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August 21 - August 27, 2000
C.O.R.N. 2000-27
A) Watch For Bean Leaf Beetle and Pod Injury
B) Soybean Aphid - A New Pest Problem
C) Sudden Death Syndrome and Brown Stem Rot
D) Harvest Corn Silage at Proper Dry Matter Content
E) Corn Leaf Diseases Continue to Spread
F) Corn Ear Rots are Being Reported
In This Issue:
In central Ohio, adult activity of the first generation
of bean leaf beetle (BLB) peaked around last week of July. By mid-August, BLB
activity of soybean sites monitored in Clark and Madison counties had declined
about 75%. In addition, adult activity of Japanese beetle (JB) had declined
about 65% based on sweep net samples collected. The combined activity of BLB
and JB in some fields had caused defoliation of about 10%, which is tolerable
but pushing the economic threshold of 15% for soybeans in the late bloom and
early pod development stages. Given the current decline in adult BLB and JB
activity, the risk for excessive defoliation has passed and level of defoliation
that may be tolerated will increase to 25% during the final stage of pod fill.
At the present time, some initial signs of pod injury have been detected that
resulted from feeding of the remaining 1st generation of BLB adults.
Since the 1st generation of adult BLB activity peaked in late July, the 2nd
generation of adult BLB activity will likely begin to appear in early September.
The abundance of 2nd generation BLB may be higher or lower than that of the
1st generation. Pod injury by 2nd generation BLB will initially appear most
severe in early planted soybean fields - especially when the foliage begins
to turn color and the green pods become more attractive to the BLB adults than
the remaining foliage. As early soybean fields dry down, populations of BLB
may migrate to later planted soybeans.
When pod injury occurs on 8 to 10% of pods, seed injury will become evident.
If the pod injury occurs during periods of wet conditions that enable infection
by seed diseases, the development of moldy bean seeds may lead to a loss in
seed quality and possible loss in yield. 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 8% and adult BLB activity is present to cause a significant
level of additional pod injury leading to additional seed injury. The collection
of 4 BLB adults per sweep will lead to an additional 5% pod injury which will
likely lead to 2% additional seed damage and 1% additional moldy seeds for each
week of active beetle feeding activity remaining before total leaf drop and
dispersal of the BLB from a field site. Thus, 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.
It should be noted that BLB activity needs to be very high to cause a significant
level of seed injury and that moldy seeds will only develop if conditions favor
disease infection. Furthermore, the current price of beans may not justify a
treatment unless all conditions clearly indicate the presence of a significant
pod injury problem that will lead to substantial seed injury. Under such conditions,
the need for treatments may be limited to beans targeted for a market having
a low tolerance for any loss in seed quality.
The Wall Street Journal article triggered the expected response of questions
from local media plus a surge in e-mail communications among field crop entomologists
and regulatory agencies. In brief, the situation regarding the new pest on soybeans
appears to be as follows:
During the current summer, University of Wisconsin entomologists and plant pathologists
were conducting studies of insect and disease interactions on soybeans when
they began to observe some unusual aphid activity in their soybean plots. In
addition, reports of unusual aphid infestations were also being detected in
a number of field sites in Wisconsin and neighboring sites in western Michigan,
northern Illinois and southeast Minnesota. Aphid infestations detected ranged
from mild to heavy with and without symptoms of economic injury. In heavily
infested fields, the presence of sticky honeydew on the foliage was a common
feature and infested foliage was exhibiting various symptoms like cupping and
yellowing. Initial observations indicate that late planted soybeans may be more
heavily infested than early planted soybeans.
The initial identification of the new aphid problem focused on the possibility
that the aphid was a cotton aphid (also known as a melon aphid), and such an
identification was used in initial newsletter reports from Wisconsin and Illinois
in early August. However, during the past week insect taxonomists of the Illinois
Natural History Survey and the USDA Systematic Entomology Laboratory in Beltsville,
Md. concluded that the aphid was actually the soybean aphid, Aphis glycines,
which is a common pest of soybeans in Asia and previously not known to be present
in the North American hemisphere. Thus, it is now established that the new aphid
problem on soybeans does not represent some unusual behavior of a local aphid
species, but that it represents the import and establishment of a pest species
commonly distributed on soybeans in China, Japan, eastern Russia, Korea, Thailand,
Borneo, Malaya, Philippines, Indonesia, and Australia.
To date, there have not been any confirmed reports that the soybean aphid is
present in either Indiana or Ohio, but given the geographic distribution of
the pest in Asia and the Pacific it should be assumed that the soybean aphid
will likely expand its current distribution in the mid-west. Current reports
from Michigan State indicate that the soybean aphid is moving eastward and may
already be present in northwest Ohio. Field observations of infestations in
Wisconsin are currently showing a decline in activity as the end of the growing
season approaches. If the aphid follows a pattern of behavior similar to that
in China, winged forms of the soybean aphid will develop to enable movement
to overwintering sites. In China, the primary overwintering site is the buckthorn,
a woody shrub that also has become well established in the north American hemisphere.
The bottom line of this story is that in the future we will have to deal with
a new addition to the soybean pest complex. Not only will this new pest cause
direct foliar injury to its soybean host crop, but it may act as a vector of
a number of plant virus diseases which is the case in Asia. The dynamics of
future infestations of this new aphid pest will likely be biologically controlled
in part by the range of aphid predators that routinely attack most aphid infestations,
but it may be assumed that this new pest probably arrived without the complex
of parasitoids that control it in Asia. Given the reports of infestations coming
from Illinois and Wisconsin, it is likely that chemical treatment may be warranted
in some cases. At the present time, no insecticides registered on soybeans will
be specifically labeled for treatment of the soybean aphid, but a number of
products registered on soybeans are known to be effective on aphids.
At the present time, the most important task will be to document the distribution
and movement of this new pest problem on soybeans. Anyone observing an unusual
infestation of aphids on soybeans with an accumulation of honeydew on the host
plant should collect a sample of infested stems and foliage, and submit the
sample to the nearest Extension office for identification and confirmation.
The soybean aphid is a small aphid ranging in color from pale green to pale
yellow. The cornicles (tailpipes) of this aphid are dark in color and the cauda
(tail like structure) is pale in color. Samples submitted to a local Extension
office should be forwarded to the OSU Extension Entomology office or the OSU
Plant and Pest Diagnostic Lab for identification. Samples initially received
by our office will be submitted to taxonomic experts for confirmation until
we become familiar with this new pest.
Soybeans with symptoms of both Sudden Death Syndrome and
Brown Stem Rot have been reported in numerous areas of the state. These are
two fungi which can infect and colonize soybean plants prior to flowering. Sudden
Death Syndrome is caused by the fungus, Fusarium solani f.sp. glycines. This
fungus is often found in fields with soybean cyst nematode. Both of these pathogens
can "knock the socks" off the soybeans but when they are together they really
hammer the plants and can cause substantial yield losses. To date - IN OHIO
- Sudden Death Syndrome is limited to a few fields and only to those areas in
the fields where the soil is compacted, along the roadsides and wet spots. Fusarium
will be in the soil for a long time, but SDS will only develop when the environmental
conditions are favorable. Soybean Cyst Nematode, will continuously rob plants
of yield. How to manage Sudden Death Syndrome in Ohio - 1) Manage Soybean Cyst
Nematode and 2) improve soil drainage. There are some varieties with resistance
and there is some data that indicates later plantings have reduced yield impacts.
Brown Stem Rot - is caused by Phialophora gregata, and is a residue-borne disease.
This fungus is appearing most often in our no-till, continuous soybean fields.
The classic diagnosis for brown stem rot is the chocolate-brown pith. However,
the fungus didn't read the fact sheet and the discoloration may be limited to
a few nodes or is not present at all. If you suspect brown stem rot, split 15
to 20 stems. The crowns will appear healthy compared to Sudden Death Syndrome.
Finally, there are fields in which both of these fungi have been found and we
have isolated both fungi from the same plant. In these situations, ROTATION,
tillage and increasing the drainage have been the primary recommendations.
Despite relatively cool conditions recently, much of the
early planted corn in SW Ohio and parts of West Central Ohio is rapidly maturing.
Corn growers planning to ensile corn should be monitoring corn fields closely
because their corn may be near or at the optimal stage for silage harvest.
Determining the proper time to harvest corn for silage is critical because whole
plant dry matter (DM) content varies with maturity and it influences fermentation.
Ensiling corn silage that is too wet produces poor fermentation, seepage losses,
and lowered animal intake. Ensiling excessively dry corn increases the risk
of heat damage and molding. Corn silage preserved between 30 and 40% DM generally
provides good fermentation and animal performance, but different storage structures
require different DM concentrations for optimal fermentation. Table 1 shows
the recommended target DM content for corn silage in different types of structures:
Table 1. Recommended Dry Matter Content for Corn Silage Stored in Different
Structures.
Structure %DM
========================= =======
Upright, Top Unloading 30 - 40
Upright, Bottom Unloading 40 - 45
Horizontal 30 - 35
========================= =======
The recommended DM content for upright, bottom unloading silos is higher to
ensure easier unloading. Horizontal silos require a lower DM content (higher
moisture content) to ensure adequate packing to eliminate oxygen and prevent
heating.
Observing the development of the corn kernel milkline has been suggested as
an easy way to estimate when corn is at the proper dry matter content for ensiling.
Generally, recommendations have been to harvest corn for silage when the milkline
is 1/2 to 2/3 of the way down the kernel. However, Ohio research has indicated
that there is a lot of variability in the relationship between the kernel milkline
and whole plant DM content. The milkline is not a very accurate or reliable
guide to gauge whole plant DM content. Hybrid, planting date, and growing season
can affect the relationship between kernel milkline position and whole plant
DM content. However, the appearance of the milkline in the upper 1/4 of the
kernel indicates that the crop is very near the optimal time to harvest. A sample
should be taken at this time and DM content determined with a commercial forage
moisture tester or microwave oven.
A limited sampling of plots last week at the Waterman Farm in Columbus indicated
that early planted (late April) corn was at the full dent stage of kernel development,
with some corn as far along as 1/4 milkline. I've received similar reports from
several seed company agronomists in SW and West Central Ohio.
Using a commercial forage moisture tester or microwave oven to determine the
DM content is the best way to accurately determine the optimal time to harvest
corn silage according to the storage structure to be used. And keep in mind
that waiting until blacklayer will almost always result in corn being too dry
for proper packing and fermentation, especially in horizontal and upright, top
unloading silos.
The persistent cool temperatures have favored the development
of several different corn leaf diseases. Nearly every commercial corn field
and garden plot has some level of common rust. The small red rust pustules can
be detected scattered over the leaf surfaces, with the lower leaves being more
heavily infected than the upper leaves. As long as the weather remains cool,
rust will continue to spread. This fungus can be very aggressive. It is capable
of producing new spores in as little as seven days after initial infection.
This short generation time indicates that disease severities can get quite high
in a very short period of time. It is much too late to apply a fungicide application
to control rust, and at the price of corn it is difficult to determine if an
application would pay. Yield loss estimates are quite variable and loss is related
to the percentage of leaf area affected on leaves above the ear and the growth
stage when the leaves become severely diseased. Estimates of reductions in yield
range from about 3 to 8% for each 10% of total leaf area affected.
Two additional cool-weather leaf diseases, eyespot and northern leaf spot, are
becoming prevalent in some fields. These diseases are usually more common in
Michigan and Wisconsin than in Ohio during normal years. Eyespot is recognized
as small (1/8 inch) diameter round spots with tan centers and a dark brown margins
scattered over the leaf surface. Northern leaf spot produces narrow linear lesions
with dark margins that extend along the leaf veins. Both of these diseases are
associated with crop residues and continuous corn. Both of these diseases will
increase as the season progresses. There are many good hybrids with resistance
to both of these diseases.
It is particularly important to note the severity of these diseases as the plants
approach denting growth stage. Premature death of the leaves will signal early
death of the stalk and the possibility of increased stalk rot and lodging. The
delay in maturation of the corn due to the lower heat units accumulated this
year will have an impact on when stalk rots will become important. Thus, it
is too early to make any predictions about stalk rots.
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: Hal Willson (Entomology), Ron Hammond (Entomology), Pat Lipps (Plant Pathology), Anne Dorrance (Plant Pathology), Erick DeWolf (Plant Pathology), Jeff Stachler (Weed Science), Peter Thomison (Corn Production) District Specialists: Ed Lentz (Agronomy) Extension Agents: Glen Arnold (Putnam), Steve Prochaska (Crawford), Clark Hutson (Seneca), Gary Wilson (Hancock), Howard Siegrist (Licking), Roger Bender (Shelby), and Andy Kleinschmidt (Van Wert).
Editor: Andy Kleinschmidt Web Editor: Tom Rosati
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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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