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For Month of January, 1998
C.O.R.N. 98-1
In This Issue:
A) What those disease ratings mean in the OSU-Soybean
Performance Trials-Sclerotinia or White Mold
B) What those disease ratings mean in the OSU-Soybean
Performance Trials-Phytophthora
C) First Year Corn Rootworm and Insecticide Treatment
D) Did 60 in January hurt wheat?
Producers will see an additional column in the 1997 Ohio Soybean Performance Trials report (Hort & Crop Sci Series 212) listing a Sclerotinia rating for soybean varieties. This rating was determined from growth chamber studies where cotyledons of 10 day old seedlings were inoculated with the fungus. This is a very severe test where many of the physical factors that help plants escape disease are bypassed. The relatively high disease ratings from these growth chamber evaluations also indicate the severity of these tests in that all of the varieties tested were susceptible. However, some soybean varieties were able to limit the extent of infection.
The mean disease ratings of the 80 varieties tested from the 3 growth chamber studies were ranked based on the percent seedlings killed and then given a rating where 1=cultivars ranked 1 to 10; 2=cultivars ranked 10.1 to 20, 3= cultivars ranked 20.1 to 30 and 4 = cultivars ranked 30.1 to 40. The ratings for varieties ranking higher than 40 are not presented because they were not evaluated or there values ranked above 40. The exception was the public lines which were used as standards to represent all levels of disease severity in these evaluations.
What do these rankings mean as far as field performance? We have some data, mainly with the public lines, that there is some correlation between growth chamber data and the field. Soybean varieties Vertex, OHFG1, and Defiance had low percent white mold in the field and the growth chamber assay. However, Stressland and General had moderate levels of white mold in field studies, but had very high levels of disease in the growth chamber test. Williams 82 was the standard susceptible variety which had very high levels of white mold in both the growth chamber and field studies. Varieties that were ranked 1 to 4 should have less disease than a more susceptible variety under similar disease pressure and environmental conditions as indicated by field evaluations in 1996 and 1997. How well these growth chamber studies predict performance in the field is an area that will be evaluated over the next few years.
As many of you know resistance to Phytophthora root rot is expressed in two ways. One is race specific, in which a specific Rps gene limits infection of only those races of Phytophthora sojae that cannot recognize it. Once an isolate can recognize the Rps gene, it is no longer effective and that soybean variety is very susceptible to infection. Thus either the resistance gene is highly effective or not effective at all against a certain race. The second way in which plants resist disease is to limit the amount of root area affected. These plants will have some roots that are attacked, but it takes a much longer period of time for those roots to turn brown thus a variety with no partial resistance. This type of resistance is called tolerance or partial resistance and it is effective against all races of Phytophthora.
For soybean producers that want to utilize specific Phytophthora resistance the Performance Trial report lists the resistance gene available in the variety tested: A resistance rating of 1 = no resistance, 2 = Rps7, 3 = Rps1-a, 5 = Rps3-a, 6 = Rps1-c, 7 = unspecified gene controlling a few races, 9 = Rps1-b, 10 = Rps1-k, 20 = unspecified genes controlling all races. Most soybean fields in Ohio have Phytophthora races that are controlled by varieties with resistance numbers of 9 or higher. Varieties with numbers of 5 and 6 can be attacked by most of the races and do not provide complete control. Numbers of 1, 2 and 3 provide no effective control in Ohio. A number of fields were identified this past summer which had races of Phytophthora that could attack the Rps1-k gene. If growers have had Phytophthora problems even when using Phytophthora resistant soybean varieties should use soybean lines with a high degree of partial resistance or tolerance.
To determine the level of partial resistance or tolerance of a soybean variety, the seeds are planted into a medium that has a race of the fungus known to recognize it. These plants will develop root rot, what the partial resistance assay tells us is how much root rot will develop. Ratings of 1-3.9 can be considered highly tolerant, and will provide good control in the field. Ratings of 4.0 to 4.9, can be considered moderately tolerant, and will provide some measure of control. Ratings over 5 can be considered as having little or no tolerance and will provide no effective Phytophthora control.
One of the best disease management options is to use soybean varieties that combine both specific resistance (Rps genes) and partial resistance into the same soybean variety. In addition growers can use Apron FL and Apron XL seed treatments at the higher label rates, tillage and improve soil drainage to limit the losses from Phytophthora root rot.
A biotype of Western Corn Rootworm has adapted to a corn/soybean rotation by laying eggs in soybean fields and hatching the following year in first year corn. Where such a strain of WCR exist root injury from feeding maybe similar to injury found in continuous corn. In 1996, activity was reported by OSU entomologist in Shelby and Mercer Counties. In 1997, additional sites in Paulding and VanWert Counties were identified. This would follow a eastward pattern of movement for this biotype from Illinois through Indiana and into Ohio over the past several years. Additional sites of first year CRW activity in Butler, Darke, Henry and Preble County are reported. Sites monitored in 1997 and found not to have the problem were Miami, Clark, Madison, Crawford, Marion and Delaware Counties.
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Western Corn Rootworm Beetle |
In Ohio, it is clear that first year corn rootworm has moved into counties west of I-75. Since first year corn rootworm requires a couple of years to become established and build up to economic levels, use of soil insecticides on corn following soybeans should be delayed until presence of the problem in an area can be confirmed. The following techniques can be used to verify first year CRW potential.
Verification of root system injury
However, as populations of first year corn rootworm multiply and diperse, preventative use of soil insecticides on first year corn will be warrented in the next couple of years. Growers have the following strategies to choose from based on risk in their area.
Marginal Risk Action (Problem not identified in county or area)
Minimum Risk Action (Problem identified near your location)
It is not unusual for wheat to have periods of green-up during the winter months. Wheat does not become fully dormant, so green-up will occur whenever temperatures stay above freezing (like the first part of the January). The tops will die-back again when the temperatures stay below freezing. Generally, periods of green-up will not cause stand reduction unless temperatures suddenly drop from 40 - 50 degrees to below 15 degrees. Snow cover would buffer a sudden temperature drop. However, each green-up event uses stored food reserves, and if these reserves become depleted before spring, some stand reduction may occur. In most cases, wheat that was planted at the right time and depth will have the reserves to survive several green-up periods during the winter months.
A more serious concern are the periods of standing water and ice that occur after the warm conditions. The standing water and ice may prevent plants from getting adequate oxygen and cause some stand loss. The damaged plants that survive may be more susceptible to various soil-borne diseases. It is too early to determine the damage caused by the standing water and ice, but a grower may want to mentally mark these fields for spring evaluation.
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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: Hal Willson (State Extension Specialist, Entomology), Peter Thomison (Extension Agronomist, Corn Production), Jay Johnson (State Extension Specialist, Soil Fertility), Jeff M. Stachler (Extension Associate, Weed Science), John Barker (Knox Co.), Greg La Barge (Fulton Co.), Troy Putnam (Highland Co.), Howard Siegrist (Licking Co.), Tom Krill (Van Wert), Max Mohler (Hardin Co.), Gary Wilson (Hancock Co.), Ray Wells (Ross Co.), Larry Lotz (Fayette Co.), Dennis Baker (Darke Co.), Barry Ward (Marion Co.), Dave Jones (Allen Co.) & Clark Hutson (Seneca Co.).
Information presented above and where trade names are used, they are supplied with the understanding that no discrimination 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.
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