C.O.R.N. Newsletter 2009-35

Dates Covered: 
October 12, 2009 - October 20, 2009
Editor: 
Curtis E. Young

Check for Phomopsis Seed Rot

Delayed harvest due to rain can set the stage for Phomopsis seed rot. This is a fungal disease of soybean seed which is characterized by chalky white, shriveled, shrunken soybeans. For more information, visit The Ohio State University soybean disease web site at the following address: http://www.oardc.ohio-state.edu/ohiofieldcropdisease/soybeans/phomopsis.htm. There are three fungi that are associated with this disease, however the seed rot is predominately caused by Phomopsis longicolla. The other two fungi involved are better known for the black dots that they form on pods and stems, Diaporthe phaseolorum var. sojae and D. phaseolorum var. caulivora. All three fungi can be isolated from poor quality seed, but Phomposis is the one most commonly recovered.

Seeds can also be infected and not show symptoms, but many of these when put through the germination tests may die. On a more positive note, the mycelium in some of the infected seeds will die when the soybeans are dried for winter storage, and germination tests may improve somewhat over the course of the winter. Thus it is critical to harvest affected fields as quickly as possible (yes, I have read the forecast for this week and it is going to be miserable).

Both seeds and soybean debris are sources of inoculum for Phomopsis. Fungicide seed treatments can greatly help with the seed born inoculum. Fludioxonil is the only fungicide that will have a measurable effect in managing this disease for seed lots with low levels of infection (20-25%). The strobilurins and metalaxyl/mefenoxam are not effective against this fungus on the seed. There is also resistance to this pathogen. Studies from Missouri definitively showed that Phomopsis can readily be managed by having resistance in the varieties one is planting. If you have a field that has a history of high levels of the disease, be sure to work with your seed dealer to get the best resistant variety possible. Fields that are heavily impacted by Phomopsis should be harvested as quickly as possible to prevent further colonization of more seed and tilled to bury the infected residue. For no-tillers, rotate this field away from soybeans for a couple of years to drop the inoculum level.

It is Time to Sample for Soybean Cyst Nematode (SCN)

Soybean cyst nematode has become a more serious problem in Ohio because more and more fields are being planted to soybeans for multiple years and the nematode has been introduced into fields by machinery. In Ohio, in most years it is difficult to observe any symptoms on affected plants other than lower yields. When nematode populations are excessively high, or during years with drought, symptoms can include stunting and yellowing.

The best way to manage SCN is to know which fields have nematodes and how many nematodes are present. Fall is the best time to sample fields for soybean cyst nematodes. In the fall the nematodes have finished producing eggs. Sampling in the fall will give an estimation of the population level on which to base management decisions for planting next spring. Soil samples can be sent to some private labs or to The Ohio State University C. Wayne Ellett Plant and Pest Diagnostic Clinic. Fees for SCN testing will be: $15 per soil sample.

C. Wayne Ellett Plant and Pest Diagnostic Clinic
110 Kottman Hall
2021 Coffey Road
Ohio State University
Columbus, OH 43210-1087
614-292-5006
http://ppdc.osu.edu

SCN Soil Sampling Instructions:
1. Use a 1-inch diameter soil probe to collect soil samples (6-8 inches in depth)
2. Following a zig-zag pattern, collect 10-20 soil cores per 10-20 acres
3. Collect cores from areas of similar soil type and crop history
4. Dump cores from each 10 to 20 acre area into a bucket or tub and mix thoroughly
5. Place 1 pint (2 cups) of mixed soil in a soil sample bag or plastic zippered bag and label with a permanent marker
6. Store sample in cool, dark place until shipped to a lab doing SCN analysis.

At trace population levels, ranging from 40-200 eggs per 250 cc of soil, some yield loss may be detected when susceptible varieties are grown. If low populations are detected, ranging from 200-2000 eggs, it is recommended to plant a SCN resistant variety, but some yield loss may occur. Under moderate levels of 2000-5000 eggs of soil it is recommended to rotate to a non-host crop like corn, wheat or alfalfa next year and return with a cyst nematode resistant soybean variety the next time soybeans are planted in the field. When high levels of nematodes are encountered, such as 5000 or more eggs, the field should be rotated to a non-host crop for 2-3 years then the field should be re-sampled before planting a nematode resistant variety to ensure the nematode population has declined enough to successfully plant soybeans again.

Fall Weed Control and Black Cutworm Prevention

A few weeks ago, Mark Loux discussed fall weed control in an article titled "Timing Fall Herbicide Treatments Based on Weed Life Cycle" in Issue 32 of this C.O.R.N. newsletter, http://corn.osu.edu/story.php?setissueID=316&storyID=1918. A point we like to make at this time is to remind growers that fall weed control is an excellent preventive management tactic that can be taken against potential problems from black cutworm in corn the following spring. By providing a weed-free seedbed in the spring, the likelihood of black cutworm problems in the spring is significant lowered. The adult black cutworm moth migrates from southern locations each spring, and lays their eggs on weeds in the spring. Insect movement to corn occurs when the weeds are killed in the spring. Chickweed is perhaps the most well-known reservoir for cutworm eggs. Thus, a fall herbicide application not only rids the field of the weeds, but also can remove potential insect egg laying sites. When considering the benefits of a fall herbicide application, do not forget the added benefit of black cutworm management.

Corn Ear Rot: Which One is It?

Several different types of ear rot problems are showing up in some of Ohio’s corn fields this year, and lots of questions are being asked about these problems and how to tell them apart. Reported symptoms range from those typical of common ear rots to relatively uncommon symptoms such as blackish mold growth on the ear. It is important to identify ear rot problems before harvest because some ear rot fungi produce mycotoxins that are harmful to livestock.

Good indicators of potential ear rot problems are:

1. Wet weather conditions late in the season
2. Frost occurring before maturity
3. Corn standing in the field for an extended period in late fall
4. Delayed maturity (dry-down)
5. Bird and insect damage
6. Hybrid susceptibility
7. Ear drying-down in an upright position.

Generally, it is fairly easy to tell ear rots apart based on the color of the fungal growth on the ear, how the mold develops, and how the moldy kernels are distributed on the ear. The three most common ear rots in Ohio are Gibberella, Diplodia and Fusarium ear rot. Gibberella ear rot, is the most prevalent of the ear rots this year, however, we have also received reports of Diplodia ear rot in some fields. With Gibberella ear rot, the fungus enters the ear tips through the silk channel. A pinkish mold starting at the tip and progressing toward the base of the ear is very typical of this disease. Gibberella ear rot develops best when cool temperatures and frequent rainfall occur during the 3-week period after silk emergence. Hybrids that dry-down with the ear in a declined position are less likely to have problems with Gibberella and other ear rots.

Like Gibberella ear rot, Fusarium ear rot also causes pinkish discoloration of infected kernels, the fungus also enters through the silk, and the disease develops best when warm, wet weather occurs during the 2 to 3 weeks period after silking. However, with Fusarium ear rot, the pink moldy kernels are usually scatters all over the ear. As the disease develops, the infected kernels may become tan or brown or have white streaks. Ears that dry-down in declined position and those free of injuries and well covered with husks are less prone to Fusarium ear rot.

Diplodia on the other hand causes a thick white mold to grow on the ear, usually starting from the base and progressing toward the tip. Infection can begin before tassel emergence up to silking, and disease development is favored by wet weather and mild temperatures during early ear development. As the disease develops, the entire husk becomes bleached, covered with whitish-gray mold, and glued to the kernels. Infected ears are usually lightweight and may become rotten. Hybrids that dry-down with the ear in an upright position are more prone to Diplodia ear rot because the fungus can penetrate the husk through the base of the ear.

Symptoms of ear rot do not always appear on the outside of the husk. This is particularly true with late infections. So, to determine if you have an ear rot problem, walk fields, strip back the husks of about 50 plants spread out across the field and look for the following telltale symptoms:

Gibberella ear rot:
* Pinkish mold starting at the tip and progressing toward the base of the ear.

Fusarium ear rot:
* Pinkish or tan moldy kernels scattered on the ear.

Diplodia ear rot:
* White mold growth starting at the base and moving up the ear
* Husk glued to kernels
* Lightweight ears

Other important, but less common ear rots:
* Aspergillus ear rot: Yellowish-green to dark green powdery mold on the ear that typically develops under hot, dry conditions. This ear rot may lead to aflatoxin contamination of grain.
* Cladosporium ear rot: Dark, greenish-black streaks scattered over the ear. This ear rot is often associated with insect, hail or frost damage.

For more on ear rots and mycotoxins visit:
http://corn.osu.edu/story.php?setissueID=324&storyID=1927
http://www.oardc.ohio-state.edu/ohiofieldcropdisease/Mycotoxins/mycopagedefault.htm
http://www.ipm.iastate.edu/ipm/icm/2002/10-21-2002/molds.html

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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.