxim	Fludioxonil	G	N	N	G
Rival	Captan, PCNB,TBZ	E	N	P	G
Stiletto	Carboxin, Thiram,Metalaxyl	G	F	G	G
Soygard	Metalaxyl, Azoxystrobin	P	F	G	G
YieldShield	Bacillus pumilus	N	N	N	F

Efficacy based on labeled rates of active ingredient for each material. Efficacy rating scale: E=excellent, G=good, F=fair, P=poor, N=no activity, ND=no data. Control of Phytophthora damping off only at the higher labeled rates. Low rates of Metalaxyl and Mefenoxam do not control Phytophthora but they do control Pythium.

 

Control Winter Annual Weeds Soon in Wheat

Authors: Mark Loux

Dense populations of winter annual weeds can compete with wheat in early spring and slow the rate of crop development. Herbicides should be applied soon to minimize the impact of these types of weed populations on the wheat. However, herbicides will be most effective where it is readily apparent that plants have resumed active growth. Winter annual grasses, such as downy brome, cheat, annual bluegrass, and annual ryegrass, should be controlled as soon as an inch of new growth has occurred. The wheat herbicides available for control of grasses, Maverick, Osprey, and Olympus, are most effective when applied in the fall, and effective spring activity is dependent upon the weeds being small. Winter annual grasses are not nearly as prevalent as winter annual broadleaf weeds, though, and the major problems in many fields consist of chickweed, purple deadnettle, henbit, marestail, etc.

Herbicides applied at this time will eventually control emerged weeds, but activity can be slow under the typically cool conditions in March. This is also too early to control dandelions, which have not resumed active growth. Fields treated at this time should be scouted again in April to determine if later-emerging populations of summer annual weeds, Canada thistle, wild garlic, or other weeds require another herbicide treatment. In fields without winter annuals, delay herbicide application until later in spring, and select herbicides based on scouting. Be sure to follow label guidelines to minimize risk of crop injury and yield loss. Labels for some products specify the number of tillers or leaves that wheat should have before treatment is allowed. Guidelines and effectiveness ratings for wheat herbicides can be found on pages 137-148 of the 2006 Weed Control Guide for Ohio and Indiana.

Is It Too Early to Apply Residual Soybean Herbicides?

Authors: Mark Loux

The short answer to this question is – “no”. Most residual soybean herbicides are being used in a planned preemergence plus postemergence approach in Roundup Ready or non-GMO soybeans. Our research indicates that in this type of approach, the residual herbicide can be applied almost anytime in the spring prior to soybean planting. Herbicides that have the best fit for application this early due to fairly long life in soil include Canopy EX, Canopy, Scepter, and FirstRate, whereas shorter-lived herbicides such as Valor and Sencor are best applied within a few weeks before soybean planting. Where no-till fields are only sparsely populated with winter weeds, it may be possible to get by with a combination of residual herbicide plus 2,4-D ester at this time. Be aware that 2,4-D will not control chickweed, and glyphosate or Gramoxone should be added as necessary to control this weed or to ensure effective control in fields with higher populations of winter weeds. Applications later this month or in early April, when dandelion has resumed growth, should include 2,4-D ester and glyphosate for most effective dandelion control.

Leaving Corn in the Field Through the Winter: How Much Do You Lose?

Authors: Peter Thomison, Allen Geyer, Rich Minyo

I recently received a question asking how much grain yield is lost when corn is left standing through the winter? In Ohio, we generally don’t see much corn standing in the field during the winter. However, late plantings combined with cool, wet conditions at maturity may result in a corn crop with excessive grain moisture that ceases to dry down in the fall when temperatures turn cold. If wet weather persists following maturity, harvesting may be delayed until late winter.

We have data from a 2004 study that may give some idea as to the yield losses that occur when corn is left in the field through the winter. This 2004 study was part of a larger research project evaluating effects of harvest date and plant population effects on the agronomic performance of four hybrids differing in maturity and stalk quality (for more on this study see “Effects of Harvest Delays on Yield, Grain Moisture and Stalk Lodging in Corn” http://corn.osu.edu/story.php?setissueID=109&storyID=631). Four plant populations were considered (24,000, 30,000, 36,000, and 42,000 plants/A). The targeted harvest dates were early Oct, November and December. However, at the Apple Creek test site in NE Ohio, the third harvest was delayed until March 22, 2005 due to above average rainfall that made soil conditions unsuitable for field harvesting.

Yield differences among hybrids and plant population were generally small on the first harvest date. With harvest delays, major yield losses occurred at the higher plant populations, especially 42,000 plants/A, due to increased stalk lodging. Yields averaged across hybrids and plant populations decreased from 207 bu/A on Oct. 13 to 185 bu/A on Nov. 9, to 158 bu/A on March 22 (for a 24% yield loss over the four month harvest delay). Between the Oct. 13 and March 22 harvest dates, yields at 24000 plants/A, averaged across hybrids, fell 15%, whereas at 42,000 plants/A yields dropped 36%. Stalk lodging increased from about 1% on Oct. 13 to 40% on Nov. 9, to 93% on March 22. Grain moisture, averaged 24.8% on Oct. 13, 20.2% on Nov. 9, and 18.9% on March 22. Test weights remained the same between the Oct and Nov harvest dates, averaging 60.7 lbs/bu but dropped to 56.8 lbs/bu on March 22. Hybrids with lower stalk quality ratings exhibited greater stalk rot, lodging and yield loss when harvesting was delayed. Averaged across plant populations, the hybrid associated with the greatest lodging on March 22 averaged yields 28% less than on Oct. 13, whereas the hybrid exhibiting the least lodging, averaged yields 16% less than on Oct. 13.

Leaving corn to stand in the field through the winter is more common in northern states. According to Dr. Joe Lauer, the corn extension agronomist at the University of Wisconsin, if stalks stay standing and there isn’t much ear drop, snow cover, or wildlife damage, a crop can get through the winter with limited yield loss. In a 2004 report (“Some Pros and Cons of Letting Corn Stand in the Field Through the Winter”; Wisconsin Crop Manager 11(26): 170-171) Dr. Lauer concluded that in years with heavy snow cover, grain yield loss can decrease significantly. At the test site he monitored during 2000, a year with heavy snow cover, grain yield by spring (April) was 37% lower than October harvest. This contrasted with the winter of 2001 (little snow cover) when grain yield by spring was only 10% lower than October harvest.