Cercospora Blight of Soybean, the Cause of Premature Leaf Browning and Defoliation
- Severe Cercospora blight on leaves and petioles.
Over the past several weeks I’ve received a tremendous number of telephone calls regarding Cercospora blight (late-season Cercospora). For whatever reason, Cercospora blight has been more severe this year then compared to the past few seasons. In particular I have seen more severe damage to leaves as a result of Cercospora blight and in some cases premature defoliation has been one of the main symptoms. Typically, Cercospora blight causes a little yellowing to the upper leaves and in some cases the petioles will turn dark purple or almost black on some varieties that possess increased susceptibility to the fungus. However, it truly appears that a variety with tolerance to the fungus that causes Cercospora blight does not exist. In addition to the appearance of disease symptoms, a tremendous amount of premature defoliation has occurred on a large number of acres as a result of the disease (see below for more on this). Personally, I have only observed severe leaf symptoms (characterized by a drying of the edge of the leaves) on one other occasion during a trip to Louisiana two years ago. In general, symptoms from Cercospora blight can appear on leaves, petioles, pods, and stems. In addition, the fungus that causes Cercospora blight also causes purple seed stain. However, a correlation between the presence of the leaf phase of Cercospora blight and purple seed stain has never been made. Essentially, two different sets of environmental conditions appear to cause the two different diseases.
The other question that keeps popping up in response to the extreme levels of Cercospora blight has to do with the application of a fungicide. When the symptoms of Cercospora blight become evident (bronzed, yellowed, purplish leaves; thicker leaf tissue; purplish petioles; purplish pods) applying a fungicide will likely not reduce the impact of the disease. Once symptoms become evident the fungicide application would be reactionary and attempting to arrest the disease would be best served by a fungicide applied prior to symptom expression. Even though the R3/R4 fungicide application appears to provide a limited amount of prevention of the disease it does not completely eliminate the impact from Cercospora blight. However, there are still questions regarding when infection by the fungus occurs and when and if a fungicide would best be applied to prevent yield loss. Moreover, in some limited past research situations a fungicide application was made that prevented a great deal of the symptoms from expressing themselves. But, additional work in successive years using the same product(s) was not able to achieve the same success.
Following Tropical Storm Lee numerous fields throughout the state dropped most of their leaf tissue especially from the upper canopy. The leaf tissue was dropped prematurely and in some cases soybean plants had barely reached R6. In some cases fields had large areas with leaves that rapidly turned brown and were shed by the plant. I’ve heard several folks mention that salt water was transported by the tropical system and deposited over Mississippi soybean fields. Rainwater is always composed of fresh water, regardless of the area where evaporation occurred. The majority of the plants in fields where this occurred were infected with the fungus that causes Cercospora blight. The fast decrease in temperature coupled with the cool north wind and the Cercospora blight that initially worked to dry out the leaves sped the process of leaf dessication and meant that excessive leaf drop followed.
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