Soybean Rust Confirmed in Southern Illinois

On September 25, 2007 soybean rust (SBR) was reported and confirmed on a soybean leaflet in Southern Illinois in Massac County. This field is not far from where it was confirmed in Kentucky last Friday. In addition to being found in Illinois, SBR was also confirmed in two counties (Pemiscot and Scott) in southeastern Missouri.

In Illinois, fungicide treatments are not recommended because most soybeans are mature or at a point in their development and growth stage past the time where soybean rust can cause economic yield loss. Soybean plants at growth stages R1 to R5 are the only ones at risk of yield loss from SBR.

However, if a double-crop soybean field is currently at a growth stage prior to R6, it is extremely important to determine the yield potential before considering applying a fungicide. If the decision to apply a fungicide is made, use either a triazole/strobilurin mixture or a triazole fungicide.

Dr. Loretta Ortiz-Ribbing, University of Illinois Extension Specialist in Macomb says, "Many beans in Western Illinois are already stressed by low moisture so their yield potential may already be lower than expected. Applying a fungicide would probably not increase yield potential or be cost effective at this late date especially in areas already stressed by low rainfall."

In addition, many fungicides have preharvest interval restrictions that do not allow application past the R6 growth stage. Any fungicide application requires reading and following the label directions. More information and updated management guidelines are available in the "management toolbox" located in the lower right-hand corner of the main page of the soybean rust pipe website (www.sbrusa.net).

In Western Illinois, fields with double-crop beans or any fields where green leaves remain might be a good place to look for soybean rust. Dr. Ortiz-Ribbing suggests to, "Look at the underside of the leaves in the lower canopy of the soybean plant and check to see if you observe any clusters of SBR pustules in small tan or red groupings. A 20X hand lens would be very helpful in this process."

"Even though soybean rust is not an economic problem right now, we would like to continue monitoring it's spread, so we can confirm and verify models used to predict SBR spore movement." Please let Dr. Ortiz-Ribbing know if you think you may have found any suspect leaves.

In addition, any leaves that are suspect for soybean rust should be taken to the nearest Illinois Extension Office. The Digital Distance Diagnostics Imaging (DDDI) system will be used to help evaluate suspicious leaves for soybean rust. Leaf samples can also be sent overnight directly to the University of Illinois Plant Clinic, 1401 W. St. Mary's Road, Urbana, IL 61802 (217-333-0519).

Logan County Extension Office

It is important to try and plant wheat on or after the fly free date, not only because of the Hessian fly, but also to reduce pressure from diseases and other insects. Lush growth in the fall favors diseases such as Septoria leaf spot. Barley yellow dwarf virus and wheat streak mosaic virus are carried by an aphid and a mite respectively. Proper seeding time usually provides a frost to control these pests.

The fly free seeding dates for Logan County range from September 29 on the northern border of the county to October 3 on the southern border. Figure Lincoln at October 1, and you can figure out your date based on the distance from north to south.

There are usually questions about later seeding due to weather being too wet or dry, or getting the field harvested and prepared. In general, yields decline very little for the first 10 days after the fly free date. It is common to drop a bushel or so per day from the 10 to 20 day period after the fly free date. From 20 to 30 days late drops about 2 bushel a day. After a month late, you can only expect about 60 percent of a crop if the wheat survives the winter.

Seeding should provide 30 to 35 seeds per square foot. As in soybeans, the seed size in wheat affects the pounds per acre that should be dropped. Very large seed (11,000 seeds per pound) would require about 127 pounds per acre fo get 32 seeds per square foot. Very small seed (17,000 seeds per pound) would require 82 pounds to get the same 32 plants.

Logan County Extension Office

Charcoal rot is evident in area soybean fields. Charcoal rot is going to be one of the major yield reducing diseases this year in our area, and to the south. Following is information from Carl Bradley on charcoal rot.

Many soybean fields in the dry portions of the state are being affected by charcoal rot, caused by the soilborne fungus Macrophomina phaseolina. Charcoal rot is a disease that can thrive under hot and dry conditions. Symptoms in fields currently affected in southern Illinois appear as patches of wilted and dead plants. In charcoal rot-affected plants, the pith of the lower stem contains tiny black structures, known as microsclerotia, that resemble charcoal dust. The microsclerotia are the survival structures of the fungus. When the epidermis of affected plants is removed, the area beneath the epidermis may have an ashy-gray appearance and contain black streaks.

Management of charcoal rot requires an integrated approach. Although no soybean varieties have complete resistance to charcoal rot, varieties can differ in their levels of susceptibility. Because the charcoal rot fungus has a wide host range, including corn, crop rotation alone will not provide complete management. Practices that reduce drought stress may help, such as avoiding high seeding rates and using conservation tillage practices that conserve soil moisture. The foliar and seed treatment fungicides currently available do not provide protection against charcoal rot.--Carl A. Bradley

Logan County Extension Office

Logan County Extension Office