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July 28, 2000
Flush of corn diseases likely; but widespread and heavy yield losses not expected Act now to prevent 2001 wheat disease Bacterial blight and bacterial pustule common in soybeans this year Insurance tables aid in assessing damage 'New' insects possible visitors to southcentral Nebraska Kansas update: Southwestern corn borers at higher numbers than ever before Silage from damaged grain Late summer alfalfa planting requires good seed beds Soybean Management Field Days August 15-18 NU Diagnostic Clinic Update Keep up with USDA research via Email Crop updates"It's an unusual year in Nebraska...... but then again, that's typical," notes NU Plant Pathologist Jim Stack, as he reviews the status of current and developing diseases in this year's corn fields. In his first four years at the South Central Research and Extension Center near Clay Center, Stack has seen a different disease profile each year, including "unexpected" diseases. Following is a synopsis of his discussion. |
The race is on between Nebraska's corn crop and developing diseases and for the most part, it looks like the state's crop will mature before yields are significantly reduced. Producers should be scouting their corn; however, since individual fields may reach threshold levels and require fungicide treatments. Determining the need for treatment in a specific field depends on several factors, all of which should be considered when assessing potential damage.
Factors specific to an individual field and disease outbreak include:
The last two winters have been particularly mild and have contributed to the survival of pathogens in crop residue or soil. Due to the early season being particularly hot and dry, many producers started irrigating much earlier than usual, providing increased opportunity for development of these pathogens.
Following is an update on the diseases affecting this year's corn. Sometimes symptoms of several diseases may look similar so you may want to have a sample diagnosed at the NU Pest and Plant Diagnostic Clinic or one of the state's other diagnostic laboratories. For more information on individual diseases and recommended treatment strategies, check out the following publications available from your local NU Cooperative Extension Office or on the Web.
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Holcus spot has flourished in south central Nebraska; however it is not expected to cause detectable yield loss. The same species of bacterium causes holcus spot in corn and bacterial leaf spot in sorghum, both of which are likely to appear 7-10 days after a heavy rain storm. While widespread this year, holcus spot is unlikely to affect yield. Damage from this disease can look a lot like herbicide injury and a diagnoses may be necessary to determine the cause of field damage.
Gray leaf spot potential was relatively high due to recent mild winters and higher than normal amounts of inoculum in the field; however, plants developing ahead of normal may outrun the disease. While gray leaf spot has traditionally appeared first in south central Nebraska, this year conditions were optimal for early development of the disease in popcorn and field corn fields near Grant and Imperial as well as in northeast Nebraska. In south central Nebraska, outbreaks are two weeks behind normal. Disease development in the next week is likely to determine whether impact on yield will occur and whether treatment is necessary in some fields. The key is for the crop to reach the dent stage before reaching treatment thresholds.
Common rust is present at levels you only see every five years in a wide swath of east and central Nebraska. Depending on the hybrid's genetics, common rust may cause yield loss. In some counties rust is visible from the top to the bottom of the plant. In some Saunders County fields, rust has reached threshold levels. Individual fields may need treatment, but generally across the state it's unlikely to cause huge yield losses this year. In this week's Iowa State University Integrated Pest Management newsletter, Charlie Martinson and Gary Munkvold, extension plant pathologists noted that common rust (Puccinia sorghi) of corn is developing early and is widespread this year in Iowa:
"Seed corn producers should scout for the rust pustules and make decisions about whether to spray a fungicide..... Relative susceptibility of the inbred to rust and the outlook for continued weather favorable for rust development should be considered in any decision-making process. Common rust develops best with cool nights and adequate moisture (dew or rainfall)."
Southern rust, which is caused by a different pathogen, can be very damaging as well. Producers should begin scouting for it as soon as possible.
This year's corn crop may win the race to achieve yield potential; however, developing disease may impact yield indirectly by predisposing the crop to stalk rot and lodging. Start scouting fields for stalk rot (Stalk Rot NebGuide) in mid-August and be prepared to harvest fields early. (For more information, see Common Stalk Rot Diseases of Corn Fields, G99-1385, available from your local Cooperative Extension office.) Fields with moderate to severe gray leaf spot are particularly vulnerable to stalk rot diseases.
Jim Stack
NU Plant Pathologist, South Central REC
Lisa Jasa, Crop Watch Editor
The most damaging wheat disease in 2000 was wheat streak mosaic. This disease caused serious losses in the southern Nebraska Panhandle. The pattern of wheat streak mosaic closely fit the pattern of hail in 1999. These hail storms produced volunteer wheat, and where this volunteer was not controlled, wheat streak mosaic was severe. The only effective control measures for wheat streak mosaic are to plant at the proper time and to control volunteer wheat in adjacent or nearby stubble fields. Any volunteer wheat that emerges within 7-10 days after a hail storm is a prime candidate for providing the summer green bridge for the curl mite and the wheat streak mosaic virus. The curl mites will carry the virus from the hailed wheat to the volunteer then to the fall sown crop. One hail storm can cost a grower two wheat crops — one, directly, from the hail and the second the next year, indirectly, from wheat streak mosaic.
It only takes one volunteer wheat field in an area to serve as a source of mites and virus for all of the surrounding fields planted this September. The "Good Neighbor" policy really comes into play in this situation. Being a good neighbor means controlling your volunteer so it doesn't become a source of mosaic for your neighbor's fields as well as your own fields.
In addition to controlling volunteer in the stubble, plant your wheat at the target date for your area. Planting early is an open invitation to problems with wheat streak mosaic, barley yellow dwarf, soil-borne wheat mosaic and crown and root rot.
John Watkins
Extension Plant Pathology
Bacterial blight and bacterial pustule have been identified in many soybean fields this year. These diseases normally don't affect yield in Nebraska, but this year they may. Both diseases are spread through infected seed and overwinter in crop residue. Of the two, bacterial blight is more commonly seed-transmitted and normally more of a problem. These diseases are favored by cool temperatures and are more frequently observed in areas that have received a lot of rain because they are spread by splashing rain and wind.
Bacterial blight symptoms start as small, angular water-soaked spots that become brown with a yellow border. Portions of the leaf tissue drop out, resulting in a shot-hole appearance. Black lesions develop on the stem and small water-soaked spots can develop on the pod. Infected seed may be shriveled.
Bacterial pustule symptoms start as small yellowish-green areas with reddish-brown centers and are more evident on upper surface. A small raised pustule on the lower leaf surface may be evident. At first glance this may look like rust, but we do not have rust on soybeans in the U.S. and there are no spores when you rub the raised lesions on the leaf.
Management/control
Plant high quality seed and don't keep seed from fields with either disease; use rotation; bury crop residue to reduce overwintering of inoculum, and do not cultivate the field when the foliage is wet.
Loren J. Giesler
Extension Plant Pathologist
Last week's devastating hail storm in southeast Nebraska was like pouring salt on a wound. On Thursday, July 20, producers in Gage, Johnson, Lancaster and Pawnee counties watched as hail the size of golf balls and 50 mph winds ravaged crops that had been enjoying a little relief from recent high temperatures. An estimated 80,000 acres of cropland in four counties was affected by the hail.
Preliminary Farm Service Agency flash reports for Gage County, the hardest hit of the four, estimated that average corn and soybean losses were as high as 65% and sorghum losses were as high as 40-45%, according to Steve Stark of the Gage County FSA office.
Some of these same fields in southeast Nebraska were among the earliest and hardest hit by this year's drought. Soil moisture reserves were still low and the area had only received 57% of normal precipitation since last September.
Hail damage at this point in the season can cause significant damage to both crop and potential yield. Sometimes damage can appear horrendous the first day or two and then the plant may start to recover, or, the plant may appear to be surviving when disease enters through a wound and begins weakening it further. In either case, waiting 7-10 days will make it easier to determine whether plant tissue is dead. Contact the appropriate government agencies before destroying fields. (See July 14 Crop Watch, Assessing hail-damage fields.)
If you're planning to chop the crop into silage, waiting may also be beneficial and allow the plant to dry to a more optimal moisture level. (See Harvesting grain crops for silage.) While the value of the grain crop may be reduced, Paul Hay, extension educator in Gage County, notes that as silage the crop may be near or at full value. He had heard reports of buyers wanting to heavily discount the price of silage, however he noted that while quantity will be reduced, quality may not be. If it's got 25 or more bushels of grain, growers should pursue high value prices.
As serious as the direct impact of the hail was in these fields, the secondary effects can be more subtle but still damaging to yield and harvest. Two consecutive mild winters have led to an increase in disease inoculum in many fields. This week's return to hot, humid conditions in addition to irrigation may create optimum conditions for disease development in the wounded crop. Hail damage can increase the incidence of corn smut, stalk rot, Goss's bacterial wilt and blight and holcus spot. In soybean fields, windy rainstorms and hail have been known to lead to increased levels of bacterial blight and bacterial pustule (See story, Bacterial blight and bacterial pustule common in soybeans this year.) Hay reports molds and diseases are already becoming evident on severely bruised plants. Even if corn plants appear to recover, Hay notes that stalk rot, lodging, and ear drop are still likely to cause problems.
In addition to disease damage, where plants have been stripped or destroyed and stands remain spotty, plant canopies will provide less shade and weeds are likely to develop quickly. These weeds will use valuable moisture and complicate harvest if not controlled while still little.
Guidelines have been developed for assessing potential yield loss from hail damage. Two factors are used to help estimate yield loss: stand reduction and plant damage at a specific growth stage. The following charts from the hail insurance industry work well because they predict percent loss compared to actual yield.
Corn
Total corn yield loss from hail damage is estimated by adding the expected yield loss caused by stand reduction and the expected loss caused by defoliation of the remaining plants. Be sure to estimate defoliation based on the adjusted stand number (total stand minus stand loss). This is only an estimate of the percent yield loss. For example, with a 20% stand loss and a 36% defoliation loss you would calculate the following:
As with undamaged corn, favorable weather during the rest of the growing season can cause actual yields to be higher than expected. Likewise, unfavorable weather can cause greater reductions. At this stage of plant development, there is a direct relationship between loss of plant stand and loss of yield. A 1% loss in plant stand translates to a 1% loss in yield.
See table, Estimated percent corn yield loss due to defoliation at various Stages of Growth.
Soybean
Yield loss predictions are based on two factors:
Plant damage is classified as leaf defoliation, stand reduction, stem damage, and pod damage. To calculate actual soybean yield loss due to hail, several factors should be considered. Actual soybean loss due to hail is the sum of direct damage and plant damage loss. Direct damage is the sum of the yield losses from stand reduction and pod damage. The plant damage — the sum of losses due to stem cutoff, broken over and defolation — is multiplied by the remaining percent stand to find the plant damage loss. At the current stage of plant development, there is a direct relationship between loss of plant stand and loss of yield. A 1% loss in plant stand translates to a 1% loss in yield. (See NebGuide G85-762, Soybean Yield Loss Due to Hail Damage, for a worksheet to calculate loss.)
See soybean yield loss/reduction tables.
Sorghum
In the short run, sorghum is apt to recover well from severe hail. Generally its leaves are more flexible and tend to sustain less severe damage; however, recovery is not without its own challenges. A perennial, sorghum will try to compensate for plant injury by sending out new suckers, all of which can lead to new heads of grain, all at different levels of maturity. While the primary head may ripen much as planned, secondary heads may still be green, creating a variety of maturity levels. If stems are healthy, producers may choose to delay harvest until frost or the majority of the crop is mature.
More information
For further information on assessing hail damage, please check the following publications, available from your local NU Cooperative Extension office or on the Web:
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Kansas State University entomologists report that southwestern corn borers have been seen in Jewell, Republic, Cloud and Mitchell counties in Kansas this year. These counties are just south of Webster, Nuckolls and Thayer counties in Nebraska. Historically, southwestern corn borer have rarely been reported in Nebraska; winter temperatures are thought to influence their northern limits.
Southwestern corn borers, Diatraea grandiosella, belong to the same insect family as European corn borer, and share many similarities in their life cycle. In Kansas and Nebraska there are two generations a year, with timing similar to European corn borers. Eggs are laid in masses and have similar appearance to European corn borer (flattened, and overlapping like fish scales). One difference is that as southwestern corn borer eggs develop, three orange-reddish lines develop across each egg. Larvae are white with large raised black spots on each segment. They are 1-1 1/4" long at maturity. The second generation of southwestern corn borer is most damaging, both from stalk boring activity and because it girdles the base of the corn stalk in preparation for overwintering. This weakens the stalk and makes stalk breakage more likely. KSU entomologists recommend applying insecticides when 20% to 25% of the corn plants are infested with eggs or newly hatched larvae. Southwestern corn borer may survive in sorghum as well as corn.
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The adults are elongate, gray beetles, 6-10 mm long, with antennae longer than the body. Adults lay eggs in the upper leaf petioles of soybean during July and August. Newly hatched larvae feed in the petiole pith initially but soon tunnel to the main stem. The trifoliate leaf where the egg hatched and the larva began feeding will wilt and die. Larvae tunnel in the stalk until they complete their development. Soybean stem borer larvae are slender, legless, and creamy white in color, reaching 6/10" at maturity. Larvae overwinter in the stalk, pupate in early summer, and adults emerge from June to September.
As they finish feeding, last stage larvae move down to the bottom of the stem, and girdle the inside of the stem 2-4.5 inches above the soil level. This predisposes the soybean plant to break at the girdled point during windy periods. Up to 10% yield reduction has been reported from the effects of larval tunneling, however, the greatest yield losses occur due to lodging. The borer girdles earlier-maturing varieties more severely and lodging is most severe on earlier-planted soybeans.
Insecticides are ineffective for control of overwintering larvae and are not recommended. Adults can be controlled with foliar sprays, but their extended period of emergence does not make treatment economically feasible.
Cultural practices suggested to reduce losses include harvest of infested fields as soon as they mature, crop rotation, control of weed hosts, and burial of infested stubble to a depth of 2 inches.
For more information and additional color photos to aid in identification of Southwestern corn borer, see http://www.oznet.ksu.edu/library/entml1/SWCB.pdf.
For more information and additional color photos to aid in identification of Soybean stem borer, see http://www.oznet.ksu.edu/dp_entm/extension/Current/soybstbr.html.
Bob Wright
Extension Entomologist, South Central REC
Normally a problem from Great Bend to Oklahoma, Southwestern corn borers are teaching north central Kansas about the biblical pairing of drought and pestilence. On the heels of an unusually warm, dry winter and spring, first-generation borers have infested corn fields in Jewell, Republic, Cloud and Mitchell counties.
"Unfortunately, the moths emerging from that first generation can fly for miles in search of attractive corn, where they'll deposit second-generation eggs. Each female can lay two to five eggs per mass and lay several masses over her lifetime. So, the resulting population could be large. We could be seeing severe damage in many more fields, including ones with little to no earlier distress," warned Kansas State University entomologist Randall Higgins.
Even more bizarre, Southwestern corn borers also have attacked several north central Kansas grain sorghum fields, planted into no- till corn stubble.
"From time to time, we've seen this kind of thing on a very limited basis. Even so, Southwestern corn borers aren't a recognized milo pest. Our records here indicate these are by far the heaviest levels ever confirmed in Kansas," said Leroy Brooks, also a K-State Research and Extension entomologist.
Mitchell and Lincoln counties have reported infested grain sorghum fields. First-generation borers were feeding on from 10% to 50% of the milo plants.
In the crop for which it's named, the Southwestern corn borer can cause significant yield losses, as well as serious harvest problems, Higgins said.
"In K-State's southwest area research fields, we've measured 70 or more bushels of grain that ended up on the ground in untreated, but heavily infested test plots," he explained.
Their chemical controls are similar, but the pest is both larger and more destructive than the European corn borer that typically attacks northern Kansas fields, Higgins said.
The window for controlling the pest is narrow, the entomologists said.
Between one and two weeks after hatching, Southwest corn borer larvae stop causing "shot hole" feeding damage in the upper plant surface. They chew into a stalk and begin tunneling from near the ear zone to ground level. They may bore out of and back into a stalk several times. They often end their journey by girdling the plant from the inside, causing stalk break (lodging) just above the soil line.
"Each larva will easily eat through 20 to 30 inches of stalk," Higgins said. "In contrast, European corn borer larvae typically make 3- to 5-inch long tunnels."
Once inside the stalk, both pests are protected from chemical controls.
"That's why north central Kansans, in particular, should start now and continue scouting for Southwestern corn borer activity through mid-August," he said. "The best time for treatment is when you expect 50% emergence — although you may very well have to retreat 7-10 days later. The economic threshold for deciding to treat is when 20%-25% of plants are infested with eggs or newly hatched larvae."
K-State recommends that farmers scout their fields at least three times at 7- to 10-day intervals, estimating the level of borer infestation each time. If borer numbers don't start at critical levels, farmers still should consider spraying if the sum of their three scouting counts ever exceeds 25%.
Most Bt corn varieties have protection against both Southwestern and European corn borer activity, Higgins said. But, hybrids containing "Bt event 176" may not have much protection against late-season, second-generation Southwestern corn borers.
"Weather will mostly determine how severe the problem is for non-Bt varieties. Moist, still nights favor Southwestern corn borer mating and egg laying. Severe storms and driving rains can kill lots of adults, thus limiting the number of larvae that can establish themselves in plants."
Brooks said the future for the sorghum-infesting borers is even further up in the air.
"We saw no sign of corn borers in milo fields just a few miles away from the infested ones," he said. "And, frankly, we don't know if the moths will stay with sorghum for their second generation.
"Our initial thought is the borers aren't likely to become more of a problem for sorghum growers. But, with no history, all we can really say is people should keep their county Extension agent informed."
Southwestern corn borer eggs are 1/8-inch wide ovals with a slightly raised (convex) upper surface. Most show up on upper leaf surfaces, slightly overlapped and often forming a chain pattern.
They start out creamy white, but develop three parallel red-orange lines as they mature.
The larvae are bright white, covered with black spots.
Kansas State University
Research and Extension Press Release
Dryland corn fields, especially in southwest Nebraska, have suffered severe drought damage. Silage can salvage this crop, but some harvest modifications may be needed.
Most of the green, low grain corn will be too wet to make good silage directly. In fact, even plants with brown leaves may be too wet because stalks hold so much moisture. Usually, 65% to 70% moisture is ideal for bunker-type silos, but green stalks and leaves of corn contain 80% to 85% water. Direct chopping and ensiling this wet corn can cause heavy seepage and a sour silage.
The easiest way, and maybe the best way, to lower moisture content is simply wait until stalks start to turn brown and die.
If waiting isn't desirable, windrowing the crop and allowing it to wilt one-half to one full day before chopping will reduce moisture. You also could mix grain or chopped hay with freshly chopped corn to lower the moisture content. It takes quite a bit of material for mixing though — about 10 bushels of grain or 500 pounds of hay to lower each ton of silage from 85% to 70% moisture.
Or, you can allow that windrowed corn to dry completely and bale it as hay. Be sure to test it for nitrates before feeding.
Silage feeding quality is determined by
Grazing might be the best way to use damaged corn, and since many pastures also are short, this is a good way to extend your grazing. Introduce livestock slowly to this new feed to reduce potential digestive problems. Strip graze the field to reduce trampling losses and get more grazing from the corn.
For more information on chopping silage, see Harvesting Corn and Sorghum for Silage, Extension NebGuide G94-1231.
Bruce Anderson
Extension Forage Specialist
Alfalfa planted in August establishes well when moisture is available. Be sure to plant early enough so alfalfa has six to eight weeks between emergence and freeze back to develop good cold tolerance. That means that if you are in northern Nebraska or southern South Dakota, you need to plant immediately. But only if you also have moisture for seeds to germinate right away. Any delay is likely to cause poorer stands. In southern Nebraska you can plant a little later. Mid-August is ideal while planting after August 31 becomes risky. In central Kansas alfalfa can be planted as late as mid-September.
Seedbed preparation is crucial for late summer plantings. Good seed-to-soil contact and weed control are critical, both when seeding into prepared seedbeds or into wheat stubble. Conserve soil moisture whenever possible, and put extra effort into getting a firm seedbed.
Whenever seeding alfalfa in August, be especially wary of grasshoppers. They sometimes seem to come from nowhere, and they love to eat new seedlings. Spray field margins with insecticides before planting if necessary.
One important caution to consider, especially this year, never plant into dry soil. August plantings into dry soil may lie dormant for several weeks and there'll be too little time for seedlings to develop good cold tolerance. Fall rains are often unreliable in Nebraska, as evidenced by the 1999 fall.
Bruce Anderson
Extension Forage Specialist
The Nebraska Soybean Management Field Days will offer valuable information for meeting the challenges of today's market. The field days will be held Aug. 15-18 at four Nebraska sites.
Maximizing profitability is essential in today's market, said Keith Glewen, event co-coordinator and University of Nebraska Cooperative Extension educator.
"Producers will take home new ideas on managing weeds, reducing harvest loss, growing value-added soybeans and controlling diseases," Glewen said. Unbiased, research-based demonstrations and information will be provided by NU specialists, educators and industry consultants. Topics include weed management technology, harvest management, soybean value enhancement, and disease and soil fertility management. Each hour-long interactive session will feature guest speakers.
The Nebraska Soybean Board is sponsoring the event in cooperation with the United Soybean Board and NU Cooperative Extension.
Nebraska Soybean Board Chairman Norm Husa of Barneston said the soybean checkoff program strives to help producers achieve greater profits from quality crops. By participating, producers can see their checkoff dollars at work bringing leading technology and ideas to producers.
The field days run 9 a.m. to 2 p.m. There is no charge for admission or lunch.
Dates and locations are:
The Nebraska Soybean Board administers the state's share of the one-half percent national check-off on soybeans produced in Nebraska.
Program details and maps can be obtained on the web, or contact the Nebraska Soybean Board at (800) 852-BEAN or (800) 852-2326 or Glewen at (800) 529-8030.
The following diseases were diagnosed in the NU Plant and Pest Diagnostic Clinic From July 19-July 25.
An alfalfa sample submitted by the Panhandle Research and Extension Center at Scottsbluff was diagnosed with spring black stem.
Corn diseases diagnosed this week were: common rust (Dakota and Washington counties), gray leaf spot (Lancaster County), holcus spot (Scottsbluff), iron deficiency (Buffalo County), Physoderma brown spot (York County) and Stewart's wilt (eastern Colorado).
A pinto bean sample was diagnosed with Rhizoctonia root and stem rot (western Kansas).
Soybean diseases identified in samples were: bacterial blight (Phelps County), Phytophthora Rot (Dakota, Gage and Jefferson counties), Rhizoctonia root and stem rot (Brown, Lancaster and Washington counties.)
Jane A. Christensen
Plant and Pest Diagnostic Clinic
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