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For Weeks of September 8 - 20
C.O.R.N. 98-26
In This Issue:
A) Now is the Time to Look for Corn Stalk Rots
B) Late Season Soybean Diseases
C) Second Generation Bean Leaf Beetle Are Active
D) European Corn Borer Injury is Very Low this Year
E) Winterize Your Forage Crops
The potential for corn stalk rots and lodging appear to be relatively high this year in different parts of the state. Weather conditions have varied greatly across the state during the growing season with some locations receiving adequate to surplus moisture and others being quite dry. Areas with adequate precipitation early in the season which fostered good early season corn growth with severe drought conditions during grain fill will see considerable stalk quality problems resulting from Gibberella stalk rot and anthracnose stalk rot. Other areas that were dry throughout the growing season will have mostly Fusarium stalk rot. Severe leaf damage due to leaf diseases like gray leaf spot or Stewart's bacterial leaf blight will predispose plants to higher levels of stalk rots. Regardless of the stalk rot disease affecting the field, the relative damage is the same: premature death of plants and weakened stalks prone to lodging.
Gibberella stalk rot is the most common and severe stalk rot. The Gibberella fungus enters the stalks via the roots and moves into the stalks when plants become stressed. The pith inside the stalks begins to disintegrate causing weakened stalks. Gibberella stalk rot can be identified by the pink discoloration of the pith when the stalks are split open. Anthracnose stalk rot is more common in continuous corn fields, especially those in reduced tillage. The anthracnose fungus survives in old corn residues and infects the leaves of the plants. Spores are rain splashed onto stalk surfaces where the fungus infects the stalks. Shiny black streaks or spots on the surface of the stalks is characteristic of anthracnose stalk rot. Fusarium stalk rot usually occurs in fields that have been stressed by drought conditions during grain fill. Plants die prematurely and the pith of the stalk disintegrates, but the pith usually remains white in color.
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Gibberella stalk rot. |
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Gibberella stalk rot. |
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Anthracnose stalk rot. |
As corn growers prepare for harvest, scouting fields is just as important as getting the combine ready. Plants that die prematurely are most prone to lodging. Look for plants with stalks that loose their green color before other plants in the field. Tear off the lower leaf sheaths to examine the stalk surface and squeeze stalks above the brace roots to see how rotted they may be. Fields should be assessed for stalk rot by examining a hundred or so plants per field. Randomly select plants, bend over and squeeze one of the lower internodes on each stalk. Record the number of soft stalks and plan to harvest those fields with the highest percentage of soft stalks before they lodge. Management of stalk rot diseases is through crop rotation, destroying old corn residues, avoiding stress through water and fertility management, and selecting hybrids resistant to lodging.
Symptoms of brown stem rot and Diaporthe stem canker can be seen now in a few locations. The most common symptom of brown stem rot is the chocolate brown to reddish discoloration of the pith. Severely infected plants may have foliar symptoms which are wilting, yellowing, and browning of the tissue between the veins. In addition, the outside of the stem has a grayish to green color which appears "greasy". This disease is caused by a fungus which can infect plants through the roots and then grows up through the xylem (water conducting vessels). This is a residue-borne fungus, so management strategies should include good crop rotations, tillage when disease severity is high, and use of resistant varieties.
Diaporthe stem canker or Northern stem canker can be found throughout Ohio at this time but is limited to very few plants in a field. Plants will appear wilted and leaves will be dried on the stems which appears very similar to another late season disease, Sclerotinia white mold. However, when the plants are examined there is a sunken, reddish-brown lesion on one of the lower nodes which girdles the stem. The area of the stem below the lesion will still be green which is a good way to separate this disease from Phytophthora stem rot. Stem canker is another fungus disease which also survives on soybean residue. In the southern parts of the United States, there is another biotype of this fungus that can cause significant yield losses. We do not have the right environmental conditions here in Ohio for the southern-biotype.
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Brown stem rot. |
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Diaporthe stem canker. |
In last week's issue of CORN, I indicated that 2nd generation bean leaf beetle should be underway and that assessment of potential pod injury may be warranted. Within hours of submitting the article to the CORN editor, a report was received from northwest Ohio that an increase in adult bean leaf beetles had been observed and that pod injury was occurring.
Assessment of potential pod injury by bean leaf beetle depends on a combination of
To determine the current level of pod injury, it is important that a number of plants be sampled to determine the proportion of pod exhibiting injury. Casual inspection of the pods in the upper canopy may lead to an over-estimate of pod injury, since beetles tend to feed more in the upper canopy. Seed injury will generally become evident as the proportion of pods exhibiting feeding scars exceeds 8 to 10 percent.
Evaluation of the relative abundance of adult bean leaf beetle activity generally requires a sweep net sampling. Significant pod injury often occurs when sweep net sampling collects five or more beetles per sweep. The potential for additional pod injury due to beetle feeding is also dependent on the period of time remaining to bean maturity. If the beans are drying rapidly and foliage is dropping, the beetles will soon be migrating to greener pastures. In contrast, if a field remains green and succulent for another week or two, the potential for additional pod injury is more likely if the beetles are present in abundance.
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Bean leaf beetle feeding on soybean pod. |
Throughout the past growing season, it has been generally recognized that both first and second brood corn borer injury was low this year. However, if one wants to know just how low corn borer activity has been, try splitting a few hundred stalks from a trial comparing Bt-corn and non-Bt corn. A recent stalk splitting exercise at the Western Branch generated about one cavity (and occasionally a larvae) for every ten non-Bt corn plants examined. In summary, both first and second brood corn borer pressure has been minimal this year. A third generation appears to be underway, but should not have any impact on field corn.
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European corn borer larva in corn stalk. |
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European corn borer adults. |
It's time to prepare forage crops for the coming winter by applying necessary fertilizer and making a timely final harvest. These two factors play a critical role in the winter hardiness of forages.
Early fall fertilization should be based on a recent soil test. Adequate potassium levels are especially critical for improving winter hardiness. Phosphorus should also be applied if levels are low.
A final harvest should be made by September 10-15 (this week). Forages, especially legumes, need a six-week period before a killing frost to accumulate carbohydrate and protein reserves for winter survival and subsequent spring growth. The fall rest period also enables plants to develop adequate cold hardiness.
Cutting during the fall rest period increases the risk of winter injury. The risk is greater when: soils are not well drained, soil fertility or soil pH are below optimal, and stands were stressed by excess water or leafhopper injury in 1998. Moderately winter hardy varieties and older stands are also more susceptible to injury. Frequent cutting during the summer increases the risk of winter injury from fall harvesting. For example, injury is more likely when cutting a 4th or 5th crop during the critical fall rest period than when a late 3rd crop is harvested.
A harvest during the critical fall rest period is always tempting when it promises to provide additional forage. But the gain this fall is often equal to the loss from reduced yields next year, and in the worst case scenario it can result in severe stand loss later this winter.
For related information see Factsheet: Fall Alfalfa ManagementReaders can subscribe electronically to this newsletter bysending an e-mail message to:
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 & Anne Dorrance (Plant Pathology), and Hal Willson (Entomology);
EXTENSION AGENTS: Larry Lotz (Fayette), Clark Hutson (Seneca), Barry Ward (Marion), Dennis Baker (Darke) and Greg La Barge (Fulton).
Editor: Clark Hutson Web Editor: Steve Lichtensteiger
Information presented above and where tradenames are used, they are supplied with the understanding that nodiscrimination 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.
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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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