http://www.ag.ohio-state.edu/~corn/ |
|
For Week of July 13-19, 1998
C.O.R.N. 98-18
In This Issue:
A) Monitor Corn Now for Gray Leaf Spot
B) Now is the Time to Scout for Soybean Cyst Nematodes!
C) Soybean Defoliators: Bean Leaf Beetle and Japanese Beetle
D) Detecting of Rootworm Injury
E) Rootworm Injury in First Year Corn Without First Year Corn Rootworm
F) Key Steps in Corn Pollination
Monitor corn fields for gray leaf spot now! Gray leaf spot has shown up in many corn fields in southern, eastern and north central Ohio over the past two weeks. Fields in continuous, reduced tillage corn are high risk fields. Initial symptoms of gray leaf spot are sometimes difficult to differentiate from other leaf diseases. In general, the first symptoms appear on the lower leaves as orange-red spots which enlarge to straight sided rectangular lesions with tan centers. It may take up to 3 weeks for lesions to reach their full size. Hybrids with some degree of resistance have yellow margins surrounding the lesions and these lesions are generally smaller than the elongated rectangular lesions on susceptible hybrids. Some hybrids produce both kinds of lesions such that the yellow lesions develop into the rectangular tan colored susceptible lesions over time.
As with most leaf diseases on corn, the earlier the upper leaves become diseased the greater the yield loss. Yield losses from gray leaf spot are greatest when the leaves above the ear become severely affected within 2-4 weeks after tasseling. If the upper leaves do not become infected until 6-7 weeks after tasseling yield losses usually will not exceed 10%. Significant early leaf damage may also predispose plants to stalk rot diseases causing severe stalk lodging by harvest.
Warm, humid weather will continue to favor gray leaf spot development. High levels of rainfall are not needed to spread the gray leaf spot fungus. The spores are spread by wind in the early afternoon. High humidity, heavy dew and fog or light rains are all that is necessary for disease spread. In general a minimum of 13 hours of leaf wetness is necessary for infection. A week or more of fog each morning will initiate a severe epidemic. The weather conditions that occur over the next month will have a direct effect on the level of gray leaf spot by harvest.
Foliar fungicides are recommended on hybrid dent corn only in fields with a history of yield losses from gray leaf spot. Of the fungicides labeled for use on hybrid corn, Tilt is the most effective against gray leaf spot. Tilt (4 fl. oz./ A) can be applied through the silking growth stage. Best time for application is at beginning silk when disease is detected on the lower leaves. Apply at least 20 gpa for ground application or 5 gpa if by air. Ground equipment should use hollow cone nozzles with at least 30-40 psi to provide adequate coverage of all leaves, especially those from the ear leaf up. Tilt will slow down the development of the disease during the first three weeks after application. Yield increases from a Tilt application on susceptible hybrids have ranged from 3-25 bu/A in 1996 and 1997.
Soybean cyst nematode females can be seen on roots of affected plants. The cysts at this time of year are white to cream colored and can be found on the roots. When scouting for cysts, carefully DIG up the soybean plants, gently remove the soil from the roots. The cysts are much smaller than the nodules (nodules are also pink in the interior). The best places to search for soybean cyst nematode are fields with consistently low yields, along fence lines, previously flooded areas, field entryways and low spots. Fields in which the soybeans are stunted, yellow and appear to have delayed development also make excellent candidates for inspection.
Soybean cyst nematodes have several life stages. The eggs which are carried in the cysts, and juveniles are very small. A dissecting scope is needed to identify them. The females, however, can be seen feeding on the roots. Soybean cyst nematode females are sedentary feeders and as the nematode is feeding, the eggs are developing within the body cavity. At the end of the season the female body becomes hard and tan colored. This protective covering allows the survival of eggs over numerous growing seasons.
Fields in which soybean cysts nematodes are found now should be followed with a soil sample in the fall. The best time to take soil samples to determine soybean cyst nematode population numbers is in the fall after the crop is finished. Crop rotation and the use of resistant varieties is the best means to manage this soybean pest, but disease management recommendations are based on the population levels that are present in the soybean cyst nematode infested fields.
Soybean cyst nematode is the number one soybean pest in many of the north-central states and is becoming increasingly important in Ohio. Research indicates that under our soil conditions, soybean cyst nematode is present but the only symptoms are reduced yield. Once above ground symptoms (stunted, yellow, poorly developed soybean plants) become visible, cyst nematode populations are very high and that field is lost to soybean production for several years. Soybean cyst nematode is a pest that you will be hearing more about over the summer. There is a project supported by Soybean checkoff dollars through the Soybean Cyst Nematode Coalition and the North Central Soybean Research Program which promotes soil sampling to identify fields with soybean cyst nematode populations. The Ohio Soybean Council, through soybean check-off dollars, will be supporting soil sample processing this fall.
First generation bean leaf beetles are beginning to appear in soybean fields and Japanese beetles appear to be more abundant than normal. As soybean plants enter the reproductive stages of development, average defoliation of soybeans should not be allowed to exceed 15%. First generation bean leaf beetle activity will most likely only be abundant in early planted fields, which were colonized by the overwintering generation.
In the case of Japanese beetles, one should recognize the fact that populations tend to be highly aggregated and injury will appear heavy in one location while minimal in other areas of the field. Thus, it is important to evaluate defoliation on a random basis when Japanese beetle is the primary defoliator. In addition, it should be noted that Japanese beetle activity may be concentrated along the perimeter of a field along with grasshopper activity. Thus, one should not make an assessment of defoliation based on observations along the edge of a field.
During the past week, rootworm feeding activity on corn root systems has almost terminated, larvae have pupated, and adult rootworm beetles have begun to appear in corn fields in which they have emerged. Dispersal to a corn or soybean field from the sites of emergence is not likely to occur until corn silks in the fields of origin have begun to turn brown.
Where significant rootworm injury has occurred, extensive growth and regeneration of root systems may mask the presence of rootworm feeding injury. However, where rootworm injury has been significant, lodging (or goose-necking) or corn plants should be obvious. Lodging of corn plants having sustained significant feeding injury may or may not readily be associated with rootworm injury depending on climatic factors - especially the soil moisture condition occurring at the point when rootworm injury is most severe. However, given the rainy conditions we experienced during late June, most root systems sustaining a level of injury of a root rating of 4 (where an entire node is missing on the Iowa 1 to 6 scale) are currently exhibiting goose-necking symptoms.
The incidence of lodging (or goose-necking) detected in a corn field can be used as an indicator of the degree of rootworm injury in a field. When rootworm injury averages around a root rating of 3, about 25% or more of the plants will exhibit a root rating of 4 or more which will result in visible lodging. Thus, if about 1 in 4 plants exhibit lodging symptoms, then one may expect that the average level of root injury is about 3, which is regarded as an economic level of injury. When the incidence of lodging is reduced to about 5% or less of the corn stand, the average level of rootworm injury in the field is likely to be a level of 2.5 or less, which is generally not regarded as economic.
When using lodging as an indicator of rootworm injury, it is important that the cause of the lodging be confirmed. Lodging of corn may also be caused by extreme weather conditions and root system injury due to herbicides. Lodged plants having severe rootworm injury should exhibit a loss of root system equivalent to one node of roots due to rootworm feeding or equivalent.
In regard to detection of first year corn rootworm that is currently migrating Eastward across Ohio, now is a good time to be inspecting first year corn fields for signs of rootworm injury. If significant rootworm injury has occurred in a first year corn field, then lodging and associated root feeding symptoms should be readily visible.
For additional information on insects in corn visit Bulletin 545: Insect Pests in Field Crops – Corn an OSU Extension publication on problem insects.
Rootworm injury in first year corn lacking the presence of first year corn rootworm may be detected. Based on past field surveys, we have assumed that the first year corn rootworm has not reached our research site at the Western Branch station in Clark County. However, research protocol for an experiment that we are currently conducting on first year corn required that we conduct root ratings on a series of treatments (with and without soil insecticide treatment on three different corn hybrids). The experiment represented our best looking set of plots and not a single plant exhibited any signs of lodging. However, after digging, washing and inspecting 180 root systems in this particular experiment, rootworm injury was detected. Injury on most of the root systems exhibiting rootworm symptoms was barely detectable. However, the presence of obvious rootworm injury on two plants (out of 180) having a rootworm injury level of 3 (one or more roots chewed down to within 1 1/2 inches) confirmed the fact that the injury symptoms we were observing were due to rootworm feeding activity.
The presence of low levels of rootworm injury in a vigorous plot of first year corn following soybeans may be interpreted as an early sign of first year corn injury. However, the presence of such low levels of injury may also be due to a low level of extended diapause (eggs overwintered 2 winters instead of 1) of northern corn rootworm or due to migration of western corn rootworm from adjacent continuous corn plots during the previous growing season. The primary point for mentioning this observation is that if one looks hard enough, one can detect a presence of rootworm activity in a first year corn field, and such low levels of injury may be due to factors other than the presence of first year corn rootworm.
For additional information on corn rootworm control visit Bulletin 545: Insect Pests in Field Crops - Corn Rootworm Larval Control.
The flowering stage in corn is the most critical period in the development of a corn plant from the standpoint of grain yield determination. Drought, high temperature stress, as well as hail damage and insect feeding have the greatest impact on yield potential during the reproductive stage. The following is an overview of key steps and phases in the corn pollination process.
Readers 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.
p>Contributors to C.O.R.N. this week include: State Specialists: Ann Dorrance (Plant Pathology), Pat Lipps (Plant Pathology), Mark Loux (Weed Science), Jeff M. Stachler (Weed Science), Peter Thomison (Corn), and Hal Willson (Entomology); District Specialists: Ed Lentz (Northwest Agronomist); Extension Agents: David A. Jones (Allen Co.), Steve Prochaska (Crawford Co.), Larry Lotz (Fayette Co.), Gary Wilson (Hancock Co.), Howard Siegrist (Licking Co.), Ray Wells (Ross Co.), and Rodger Bender (Shelby 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.
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.
TDD # 1 (800) 589-8292 (Ohio only) or (614) 292-1868
| C.O.R.N. | Newsletter | Archive | Search | Questions? | Ohioline | Publications |