Update on Soybean Diseases By Laura Sweets

We are getting an increasing number of calls related to yellow soybean plants, yellow leaves on soybean plants, wilting plants and dying or dead soybean plants. In some cases these symptoms may be occurring on scattered plants in fields and in other cases the symptoms are in patterns related to soil type and soil moisture conditions in the field.

The yellowing may be a marginal yellowing of lower leaves suggestive of a potassium deficiency, it may be a marginal yellowing followed by browning of those tissues more suggestive of scorch or it may be a general yellowing followed by wilting and death of leaves or entire plant.

When troubleshooting in these fields it is important to dig up a range of plants and check the root systems carefully. Although potassium deficiencies certainly can occur and can cause symptoms on soybean plants, similar symptoms may also occur if the plants have poorly developed or rotted root systems and can not absorb and translocate nutrients that may be available in the soil. This season both Fusarium root rot and Rhizoctonia root rot have been widespread and in some fields quite severe. Fusarium root rot tends to cause a rotting of the main taproot. The taproot may be discolored and beginning to shrivel or it may be completely rotted so that it breaks off when the plant is pulled from the ground. Rhizoctonia root rot tends to cause a brick red to bright red discoloration of the stem tissue near the soil line. Plants in one field in Union County, Missouri showed red discoloration of almost the entire lower stem. Stems may be somewhat constricted or shrunken and stem tissues may show more cracking than normal. With both Fusarium root rot and Rhizoctonia root rot the plants may send out adventitious or secondary roots near the soil surface trying to compensate for loss of primary root tissue. If weather patterns were to change so that conditions were favorable for soybean growth and development, in particular more frequent rain and moderate temperatures, plants with low to moderate levels of these root rot diseases might recover. However, if hot, dry conditions continue, these plants are more likely to show severe scorch symptoms or simply wilt and die. There are no management strategies that can be implemented at this point in the growing season.

When checking affected plants, take time to observe overall root development and health. In addition to root rot symptoms, some plants have very shallow root systems or root systems that are bent or J-shaped suggesting compaction, a hard pan layer, or other site related problems.

Damage from soybean cyst nematode can be more severe under periods of moisture stress or drought conditions. If plants are carefully dug up and soil gently removed from the root system, it may be possible to see the bodies of female soybean cyst nematode on the roots. This time of the year SCN females might be more readily found on new roots formed deeper in the soil profile or on lateral roots. Submitting soil samples for SCN analysis would be another means of determining if SCN is contributing to yellowing, stunting or premature death of plants. See information on submitting samples for SCN analysis in this issue of the newsletter.

Septoria Brown Spot
Samples with Septoria brown spot and questions related to Septoria brown spot continue to come in from all parts of the state. Much of the state did receive rain over a several day period early in June. This rain apparently triggered a flush of Septoria brown spot. The dry conditions over most of the state for the last 3-4 weeks have slowed or stopped the development of this disease in most fields. At this time Septoria brown spot is most evident low in the canopy on the first one to two sets of trifoliolate leaves. These leaves may be heavily spotted with individual spots ranging from very small reddish brown to brown spots to larger, more irregularly shaped lesions. Leaves may exhibit a general yellowing and may drop prematurely. In many fields the plants have not been growing vigorously and rows still haven’t canopied over. This makes the leaf yellowing or leaf drop from Septoria brown spot much more obvious. If the rows were closed, symptoms on those lower leaves wouldn’t be as evident from a distance. A change in weather to periods of frequent rains, heavy dews and moisture that stays in the canopy could lead to the buildup of Septoria brown spot and another outbreak of symptoms on newer growth. But continued periods of hot, dry weather with little or no dew formation will not be favorable for Septoria brown spot. Quadris and Headline as well as chlorothalonil products such as Brave WeatherStik and Echo, do have full federal registration for use on soybean against a number of diseases including Septoria brown spot. Benefit from an application of these materials under the current weather pattern seems questionable.

Update on Soybean Rust
Soybean rust has not been detected in any of the Missouri sentinel plots thus far this season.

There have been a few changes in the status of soybean rust since the article in the last issue of the Integrated Pest and Crop Management Newsletter.

As of July 12, 2005, soybean rust has been confirmed on soybeans in sentinel plots in Baldwin County, Alabama and in Marion County, Florida. Soybean rust had been reported earlier in the season on volunteer soybeans in Seminole County, Georgia and these volunteer soybeans have since been destroyed. Soybean rust has also been reported on kudzu in seven counties in Florida, i.e. Dade, Hernando, Pasco, Marion, Jefferson, Leon and Gadsen Counties.

Over the last week there have also been reports that spores suspected of being spores of the soybean rust pathogen were identified from spore traps in Kentucky and Tennessee. The spore trapping is part of a multi-state project established by Syngenta Crop Protection, Inc. Syngenta located approximately 100 spore traps containing vaseline coated microscope slides throughout the southern United States (none of the original locations were in Missouri). The slides are replaced at regular intervals and the collected slides sent to a central location for examination. Spores resembling the Phakopsora pachyrhizi have been detected on slides from about ten sites including one in western Tennessee and one in Woodburn, Warren County, Kentucky. Follow-up surveying in areas close to the spore traps has not detected soybean rust in sentinel plots or production areas close to the spore traps. Surveying of sentinel plots has intensified in these areas with most being scouted on a 3-4 day interval.

Certainly Hurricane Dennis has also lead to increased concern about the movement of soybean rust. Hurricane Dennis touched ground in the western panhandle of Florida and the eastern portion of Alabama. Rains from this storm system reached portions of Missouri on Monday, July 11th. But there are several points that are important to keep in mind. First the inoculum level in the southern United States has remained very low so far in the 2005 season. Although there have been detections of soybean rust in two soybean sentinel plots, one area of volunteer soybeans and seven areas of kudzu, the disease has not built up to high levels in those areas and has not spread significantly beyond those initial detection areas. So there don’t seem to be high levels of spore inoculum for Hurricane Dennis to bring north. In addition to having weather patterns, such as Hurricane Dennis, which might spread inoculum through large geographic areas, the weather conditions in the areas to which inoculum is carried must be favorable for infection to occur and the disease to develop. On July 7, as Hurricane Dennis was approaching the Florida Keys, 23 Missouri counties were put under drought alert. If Hurricane Dennis brings 1or 2 days of rain and the weather pattern reverts back to the dry conditions which preceded it, an immediate epidemic of soybean rust in much of Missouri is very unlikely. If weather conditions after the remnants of Hurricane Dennis pass through change, so that there are periods of more extended rainfall, heavy dews and moisture that stays in the canopy, soybean rust infections would be more of a possibility.

So for right now it is important to continue to monitor events in the southern United States. (There are a number of web sites which post information on soybean rust including the USDA/APHIS site at www.sbrusa.net) and to monitor weather conditions in the southern United States as well as locally. Rainfall events in Missouri have been scattered and localized this season. And it is important to continue to scout or monitor fields. Once spores are deposited on susceptible leaf tissue, lesions may form in 5-10 days but spores may not be evident until 10-14 days after infection.

Laura Sweets
Plant Pathologist
573-884-7307