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http://www.ag.ohio-state.edu/~corn/ |
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August 13 to August 19, 2001
C.O.R.N. 2001-26
In This Issue:
A) Soybean Aphid Update
B) Bean Leaf Beetle
C) Corn Disease Update: Begin Scouting Now
D) Wheat Variety Disease Reactions Available
E) Sudden Death Syndrome Showing Up Early This Year
F) Assessing Potential For Nitrate Problems In Corn
Soybean aphids invade Toronto! Aphids attack Clevelanders!
Akron gets swarmed! Yes, those stories are true. These cities had swarms of
winged soybean aphids throughout over the past week. As mentioned in last week's
CORN newsletter, we were expecting populations to go down in some fields, and
observations last week suggest this has occurred. A primary reason for drop
is that overcrowding caused winged aphids to develop and disperse from the fields.
This occurred throughout much of northern Ohio (west of Cleveland and Akron)
and apparently Ontario, Canada, which is west of Toronto. Much to everyone's
surprise, the aphids migrated into the city, most likely on wind currents. Reports
of huge numbers of winged soybean aphids were obtained from these cities Ohio.
Winged aphids actually caused umpires at the Toronto Skydome to order the roof
closed during a Blue Jay baseball game.
This is linked to what is happening in soybean fields. Fields that had large populations a few weeks ago have been observed to have fewer aphids last week. Whether aphid populations in these fields build up again remains to be seen; observations from last year in other states does suggest the possibility of a second, albeit lower peak. Of concern is that these winged aphids that have taken flight have also entered fields previously without large aphid populations. Within the past week, we have seen numerous winged aphids, and very small, newly laid aphids, in fields, which had very low aphid populations only 2-3 weeks ago. Whether these aphids reach large numbers in these fields is unknown, but these recent developments suggest that growers closely monitor the fields and take action if warranted. This is especially important because many of these fields were later planted fields and are still in the R2 or flowering stages. See previous CORN newsletters for aphid management decisions.
The concern should be shifting to the second generation, which will begin feeding
on pods shortly. Pod injury by this generation BLB will initially appear in
early-planted soybean fields - especially when the green pods become more attractive
to the BLB adults than the foliage. As early soybean
fields dry down, populations of BLB may migrate to later planted soybeans.
When pod injury occurs on 10-15% of pods, seed injury will become evident and yield losses are possible. If the pod injury occurs during periods of wet conditions that enable infection by seed diseases, the development of moldy bean seeds may also lead to a loss in seed quality. However, if dry field conditions prevail when pod injury due to BLB feeding occurs, the potential for development of moldy seeds in injured pods may be minimal.
Rescue treatment to prevent excessive development of seed damage may be warranted when pod injury exceeds 10-15% and adult BLBs are still present and actively feeding enough to cause additional pod injury. The assessment of a field infestation depends on (1) determination of the current level of pod injury, (2) the abundance of adult BLB activity using a sweep net, (3) consideration of weather factors that may enable infection of the damaged pods by disease agents, and (4) the amount of time remaining before total leaf drop and dispersal of a BLB population from the field.
Growers should start sampling their fields weekly for the remainder of the
summer. The percent pod injury should be determined by random sampling about
twenty plants and counting on each plant, the number of pods with feeding injury
and the total number of pods. Adult BLB population should be estimated by taking
10 sweeps with a sweep net at three to four locations in a field. Less than
five BLB per sweep are unlikely to cause significant injury. Five to ten BLB
per sweep indicates a potential problem possibly warranting rescue treatment.
More than ten BLB per sweep will result in significant injury especially if
two or more weeks remain until leaf drop.
When the foliage dries and drops, beetles exit the field. Thus, the time remaining
for BLB feeding is a key factor in the occurrence of pod injury.
Corn growth and development varies greatly from one location to another in the state this year due to the great variability in rainfall. Corn in some areas is doing very well, but in drought stricken areas the corn is barely alive. Corn diseases have been greatly affected by this diverse rainfall pattern and it is difficult to predict when and where diseases will begin to show up. Regardless, this is the time when consultants and growers should begin to scout for disease problems. Up to this point disease problems have been low. Recent rains may change this by promoting development of certain diseases. Begin to look for signs of leaf diseases including gray leaf spot and anthracnose. Also, the first signs of ears affected by Diplodia ear rot may be seen.
Anthracnose leaf blight was very common in fields by late May and early June, especially in fields planted into corn residue. The leaf blight phase of this disease seldom causes yield loss by itself, but growers should be concerned with the leaf blight because diseased leaves provide the spores of the fungus that are deposited on stalk tissues by rain splash. Leaf lesions appear a irregular dark brown spots usually near the tip of the leaves and are surrounded by yellow or orange colored tissue. Stalk rot and top dieback are important phases of anthracnose. As soon as 4 weeks past pollination, some premature death of plants can be detected in severely diseased fields. The first symptoms of anthracnose stalk rot is the premature death of plants. Diseased plants will begin to lose their green color, the stalks will turn a pale green to light yellow color and within a short time afterward the stalk will die. Premature death will prevent complete filling of the grain leading to yield loss. The earlier premature death occurs the greater the yield loss. The anthracnose fungus survives in surface corn residues, so crop rotation and destruction of corn residues are important to disease control. There are hybrids available with improved resistance to anthracnose.
Gray leaf spot is beginning to be a problem in some areas of the state due to adequate moisture provided by morning fog. Fortunately, gray leaf spot has not gotten an early start, but continued foggy mornings and warm weather will promote spread throughout the rest of the growing season. Like anthracnose, expect more pressure from gray leaf spot in fields with continuous corn or in reduced tillage fields with corn residue on the soil surface. Symptoms of gray leaf spot can very greatly from one hybrid to another, but spots on most hybrids are rectangular in shape with tan to brown centers. Spots on some hybrids also appear yellow to orange in color. Gray leaf spot causes yield losses when the middle leaves of the plant become diseased. These leaves, just above and below the ear are very critical to grain filling. If these leaves do not become diseased until 5 to 6 weeks after pollination, then yield losses are minimal. Severe leaf blight is also a sign of possible severe lodging prior to harvest.
Diplodia ear rot has already been detected in a few fields in southern Ohio. Only areas that received significant rainfall during the weeks just prior to tasseling have the potential for seeing this disease. This is another disease associated with reduced tillage and continuous corn. The first symptoms growers will see is premature discoloring of the husks surrounding affected ears. Pealing back the husks will reveal a thick mass of mold near the butt end of the ear. Fields with significant level of affected ears should be noted for early harvest and drying of grain to prevent additional yield and grain quality losses.
For additional information on these corn diseases, with pictures and control recommendations visit the Ohio State Field Crop Disease web site (http://www.oardc.ohio-state.edu/ohiofieldcropdisease/).
Wheat growers interested in obtaining information about
the reaction of various varieties to diseases like powdery mildew, Stagonospora
blotch, leaf rust, head scab and wheat spindle streak mosaic can visit the Ohio
State Field Crop Disease web site:
(http://www.oardc.ohio-state.edu/ohiofieldcropdisease/).
On the web site home page click on 'Wheat' and then choose the disease of interest.
In the disease management section click on 'resistant varieties' and the current
listing of wheat varieties with their reaction to the various diseases will
come up. Remember: using disease resistant varieties is the cheapest and most
effective way to manage disease.
Sudden Death Syndrome, caused by Fusarium solani f. sp. glycines, was identified early this year in SW Ohio. Symptoms include, bright yellow leaf spots which turn brown. The stem pith is white and healthy but the tap roots will be an off-gray color. If you are fortunate and dig the plants after some heavy rains, you may be able to see the blue, iridescent spores of the fungus on the outside of the tap root. Sudden Death is occurring in the areas of the fields where the drainage is poor or the soil is compacted, high moisture can favor infection. This Fusarium is specific to soybeans, and has only been documented in approximately 300 acres total in Ohio. IN ALL of these fields in Ohio with this pathogen, Soybean Cyst Nematode also exists at very high populations. There appears to be an interaction that in the presence of cyst the above ground symptoms are very dramatic. Management of Sudden Death Syndrome at this time in Ohio is aimed at managing Soybean Cyst Nematode. We are using the occurrence of the fungal pathogen as a bioindicator of where our problem cyst fields are. Crop rotation which will prevent the rapid build-up of the Fusarium spp. and tactics to improve soil drainage will limit the amount of damage to soybean directly caused by this pathogen. Target fields with Sudden Death Syndrome for Cyst sampling following soybean harvest.
The highest levels of nitrate accumulate when drought occurs during a period of heavy nitrate uptake by the corn plant. A drought during or immediately after pollination is often associated with the highest accumulations of nitrates. Extended drought prior to pollination is not necessarily a prelude to high accumulations of nitrate. The resumption of normal plant growth from a heavy rainfall will reduce nitrate accumulation in corn plants, and harvest should be delayed for at least 1 to 2 weeks after the rainfall. Not all drought conditions cause high nitrate levels in plant. If the supply of soil nitrates is in the dry soil surface, plant roots will not absorb nitrates. Some soil moisture is necessary for absorption and accumulation of the nitrates.
If growers want to salvage part of their drought damaged corn crop as silage,
it's best to delay harvesting to maximize grain filling. Even though leaves
may be dying the stalk and ear have enough extra water for good keep. Kernels
will continue to fill and the increases in dry matter will more than compensate
for leaf loss unless plants are actually dying or dead. Moreover if nitrate
levels are high or questionable, they will decrease as plant get older and nitrates
are converted to proteins in the ear.
The kernel milk line can be used as a guide in determining the best time to
cut corn for silage. When the kernel milk line has moved 1/4 to the distance
from the top (or crown) of the kernel to the base, the whole plant generally
contains about 60-70% moisture -- usually the recommended moisture range for
making corn silage.
If the corn is to be harvested for silage it is important to test the nitrate levels at the cutting height of the forage chopper to determine the highest levels of plant nitrate.
If the plant is marginal to high in nitrates then a lab analysis is recommended before feeding. To get a quantitative nitrate determination, send or deliver a sample of 6 to 10 plants (cut at the stalk height of the forage chopper) to a plant analysis laboratory. Lists are available from your county extension office. Results of the test can generally be obtained over the phone, fax, or email when tests are completed.
The following feed laboratories will test corn samples for nitrate:
Holmes Labs (Millersburg, OH) 800/344-1101
Dairy One (Ithaca, NY) 800/344-2697
CVAS (Maugansville MD) 800/282-7522
Nitrate Field Test Kits are available from:
Nitrate Elimination Co., Inc. 888/648-7283
For more information on feeding corn with varying nitrate-nitrogen levels, consult: Nitrates in Dairy Rations, Ohio State University Fact Sheet AS-0003-99 available on-line at http://ohioline.ag.ohio-state.edu/as-fact/0003.html
There are a number of good extension publications on this topic - the following is one I recently came across from Kansas which is available on-line.
Nitrate and Prussic Acid Toxicity in Forage - Causes, Prevention, and Feeding
Management. Kansas State University Extension MF-108
http://www.oznet.ksu.edu/library/crpsl2/MF1018.PDF
Readers can subscribe electronically to this newsletter by sending an e-mail message to: corn-out-on@postoffice.ag.ohio-state.edu. A successful subscription message will receive by an automatic reply from the listserv. Contact your local Ohio State University Extension Office or e-mail labarge.1@osu.edu if you have problems subscribing.
Past versions of C.O.R.N. can be found on the World Wide Web at: http:/www.ag.ohio-state.edu/~corn/archive/
C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.
Contributors to C.O.R.N. this week include: State Specialists: Pat Lipps, and Ann Dorrance, (Plant Pathology), Ron Hammond and Bruce Eisley (Entomology), and Peter Thomison (Corn Production.) District Specialists: Ed Lentz, (Northwest District). Extension Agents: Dave Jones (Allen), Barry Ward (Champaign), Steve Prochaska (Crawford), Greg Labarge (Fulton), Howard Siegrist (Licking), Glen Arnold (Putnam), Clark Hutson (Seneca), and Roger Bender (Shelby).Editor: David Jones Web Editor: Tom Rosati
Information presented above and where trade names are used, they are supplied with the understanding that no discrimination 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.
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