Southwest Louisiana is an excellent location for rice production because of the climatic conditions, predominantly silt loam soils, gently sloping land and generally abundant surface and aquifer supplied water sources. However, the warm conditions during the growing season, frequent rainfall, heavy morning dews and very high humidity also make the rice in this region subject to high pressure from rice diseases. Numerous diseases can infect rice plants. While some are typically considered major, others are normally of only minor consequence. The major rice diseases occur fairly frequently and when they do occur can have a major negative impact on rice yield and quality. Minor diseases are those that even when they are present typically have little or infrequent effect on productivity.

Sheath blight is one of the diseases classified in the major category. The disease is caused by Rhizoctonia solani, a fungal pathogen. The disease has been the most economically significant in Louisiana rice production since the early 1970s. While this disease can infect any rice variety, it is typically more damaging on the long-grain, pure-line semidwarf varieties that have become predominate in this region in the past few years. Another important factor is that this same organism also causes aerial blight on soybeans, which have become the major rotation with rice on many farms in southwest Louisiana during the past 40 years.

A major breakthrough in the control of this important disease was the research on and ultimate registration of strobilurin fungicides in rice production. Strobilurins are one of the most important classes of fungicides in the world. Their invention was inspired by the discovery of the fungicidal activity of a group of naturally occurring products. The naturally occurring products are produced by a group of wood-rotting fungi. After the disease controlling capabilities of the naturally occurring products were discovered, intensive research led to the synthesis of similar products that could then be tested for fungicidal activity on different crops. Much of this research on rice was conducted by Dr. Don Groth and others at the Rice Research Station. This and much other research eventually led to the registration of two strobilurin-based fungicides that have been widely used in Louisiana rice production over the past few years. While these fungicides control a number of important rice diseases, by and large the most important use is for the control of sheath blight.

This past growing season, we (LSU AgCenter research and extension scientists) began to receive questions about reduced levels of control of sheath blight. These issues appeared to be fairly localized in a small geographic area between Crowley and Eunice. As always, when issues such as this arise, there is a concerted effort among extension personnel, farmers and consultants, as well as representatives from the crop protectant chemical companies, to attempt to identify the problem. This is typically a detailed endeavor to rule out application problems, issues such as rainfall events shortly after application that can wash off the fungicide, and/or any unique issue that could affect the efficacy of the applied product. Ultimately, leaf samples were taken and sent to a lab to evaluate the activity of the fungicide on the organism. These analyses involved testing the sensitivity of the Rhizoctonia solani material collected from these fields against the sensitivity of other isolates of the disease known to be controlled by the fungicide. The results of this testing revealed that the disease organisms collected from the fields appeared to be less sensitive to the fungicide. The bottom-line of this preliminary work is that the fungicide may not be as effective in controlling the disease in these fields. However, this does not mean that the fungicide is totally ineffective, just perhaps not as effective.

As this article is being written, there is a concerted effort to collect diseased tissue samples from both rice and soybean plants across Louisiana. These samples will be analyzed over the next few months. This will give us an idea of how widespread or, we hope, localized this decrease in sensitivity to the fungicide is. The good news is that this is not a wind-borne organism that can be disseminated by air currents. This organism typically must be physically moved from one field to another by movement of plant tissue or soil. We are also working on guidelines to minimize the spread of these less sensitive isolates of the disease to other fields and on alternative control measures.

This decrease in sensitivity to a fungicide is certainly not unprecedented. All living organisms are cons tantly mutating and evolving, and changes in sensitivity to pesticides are common not only in fungicides but also in insecticides and herbicides. This further illustrates the importance of ongoing research in areas such as rice disease control at centers such as the Rice Research Station.

Permission granted (9/21/2011) by B Leonards (LA Farm & Ranch) to republish article on www.lsuagcenter.com