Soybean Disease Update

Bob Mulrooney, Extension Plant Pathologist;

Powdery mildew is present on some varieties. It is not a yield limiting disease and control is not warranted. See the article in WCU 17:23 for more information on this disease. Downy mildew on soybeans is very common on varieties with no or limited resistance. It too is not thought to be yield limiting here. A few growers have asked about white mold in soybeans. This disease is caused by Sclerotinia sclerotiorum, the same fungus that causes white mold in snap beans, lima beans and peas. Soybeans that are rotated with these crops in fields that have had the disease in the past are most at risk. This disease occurs sporadically in Delaware. Conditions favoring white mold are usually present when we have had lots of rain and the temperatures are moderate during flowering. Crops that are lush and dense with tight canopies are most at risk since the lower parts of the plant stay wet for long periods of time. During dry seasons, growers that over-irrigate soybeans during the late flowering to early pod fill have induced it on fields with a history of white mold. The fungicides that are used on soybeans for disease control and plant health are not effective for white mold. Fungicide applications have not been very successful at controlling white mold.

Sudden death syndrome (SDS) was confirmed from the two finds last week from New Castle and Sussex counties. Several more fields have been diagnosed this week. We have not seen this disease in Delaware since 2000 when it was first identified. The reason we are seeing it again is that we had weather conditions that were very favorable for SDS, just like in 2000. It has to be cooler and wetter than normal for us in the early part of the season for SDS to appear. What does SDS look like in the field? Yellow blotches form between the veins, usually developing first on the uppermost leaves. In a few days the yellow blotches will coalesce and begin to turn brown. The end stage is complete tissue death between the veins, with the only green tissue remaining being that associated with the primary leaf veins. The edges of severely diseased leaves will roll inward. Over time, the diseased leaflets may fall off the leaf stalks (petioles) or they may remain attached to the plant. When you dig up the infected plants primary, secondary and tertiary roots are severely rotted. Nitrogen-fixing nodules are mushy. The exterior of the stem appears healthy but the interior of the stem is a milky-brown to gray color, compared to the yellow-white color of a healthy stem. Serious yield loss usually only occurs when plants are exhibiting serious foliar symptoms BEFORE mid-pod fill. After that time, plants can look pretty rough, but yields may not be affected much. Individual and groups of plants, 10-50 feet in radius, usually show a range of symptoms ranging from some leaf spotting to complete defoliation. Wet or otherwise stressed areas of fields from compaction or other causes, such as along field edges, will usually be the first to develop symptoms. In extreme cases, entire fields may show symptoms. When SDS is severe, symptoms will first develop in “hot spots” and later progress into other areas. This gives the effect that the disease is spreading, but in reality it is not. Rather the time of infection, crop health, and field conditions vary, so disease symptoms are expressed at varying times and rates.

Control of SDS
The only control is reducing plant stress by reducing compaction, and planting resistant or tolerant varieties. Rotation is of little to no value in controlling SDS. Be careful to check plants carefully for these symptoms because stem canker can also produce similar symptoms. The following pictures will give you a good idea of what SDS looks like in the field:

field shot of soybeans with SDSField shot showing the browning leaves and some defoliation

early leaf symptoms of SDSEarly leaf symptoms

later leaf symptoms of SDSLater leaf symptoms showing interveinal necrosis

SDS causes leaves but not petioles to fall off the plantLeaves will fall off but the petioles (leaf stalks) remain attached

stem of healthy soybean (lower) and soybean with SDS (upper)The bark has been removed from the lower stem and tap root to show the brown discoloration of the vascular tissue under the bark of the diseased soybean compared to the healthy white stem on the bottom plant.

Soybean Rust Report
On September 2, soybean rust was reported on soybean from a sentinel plot in Dorchester County, South Carolina just 30 miles from the Atlantic Ocean. This area has received abundant rainfall recently while the rest of the state has been dry. The amount of rust at this site is very low at the present time. On September 1, soybean rust was reported on soybeans in Jefferson and Phillips Counties, Arkansas; Panola County, Mississippi; and Macon and Miller Counties, Georgia. The disease was also observed in soybean sentinel plots in Lee and Macon Counties, Alabama, as well as on kudzu in Crenshaw County in that state. On August 29, soybean rust was detected on kudzu in Jackson County, Florida. On August 28, soybean rust was reported in Drew, Lincoln, Desha and Lee counties in Arkansas; Lafayette, Morehouse and West Carroll parishes in Louisiana; Attala, De Soto and Madison counties in Mississippi; and in Greene County, Alabama.

The total number of counties reporting soybean rust in 2009 has more than doubled the number that had reported rust on the same date in 2008. Rust is moving in areas that have had both the moisture and temperatures that favor rust in the South. Monitoring will continue here in Delaware. Many Group 3 and 4 soybeans planted in mid to late May are approaching R6 or later and would not be at risk if SBR would move north in a tropical storm event. Late double crop soybeans will be the most at risk if the weather pattern should change and we get some tropical storms or a hurricane. Most of the rust activity is in the Mississippi River Valley heading north into Arkansas. Besides the scattered reports of SDS, powdery mildew and downy mildew soybeans look good from a plant disease perspective.