http://www.ag.ohio-state.edu/~corn/ |
|
For Week of May 25-31, 1998
C.O.R.N. 98-11
In This Issue:
A) The Little Green Menace is Back: Time to Monitor Potato Leafhopper
on Alfalfa
B) Black Cutworm Cutting Corn
C) Bean Leaf Beetle on Soybeans
D) Cereal Leaf Beetle on Small Grains
E) Where’s My Stand?: Seedling Blight Diseases
F) Wheat Disease Update
G) Do We Need to Worry About Stewart’s Bacterial Leaf Blight on Corn
During the past week, potato leafhopper (PLH) was observed in abundance in alfalfa. Prior to the past week, detection of PLH activity in alfalfa was minimal. Alfalfa fields planted in the early spring and regrowth of established stands recently harvested should now be sweep net sampled on a periodic basis to determine whether rescue treatment is warranted.
The optimal method for assessment of PLH on alfalfa continues to be the use of a sweep net. Ten sweep samples should be taken at three or more locations in a field. If the number of PLH per ten sweeps exceeds the height of an alfalfa stand expressed in inches, then a rescue treatment is warranted. Last week, I collected 15 PLH adults from a stand of 22 inch high alfalfa. Such a catch is not significant on a first cutting stand which is near harvest. However, if such a catch was collected from a new alfalfa stand or a second cutting with a height less than 15 inches, then a rescue treatment would be warranted.
In the case of stands of PLH resistant alfalfa, higher levels of PLH activity may be tolerated. In general, it may be assumed that PLH resistant alfalfa will tolerate about twice the level of PLH activity of non-resistant alfalfa.
While sampling alfalfa at this time, various stages of alfalfa weevil may be detected. Although, activity of alfalfa weevil will likely have concluded at this point in time, it should be noted that the warm spring conditions may trigger development of a partial second generation.
Corn is emerging statewide and early cutting and basal tunneling by black cutworm can be detected at this time. Since it has been a warm spring and weed growth was above normal, corn should be scouted to determine whether a rescue treatment is warranted. It should be emphasized that post-emergence treatments for cutworm will be most effective if an economic infestation is detected early.
Bean leaf beetle (BLB) has been very active on early planted soybeans. As more acreage of soybeans emerge, the beetles will likely disperse and the impact of BLB will diminish. Early stages of soybeans can tolerate a significant level of defoliation, but if stand losses appear evident, a rescue treatment may be warranted.
Reports have been received that cereal leaf beetle (CLB) is present in abundance in some areas on oats and wheat. Although CLB activity is generally controlled by beneficial parasites, economic outbreaks of CLB often occur following mild winters and during spring seasons associated with warm weather. Larvae of CLB appear like small slugs on the foliage since they accumulate their feces in a liquid on their backs. Early stages of larvae are about an 1/8 inch in size, where as late instar larvae are about 1/4 to 3/8 inch long.
Corn and soybeans that were planted in very wet, cool soils earlier in the year or plantings that were followed by very heavy rains may begin to show the effects of seed rot and seedling damping off diseases. These diseases are caused by fungi, mainly Pythium and Phytophthora. Both of these fungi require very wet, saturated soil conditions while Pythium prefers cool soils (around 50-55º F) and Phytophthora is most active around 60º F. Symptoms in the field at this time will first appear as areas in the fields where plants have failed to emerge, seeds will be rotted, or where the seeds did germinate, the roots will be brown and collapsed or missing. These diseases are managed by using seed treatments that contain Apron. Phytophthora root rot of soybeans is also managed by planting soybean varieties that are resistant to this fungus; either single Rps genes or partial resistance.
Soybeans that were planted in fields that have remained dry followed by intermittent rains may begin to show symptoms of Rhizoctonia seedling blight. This is another fungal disease in which the symptoms are a brick red sunken lesions of the hypocotyl at or below the soil line. Seedlings will die as the fungus girdles the hypocotyl, the seedlings will become chlorotic and will wilt as the lesions grow. Seed treatments which include PCNB, Agrosol or Rival are effective to only a limited extent for this fungal pathogen. Good cultural practices which include 1-2 year rotations, improved soil drainage, and adequate but not over fertilization may help reduce yield losses.
HEAD SCAB - Dry weather during the flowering period has not been favorable for head scab development. However, there may be certain areas of the state that may see some level of head scab. Symptoms of head scab should become visible within the next week in southern Ohio if it is present. It may be two weeks or more before symptoms become visible in fields in northern Ohio if it shows up at all. Overall, we expect only limited amounts of scab in the crop.
POWDERY MILDEW - Mildew has continued to develop on susceptible varieties because night time temperatures have been favorable for infections. High relative humidity and temperatures in the low 70º F range are most favorable. However, if mildew has not been detected in the field it will not be a problem this year.
STAGONOSPORA GLUME BLOTCH - Stagonospora (Septoria) nodorum will continue to spread within fields whenever there is significant rain showers. This disease is present on the leaves at some level in most fields in Ohio and it will continue to spread until plants begin to mature. The fungus will spread to the heads with continued rain showers. Most varieties have some degree of susceptibility. Look for brown areas on the glume tips of the heads. Severely affected heads will have distinct discoloration. Head infection leads to low test weights and shriveled grain.
BARLEY YELLOW DWARF - Plants with red to purple leaf tips can be seen scattered around some early planted wheat fields. Barley Yellow Dwarf affected plants may have an effect on overall yield if the number of plants affected is sufficiently high. We have no yield loss estimates for this virus disease, but the amount of yield loss is usually proportional to the numbers of plants affected and the severity of the symptoms.
WHEAT SPINDLE STREAK MOSAIC/ WHEAT YELLOW MOSAIC - Symptoms of this disease are beginning to disappear in some affected fields. Warmer temperatures generally cause remission of symptoms. However, plants with extreme yellowing may not recover. Remember to plant varieties resistant to this disease next year. There are no other reliable ways to control this virus disease.
Adult corn flea beetles are the vectors for the bacterium that causes Stewart’s leaf blight. In spring the beetle feeds on young corn plants and transmits the bacterium. Generally, high populations of flea beetles survive mild winters which results in higher than normal levels of the bacterial disease. Typical disease lesions can be seen on young plants in the field within a week or so after flea beetles begin to feed. Most field corn has moderate to high levels of resistance to this disease, especially in comparison to sweet corn and popcorn. However, there are some corn hybrids that may be damaged by the bacterial disease at a very young age. Look for long yellow stripes on the leaves associated with flea beetle feeding tracks. As stripes mature the tissue dies and turns brown. Significant leaf injury may lead to reduced yields. Spraying for control of flea beetles in field corn has usually not been economic, but if you are concerned with the level of damage in the field, contact your seed dealer to find out the level of susceptibility of the particular hybrid. The disease will continue to spread throughout the season on susceptible hybrids as long as the beetles remain active in the field.
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.
Contributors to C.O.R.N. this week include: State Specialists: Anne Dorrance and Pat Lipps (Plant Pathology), Hal Willson (Entomology); District Specialist: Curtis Young (IPMP); Extension Agents: Steve Bartels (Butler), Roger Bender (Shelby), Dave Jones (Allen), Larry Lotz (Fayette), Max Mohler (Lima Branch), Steve Prochaska (Crawford), Howard Siegrist (Licking), Barry Ward (Marion), Ray Wells (Ross), John Barker (Knox), Dennis Baker (Darke), Gary Wilson (Hancock).
Editor: Curtis Young Web Editor: Nathan Watermeier
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 |