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For Week of July 27-August 2, 1998
C.O.R.N. 98-20
In This Issue:
A) Identifying Kernal Development Stages in Corn
B) Disease Progression in Corn and Soybeans
C) Wheat Seed Quality and Seed Treatments Materials
D) Discriminating Between: Corn Rootworm or Weather Injury
E) Bean Leaf Beetle and Other Defoliators on Soybeans Update
F) Potato Leafhopper on Alfalfa Update
Most of the corn in Ohio planted in April or early-to-mid May has reached or passed through the pollination stage. Following pollination, kernel development (or grain fill) is the most critical period in the development of the corn plant for the determination of grain yield. Kernel development proceeds through a number of stages which have been characterized by such terms as blister, milk, roasting ear, soft dough, dent, etc. Since these descriptive terms can sometimes be difficult to interpret, alternative systems have been proposed. A staging system widely used by agronomists and crop consultants divides kernel development into 6 stages, designated numerically as R1, R2, through R6. The table below lists kernel developmental stages in sequence and provides a brief description of each phase.
Kernal Development in Corn| Stage* | from Silking |
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| Silking (R1) | no visible blisters |
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| Blister (R2) | abundant fluid |
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| Milk (R3) | white fluid, no solids yet (true roasting ears) |
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| Soft Dough (R4) | edible), no visible denting |
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| Dent (R5) | dented or denting |
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| Black Layer (R6) | kernel moisture 27-32% |
The corn crop continues to develop rapidly with reports of growth stages ranging from late pollination to the milk growth stage. Most leaf diseases of corn do not develop until after these stages. Stewart's bacterial leaf blight and anthracnose usually show up early, when the plants are small and disease development remains slow until sometime after pollination. Leaf diseases like gray leaf spot and northern leaf spot (Carbonum leaf spot) usually progress slowly throughout the tasseling/pollination growth stage. Warm humid days and morning fog will promote the continued spread of gray leaf spot. The usual increase in leaf diseases as the season progresses is due to general senescence of the corn plant and translocation of nutrients from leaves to the developing kernels. Over the next few weeks look for increased development of gray leaf spot, northern leaf spot and Stewart's bacterial leaf blight. Many fields had high levels of Stewart's bacterial leaf blight early in the season. You may expect the disease to continue to spread within the field as insects (corn flea beetle) remain active. Susceptible hybrids will have more damage than more resistant hybrids. Gray leaf spot is still the number one threat to corn this year.
Over the next few weeks a number of soybean diseases may become evident. Plants that were affected by Phytophthora root rot earlier in the season will continue to wilt in the field as roots die. We usually see the first plants affected with Sclerotinia white mold during the first two weeks of August. You will notice a few tall erect plants wilting in the field. Examine the stems for white mold near the lower nodes. Stunted or yellow plants may be an indication of Soybean Cyst Nematode (SCN). The white female cysts can be seen on the roots of soybean plants during the first few weeks of August. Dig plants carefully and gently remove the soil. White female cysts can be seen in contrast to the larger pinkish Rhizobium nodules. As the month progresses we will be looking for plants affected by various root infecting pathogens. These include brown stem rot and sudden death syndrome. These root diseases cause various symptoms, but brown blotches between the veins on the leaves are a good indicator of one of these root problems. Watch soybean fields over the next few weeks for development of disease problems. The earlier any of these diseases show up the greater the yield loss.
The favorable growing conditions, relatively high yield and good test weights from this year's wheat crop indicate that wheat seed quality should be good too excellent this year. Although seed is of high quality, we still recommend all seed be treated with a seed treatment fungicide prior to planting this fall. Seed treatments help protect against the common seed-borne and soil- borne diseases found in Ohio. Seed treatments control the important diseases and help insure rapid and even emergence of seedlings. The improved health of the seedlings also helps insure overwinter survival and rapid growth in the spring. Overall good health of the crop is essential for highest yields.
Last year we saw the introduction of two new seed treatment products on the market; Dividend XL and Raxil-thiram. Both of these products contain systemic fungicides that provide excellent control of seed-borne smut diseases (loose smut and common bunt). In tests at OARDC, both products have also provided good control of seed-borne Stagonospora nodorum that causes a significant leaf and glume blotch disease in our state. In years when Fusarium head scab is a significant problem, we would recommend the addition of thiabendazole (TBZ sold as Gustafson LSP) to the seed treatment slurry. However, this may not be necessary this year because head scab was not a problem in Ohio.
These seed treatment products should be applied by the seed processor with modern equipment to ensure the seed is thoroughly covered with the correct rate. Coverage can be assessed by the grain grower when viewing seed in the bag or bulk ben. Each seed should be evenly covered with the red indicator dye. Poor coverage is an indicator of poor application and may result in poor performance of the fungicide treatment.
Severe weather storms crossing Ohio during the past week have caused sporadic injury to many corn fields resulting in lodged corn and a lot of stalk breakage. Where such storm injury has occurred, field personnel should not assume rootworm problems unless significant rootworm injury is found to be associated with lodged corn. Where rootworm has caused lodging, rootworm injury at a rating level of 4 should be readily detected. Such a level of rootworm injury will exhibit root injury equivalent to the loss of an entire node of the root system and goose-necking of the lower stalk that likely occurred in late June or early July.
Corn silks in many fields are currently turning brown and migration of adult rootworms from fields in which they have emerged should become evident during the next week or two. Thus, if one is currently monitoring adult rootworm activity in soybeans with either yellow sticky traps or sweep-net sampling, the adult rootworm beetles should begin appearing in soybean fields if first year corn rootworm behavior is exhibited. In soybean fields without first year corn rootworm activity, collection of western corn rootworm (WCR) beetles on sticky traps will be minimal - especially on the Pherocon AM traps, and catches of the beetles in sweep net samples will also be minimal. However, if two or more WCR adults are accumulated per day on Pherocon AM traps stationed in soybean fields or sweep net collections of WCR adults average one or more WCR adults per sweep on soybeans, then one may assume that an unusual level of adult rootworm activity is occurring in a soybean field.
Soybeans are beginning to enter early pod development stages and defoliation should not be allowed to exceed 15% (assuming that the defoliation estimate is based on a random inspection of foliage). Since many soybeans were planted late this season, first generation bean leaf beetle (BLB) activity is unlikely to be very severe this year. However, given the above normal temperatures this summer, a significant second generation of BLB could develop by late summer.
Leafhopper feeding continues to be a problem this summer on alfalfa. Untreated check plots in the 2nd cutting of our new alfalfa seeding are currently exhibiting significant hopper burn symptoms and the height of the untreated plots is visibly less than that of the treated plots, which exhibited minimal leafhopper activity 14 days after treatment. In general, most treatments for leafhopper are very effective if applied on a timely basis. Differences among treatments can be observed in regard to effective residual activity and resurgence of leafhopper activity, but all treatments significantly reduce leafhopper development and protect stand development.
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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 (Plant Pathology), Jeff M. Stachler (Weed Science), Peter Thomison (Corn), and Hal Willson (Entomology); Extension Agents: David A. Jones (Allen Co.), and Greg LaBarge (Fulton Co.).
Editor: David A. Jones - 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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