"At Your Service" Newspaper Column

 

Michael R. Milam

Agronomy Specialist and County Program Director

For Daily Dunklin Democrat-At Your Service

October 12, 2007

Soybean rust continues to spread

            I have been keeping up with the development of Asian soybean rust and I can now say that we have confirmed rust in Dunklin County. Last Friday, I met with Dr. Laura Sweets, MU Extension plant pathologist and a student worker, Nathan Gross, who were visiting this area to look for soybean rust. Bruce Beck, Butler County extension agronomist, took them around that morning. They found several fields with low levels of infection in Butler County. After lunch, we stopped at several fields of soybeans in the northern part of Dunklin County. We were able to confirm rust in three of the five fields visited. In all cases, the rust was at a low level with only a few leaves per plant and a small number of pustules on the affected leaves.

            I have some observations on our field visits. One of the first leaves that I looked at found near the bottom of the plant had the typical halos developing around the rust areas. I thought that I could make out the rust pattern on the underside of the leaf, but since I didn’t have my hand lens, I passed the leaflet to Nathan. He and Laura both confirmed the presence of pustules on the underside of the leaves. I looked at several of the leaves that they found and I was able to see the pustules easily. However, much of the samples that were sent in as part of the university’s monitoring program, were looked at under high magnification. It is really hard to look at the early development of soybean rust with a hand lens and see them. For example, the visits to several fields in Mississippi County did not find any rust. They pulled about 200 leaflets and took them back to Columbia. They looked at them under high magnification and they were only able to find a single pustule on one leaf. Soybean rust is the only soybean disease with the raised pustules.

            As of October 5, soybean rust has been confirmed in 17 states and 200 counties, including 14 in Missouri. It has spread to the Northwest region of the state. Again, in all cases, the incidence is very low to moderate and only a few soybean fields are still vulnerable to the disease.

            On September 25, soybean samples from Pemiscot and Scott Counties were confirmed positive for soybean rust. Incidence and severity were low in both samples (3 infected leaflets out of 100 and 2 infected leaflets out of 100). On September 28, soybean samples from Lawrence and Vernon Counties (both in southwestern Missouri) were confirmed positive for soybean rust. Incidence and severity were low in both samples with only a few pustules present on a few leaflets in each sample.

Since September 28, soybean rust has been confirmed in Jasper and Barton Counties in southwestern Missouri and in Dunklin, New Madrid, Mississippi, Stoddard and Butler Counties in southeastern Missouri. Incidence ranged from quite low to moderate in one Stoddard County field. Growth stage varies from R4/R5 in southwestern Missouri to R6 and beginning to drop leaves in southeastern Missouri.

Over the last few days there have been reports of soybean rust in counties in Nebraska, Kansas, Kentucky, Tennessee and Illinois as well as additional counties in Arkansas, Oklahoma and Louisiana. Incidence and severity appears to be low at most of these sites although some sites have reported active sporulation on infected plants. Current model forecasts show a risk for soybean rust throughout much of the Midwest.

The bottom line is that soybean rust has now been confirmed in fourteen counties in Missouri. Continued scouting may result in additional positive finds. However, at the slow rate that the disease appears to be moving and building up, the risk of significant losses from soybean rust decreases each day.

This means that only a few fields in Dunklin County are susceptible to soybean rust. Those are the plants at the R4 or R5 stage. These are the soybeans planted after wheat. However, most of these fields do not have 40 bushel yield potential.

I have been asked about the impact that soybean rust will have on soybeans next year. The good news is that the rust does not overwinter here on either soybeans or kudzu. So it all depends on wind currents to get the spores here. Even if we have spores, we would need favorable weather conditions for the disease to spread.

Keep in mind that rust was first found in Missouri in 2004, was not found in 2005, found late in 2006 and again this year. Soybean rust is like other disease organisms in that under the right set of circumstances, it could spread. Some years we have wheat scab and rust and most years we don’t. So it could happen. The good news is that some producers are already spraying soybeans for disease and are increasing their yields. Other producers like to treat soybeans as a low input crop because the cost of growing soybeans are a lot less than corn or cotton.

Finding rust at this time of the year is mostly of academic importance. From a practical standpoint, it is unlikely to have any impact on this year’s soybean crop, even in areas with late planted beans.

 

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