C.O.R.N. Newsletter 2007-11

Dates Covered: 
May 1, 2007 - May 7, 2007
Steve Prochaska

Understanding Phytophthora and Soybean Cyst Nematode Resistance in Soybeans

Authors: Anne Dorrance

Questions have been raised on how to assess what races of soybean cyst nematode (SCN) and/or Phytophthora are present in specific farmer’s field. Back 20 or so years ago, tests would show that only 1 or 2 races of Phytophthora were present in many Ohio fields. In the last screening for Phytophthora, more than 50 races were identified in one field and for SCN, there were more than 10 races. These tests are very expensive and in reality only provide a snapshot of the variability that we are detecting in these fields. The best information on how a soybean variety is performing is in the yield column of your annual field records.

For SCN in particular, one should be very aware of which fields have this problem through historical yield records, soil testing for SCN and varieties planted. Follow historical yields of SCN resistant varieties on your farm or in your area. Are SCN resistant varieties reaching their true yield potential or are they falling behind what your yield records indicate for a good soybean yield (for a specific field)? If yields fall short of your records, then nematode populations may be adapting to the set of SCN resistant genes being used by the soybean variety.

Almost 95 percent of the SCN varieties available in the state are developed from one source of resistance, PI88788. Interestingly, this source has 4 to 6 genes that contribute to resistance. When a variety is developed, only 2 to 3 of these genes may make it to the final variety. Another tactic in managing SCN with resistance is to plant a resistant variety from the 88788 source, but use one that has a different set of the resistant genes. One will have to work with a seed dealer, to be sure a different set of genes is used as opposed to a sister line.

Recall that SCN does reproduce on the winter annuals. If your SCN populations are creeping up, be sure it is not from the winter annuals in your fields. If SCN numbers are going up, your resistant package may still be effective. Again check your yield history.

If you have any doubts about your SCN resistant varieties – sample and get a nematode count. The SCN fact sheet provides guidelines for numbers to determine management programs http://ohioline.osu.edu/ac-fact/0039.html . Fall is the best time to sample, but you can sample at any time, just add 10% to the number for winter kill.

For Phytophthora, even though there were over 50 races in one sample field, Rps1k provided excellent resistance to more than half of the sample field as would have Rps1c and/or Rps3. Our general recommendation for Ohio farmers, be sure your soybean variety has a resistance gene of either Rps1k and/or Rps1c and/or Rps3 and/or Rps6 (as these will provide good protection against most P. sojae populations that are present in many Ohio fields), and combine these resistance genes with high levels of partial resistance (also called field resistance, tolerance and general resistance). Varieties with the combination of a single resistance gene with high levels of partial resistance provide the best yield protection over a number of Ohio field environments.

Air Temperature and Corn Emergence

Authors: Peter Thomison

According to the USDA-NASS http://www.nass.usda.gov/ as of Sunday, April 29, corn planted in Ohio was at 19 percent, 4 days behind last year and 6 days behind the five-year average. Cooler and wetter than normal weather has slowed corn planting down considerable compared to recent years. What impact will these conditions have on corn that has already been planted?

In past years, we have observed that early planted corn that was in the process of germinating or as far along as the V1 stage (one leaf collar visible) survived freezing temperatures in late April with little impact on crop performance or plant stand. Agronomists generally downplay the impact of low temperature injury in corn because the growing point is at or below the soil surface until V6 (six leaf collars visible), and thereby relatively safe from freezing air temperatures. However when dry corn seed absorbs cold water as a result of a cold rain or melting snow, “imbibitional chilling injury” may result. Cold water can cause similar injury to seedling structures as they emerge during germination. Such physiological injury was widely observed in 2005 when early planted corn in various stages of germination and emergence was subjected to a period freezing rain and snow followed by temperatures at or below 50 degree F for about 10 days. What we’ve experienced thus far in 2007 is mild in comparison to 2005.

To assess the impact of these freezing temperatures on emerged corn, check plants about 5 days after the freezing injury occurred (and preferably when growing conditions conducive for regrowth have occurred). New leaf tissue should be emerging from the whorl. You can also observe the condition of the growing point (usually located ½ in to 3/4 in below the soil surface) by splitting seedlings lengthwise. If the growing point appears white to light yellow and firm several days after the frost, prognosis for recovery is good.

Of greater concern with regard to the viability of germinating and emerging corn is how long soils will remain saturated. Cool temperatures and wet weather provide the right conditions for the development of seedling blight diseases. Cold temperature injury can play a significant role in predisposing plants to root infection and blight. Under normal conditions plants can continue to grow and produce new roots, but when other injuries occur, new roots cannot develop rapidly and Pythium and other soil fungi can kill stressed plants. Seed treatment fungicides generally remain effective from 10 to 14 days but under saturated conditions the duration of protection may be shorter.

For more detailed information on corn germination and emergence, I’d encourage you to check out a series of excellent articles (noted below) which Dr. Bob Nielsen, my counterpart at Purdue University has recently written. These articles include great photos that will assist your understanding of these growth and development processes.

Nielsen, RL (Bob). 2007. Germination Events in Corn. Corny News Network, Purdue Univ. [On-Line]. Available at: http://www.kingcorn.org/news/timeless/GerminationEvents.html . (URL verified 4/23/07).

Nielsen, RL (Bob). 2007. The Emergence Process in Corn. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/timeless/Emergence.html . (URL verified 4/23/07).

Nielsen, RL (Bob). 2007. Requirements for Uniform Germination and Emergence of Corn. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/timeless/GermEmergReq.html . (URL verified 4/23/07).

Nielsen, RL (Bob). 2004. Corkscrewed Corn Seedlings. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/articles.04/Corkscrew-0501.html . (URL verified 4/23/07).

Soybean Germination

Authors: Jim Beuerlein

The amount of time required for soybeans to germination and emerge is controlled by soil water content and soil temperature. As we enter the month of May there is adequate water for the seed to adsorb for germination, leaving soil temperature as the factor controlling the amount of time between planting and emergence. Currently the soil temperature is in the 50-55 degree range at the 2-inch depth. The time to emergence at that temperature is 14 to 20 days. However as the soil dries, the soil temperature will rise rapidly and shorten the time to emergence. For a soil temperature of 60-65 degrees, the time needed for emergence is 7 to 10 days.

It is Time to Start Scouting for Wheat Diseases

Authors: Dennis Mills, Pierce Paul

It is time to start scouting wheat fields for leaf diseases. The wheat is now between Feekes growth stages 6 and 8. Don’t be deceived by the height of the plants. Given the cool temperatures we have had, plants may be shorter than normal; however, crop development may still be close to normal for this time of year. Feekes growth stage 8 is early flag leaf emergence. A mature wheat tiller has four nodes on the stem that correspond to the presence of four main leaves. The top leaf is the flag leaf. Flag leaf emergence occurs in Ohio in late April or early May depending on location and planting date.

To determine growth stage, pull some of the larger tillers from various locations in a field. Strip the lower leaves off the stem to expose the lower portion of the stem. The first node is usually from a half inch to three inches from the stem base. Locate this first node and then count the number of leaves above this node. At Growth Stage 8 the flag leaf is just emerging at the top and there will be three leaves between the flag leaf and the first node. If only three leaves are counted on the stem then the wheat is at an earlier growth stage, probably at Growth Stage 6 with one node detectable or Growth Stage 7 with two nodes detectable. Even if the crop is still at growth stages 6 or 7, especially in Northern counties, it should reach growth stage 8 toward the end of this week, if it temperatures remain warm. At this time of year the time between growth stages can be only a few days when the soil temperature is above 55 F and the air temperature is in the 60 F to 70 F range.

Growth stage 6 to 8 marks the beginning of the period during which we recommend that fields be scouted to determine which disease is present and at what level. Although no significant disease problems have been reported in the state so far, leaf diseases may soon begin to show up on susceptible varieties in some fields. It would be wise to begin to visit fields at flag leaf emergence (growth stage 8) to check for the presence of diseases like powdery mildew. Fungicides are available to control foliar diseases in Ohio; however the decision to use these fungicides should be based on the susceptibility of the variety planted, the level of disease in the field, weather conditions, the yield potential of the field, and the market price of wheat. So, as you scout the fields, make a note of the level of disease and return to the field in the next week or so to see if disease is progressing. If disease continues to progress up the plants a fungicide application may be warranted. This is extremely important if you are growing one of the more susceptible varieties currently available. Go to the following website for information on wheat variety susceptibility to various wheat diseases: http://www.oardc.ohio-state.edu/ohiofieldcropdisease/wheat/wheatreactions2006.htm .

2007 Conservation Tillage & Technology Conference (CTTC) Presentations Available

Authors: Jim Hoorman, Gary Wilson

Over 684 participants (farmers, consultants, and agency personnel) attended the 2007 CTTC. Power-point presentations from the speakers are now available at the following website: http://limacenter-cms.ag.ohio-state.edu/jim-hoorman/cttc/2007-conservation-tillage-technology-ctc-conference-powerpoint-presentations .

In addition, the 2007 CTTC Impact Report from a survey of 184 participants is available at the same web page. The 2008 CTTC will be held February 21-22 at Ohio Northern University, Ada, Ohio.

Soybean Rust –No New Finds

Authors: Anne Dorrance

Due to late freezes in Georgia and Florida at the end of March beginning of April, scouting reports from last week are still negative. Sentinel plots are being monitored weekly from Texas to Florida across the south and Ohio’s first sentinel plots were planted last week.

Archive Issue Contributors: 

Jim Beuerlein (Soybean and Wheat Production), Anne Dorrance, Pierce Paul and Dennis Mills (Plant Pathology), Peter Thomison and R. L. Nielsen (Corn Production), and Ron Hammond (Entomology). Extension Educators and Assistants: Woody Joslin (Shelby), Howard Siegrist (Licking), Glen Arnold (Putnam), Keith Diedrick (Wayne), Todd Mangen (Mercer), Mark Koenig (Sandusky), Jonah Johnson (Clark), Mike Gastier (Huron), Ed Lentz (Seneca), Harold Watters (Champaign), Steve Bartels (Butler), Wes Hahn ( Logan), Jim Lopshire (Paulding), Gary Wilson (Hancock) and Steve Prochaska (Crawford)

About the C.O.R.N. Newsletter

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.