Bacterial Panicle Blight of Rice

Dr. Yeshi Wamishe, University of Arkansas, Rice Research and Extension Center

History of Panicle Blight and Introduction to the USA

Panicle blight was first identified as a disease of rice in Japan in 1956 known to cause grain rotting and seedling blight in seedling beds for transplanting. The disease under the name “grain rot” has also been present in several Asian countries for many years. It has been an increasing problem in Arkansas, in the United States, as well as other mid-south rice producing states since 1995 when it heavily damaged the panicles of rice cultivars, Bengal and Cypress.

In the preceding years, panicle blight was blamed on high temperature, water stress, poor pollination and toxic chemicals near the root zone. The name “panicle blight” had been used for over 50 years prior to its identification in the U.S.A in1996/1997 as caused by bacteria. The pathogens were isolated from symptomatic plants and since then the disease has been called “bacterial panicle blight” to distinguish it from similar symptoms essentially caused by environmental variables.

High Temperatures and Increased Presence of Panicle Blight

Research and observations show that bacterial panicle blight is associated with extended hot and dry summer climatic patterns.  In particular, higher night temperatures appeared to have more association with the prevalence and severity of the disease. In mid-south rice producing states, we experienced abnormally hot summer weather in 2010 and 2011.  The summer day and night mean temperatures were far beyond the 43 year average prior to 2009.  The temperature extremes in the last two rice growing seasons have resulted in significant negative effects on grain quality and yield reduction up to 50 percent.

Bacterial panicle blight is considered a seed-borne disease, with no good practical control options once rice is planted. The rod-shaped bacteria responsible for panicle blight destroy or rot the developing rice grains, resulting in what’s known as kernel blanking, or partial blanking and occasionally sheath rotting (Fig. 1a and b). Symptoms are confused with many other factors.  There are hints that the bacteria can also survive from season to season in crop residues, and they have been isolated from water and soil in rice fields. Most of the current commercial and conventional cultivars are susceptible since they were not selected against the disease. Hybrids and a few other cultivars currently appear to be resistant or moderately resistant. Cultivars planted early seem to have a better chance to escape the severe heat of late July and early August and hence the disease. Fields well managed with water and potassium had better panicles with full kernels, while too much nitrogen fertilization appears to enhance disease severity. No chemical options are registered to manage the disease in U.S., while the antibiotics oxolinic acid and kasugamycin have shown activity in seed and foliar treatment resulting in less panicle blanking in Asia and South America.

Conclusions

Bacterial panicle blight is currently a rising issue and a very important disease of rice. Efforts towards studying the biology of the bacteria and finding for disease management options are being carried out in several rice research facilities in rice producing states of U.S.A. Resistance would be the best option against this yield-robbing bacterial disease both in developed and developing countries.