Vol. 20, No. 18, September 22, 2005

In this Issue
Should you plant transgenic rootworm corn?
Asian soybean rust sentinel plot scouting report
Welcome to Diane Brown-Rytlewski
End of season diseases
A taste of fall?

This is the last issue of the 2005 Field Crop CAT Alert. Included in this issue is an index of articles from this season to help guide readers through the topics we have covered. Have a great winter – Editorial staff.

Should you plant transgenic rootworm corn?

Christina DiFonzo, Entomology

This fall I have had a number of phone conversations and field meetings with growers and agronomists feeling pressured to use transgenic corn in 2006 for rootworm control. In many cases, salesmen are indicating that a particular region has a rootworm problem, thus everyone in the area should purchase Bt rootworm corn. Since seed selection happens over the next few months, now is the time to make an informed decision about rootworm control next season.

Rootworm is a growing problem in Michigan. This past season was a good year for rootworm in continuous corn, perhaps reflecting favorable conditions for egg laying last summer, high survival of rootworm eggs in the soil over the winter and high survival of larvae in the spring. In first year corn, we can detect variant rootworm feeding on corn roots across much of Lower Michigan. This year we also noticed many rootworms in soybean fields. However, economic damage is still limited to southwest Michigan above the Indiana border, particularly in Berrien and Cass counties. There are also some odd hotspots for example in Monroe County and the dairy/ silage area around St. Johns with elevated feeding and rootworm numbers in first-year corn. Usually, these areas do not have economic damage. Bottom line: rootworm numbers and damage appear to be on the rise, and variant rootworm can be found in first year corn and soybean. But economic damage is still generally limited only to continuous corn and to first-year fields in southwest Michigan.

Can a seed salesman predict your need for rootworm control right now, in the fall of 2005, for 2006? Not unless you or he sampled your fields this summer. Rootworm scouting must be done in the summer to make a decision about insecticide for the following season. In continuous corn, fields are scouted to determine the beetle number per plant in July and August (the threshold is one beetle per plant). In soybean, yellow sticky cards catch beetles in July and August (the threshold is five to six beetles per trap per day).

You walked the field in August and remember seeing gobs of adult beetles.

You dug and washed roots from the field in August and found corn rootworm pruning damage.

You dug and washed roots yesterday and found corn rootworm pruning damage, although now isn’t the optimal time frame to dig roots. Root regrowth in late summer obscures damage, and in the fall root rot and dehydration makes it difficult to diagnose rootworm injury.

You noticed extensive lodging in late summer or fall as you harvested (but remember, there are several causes of lodging, and rootworm is only one of them).

The field and surrounding area has a history of corn rootworm damage, heavy rootworm populations, lodging, etc.

Bottom line: Scouting in 2005 is the only way to predict with a good level of certainty the need for insecticide in 2006. In the absence of scouting, you or a salesman are making a guess.

If you decide to use an insecticide in 2006, remember you have a variety of options now for corn rootworm control, ranging from granular insecticide, to seed treatments, to transgenic corn, each with benefits and limitations.

Limitations. Calibration, handling, storage. Can move off-target under wet conditions; product may not move around enough under dry conditions; may break down before rootworm hatch is complete (especially with early planting).

Limitations. Some replant restrictions. Saving old seed isn’t recommended (germination issues); product may not move around enough under dry conditions; may break down before rootworm hatch is complete (especially with early planting); cannot do-it-yourself – must order treated seed or have seed treated locally; potential variation in quality of treatment from commercial seed treaters; may need to adjust planter/add talc for optimal seed drop.

Limitations. Limited hybrids and companies; cannot do-it-yourself – must order transgenic seed, often months in advance of season; potential marketing issues; refuge requirements; tech fee; seed is still treated with insecticide to control soil pests; root pruning may still occur, especially later in the season.

To complicate things, there will be an additional management option choice in 2006. Up to now, Yieldgard Rootworm (YGRW), developed by Monsanto, was the sole choice for transgenic rootworm control. YGRW corn contains a gene for a Bt toxin specific to rootworms. YGRW hybrids also come doubled stacked with the Roundup-Ready gene or the Yieldgard corn borer gene, and as a triple stack with Roundup- Ready gene / Yieldgard corn borer. This month, EPA approved a different Bt transgenic trait against rootworm, Herculex RW, developed by Pioneer/ DowAgrosciences. This trait will be available in certain hybrids for the 2006 season. In an MSU field trial in central Michigan this summer, Herculex RW provided excellent root protection compared to Force-treated and untreated plots. The refuge requirements for Herculex RW will be similar to that for Yieldgard corn, and it will be seed treated. See this web site for more information: http://www.pioneer.com/Hx/default.htm.

Sandy Perry, SBR Sentinel Plot Coordinator
Jan Byrne, MSU Diagnostic Services

The soybean rust scouting program has come to a close for 2005. No soybean rust was found on soybeans or other plants in Michigan or anywhere in the North Central Region, including Ontario, Canada.

The scouting program worked out very well in 2005. Eleven MSU county Extension staff with expertise in field crops scouted 20 sentinel plots in 16 counties for a total of 214 scouting reports over 16 weeks. They were supplied with Federal Express boxes and an account number so that soybean plants suspected of having soybean rust could be easily and quickly shipped to MSU Diagnostic Services. Suspect plants underwent molecular testing with real-time PCR. Six soybean samples were submitted from the sentinel plots during the season, and all samples tested negative for soybean rust. The scouts submitted reports to Diagnostic Services weekly for 16 weeks. The reports provided a fairly clear picture of background disease levels in the sentinel plots as well as the levels of soybean aphids present. Scouting data was regularly uploaded to the USDA-APHIS web site where the scouted counties in 35 states and Ontario, Canada were tracked for soybean rust development and spread. The maps are available at http://www.sbrusa.net/

Diane Brown-Rytlewski has begun work in the area of extension and applied research for field crop diseases as of September 1, 2005. She has joined Willie Kirk’s lab, which will be adding fungicide evaluation trials for field crops and the development of practical decision-aid models for important diseases of soybeans and wheat to their current responsibilities. Among Diane’s responsibilities will be working on the fungicide trials and developing and presenting field crop Extension programs that focus on management of field crop diseases.

Diane worked for the last five years as Nursery and Landscape IPM integrator at MSU. She received a B.S. degree in entomology and an M.S. degree in plant pathology from the University of Wisconsin Madison.

Diane Brown-Rytlewski
Plant Pathology

Downy mildew on soybeans is caused by the fungus Peronospora manshurica, one of the group of watermolds that also includes Pythium and Phytophthora. Downy mildew survives as oospores on the surface of seed and in crop residue. Downy mildew was reported in Michigan’s soybean rust sentinel plots starting in early July and continued during August and early September. According to the University of Wisconsin soybean health website, (http://www.plantpath.wisc.edu/soyhealth/minordiseases/downy.htm) leaf infection doesn’t usually affect yield, although downy mildew is a fairly common disease of soybeans. However, planting contaminated seed can result in systemically infected seedlings, producing stunted plants with mottled leaves. Recommended control practices for downy mildew include crop rotation or plowing under crop residue to reduce overwintering sources of infection, using fungicide seed treatments and planting pathogen-free seed.

White mold, caused by the fungus Sclerotinia sclerotiorum has a number of hosts, including several field crops that are important in Michigan-soybean, dry beans and potato. Corn, wheat and oats are among field crops that are not susceptible. White mold on soybean is a disease of increasing frequency and increasing concern in the Great Lake states. Planting and cultural practices that favor dense canopies, lush growth and a cool, moist environment within the crop canopy provide a favorable environment for development of the disease. Rotating between soybean and other susceptible hosts increases the risk of the disease. The more frequently a host crop is present, the more opportunity the white mold pathogen has to reproduce and reach population densities in soil that can result in outbreaks of white mold . White mold can be difficult to control. The fungus can survive in soil for five years or more as sclerotia – hard, black, seed-like, fungal masses that are dormant. An excellent resource for assessing white mold risk and management guidelines based on field history can be found on the University of Wisconsin soybean health web site at: http://www.plantpath.wisc.edu/soyhealth/risk.htm

Jeff Andresen
Agricultural Meteorology
Geography

The jet stream will take on a split flow pattern across North America during the next week, with a troughing pattern over the western United States merging with a fast, west to east branch of the flow over the Great Lakes region. This pattern should lead to highly variable weather with frequent chances for precipitation.

In the short term, a cool front will move through the state Thursday (September 22), clearing the southeast corner by early Friday. The front will bring widespread rainfall to most areas through Thursday evening. Best chances for rainfall will be across central and southern sections of the state, where some scattered 0.5 to 1.0 inch totals are possible. Some lingering showers will be possible Friday morning in southern sections of Lower Michigan – otherwise much cooler and dry weather is expected. Dry weather is also in the forecast for most of the day Saturday. With mostly clear and calm conditions expected overnight Thursday and Friday nights in much of the state, some frost or even freezing temperatures are possible in western and central interior areas of the Upper Peninsula, where minimum temperatures could fall to the low 30s. Another frontal system will approach the state by late Saturday and Sunday with the next chance for precipitation.

The forecast for early next week will depend on the track of Hurricane Rita, which is expected to impact areas of the western Gulf of Mexico by early Saturday. Current forecast guidance suggests that the remnants should remain well south of Michigan due to a large area of high pressure over the Great Lakes region. That same area of high pressure, accompanied by clearing skies and light winds, could also lead to the first chances for frost in interior sections of Lower Michigan by next Tuesday or Wednesday morning. Temperatures during the next few days will gradually fall from the 70s on Thursday back to the low 60s north to low 70s south on Friday. Low temperatures will generally range from the mid and upper 50s midweek, falling back to the 30s north to mid-40s south by the weekend.

Medium-range forecast guidance suggests the continuation of an active weather pattern for the Upper Midwest. Temperatures for both the 6-10 day and 8-14 day time frames, covering September 27 through October 1 and September 29 through October 5 are forecast to range from near normal in extreme southern sections of the state to above normal elsewhere. Due to an active storm track, precipitation totals in both time frames are expected to reach above normal levels.