**6. Conclusion and prospects**

*Protecting Rice Grains in the Post-Genomic Era*

communication).

**5.4 Biological control**

**5.5 Cultural practice**

oxychloride), and Top Cop® (8.4% tric basic copper sulfate). In the field trials of Louisiana, a single application of Kocide® 2000 or Top Cop® at the boot stage reduced the BPB severity as much as 75%, and grain yield and milling quality were improved [86]. In our multiyear field trials conducted in Texas, single applications of Kocide® 3000, Badge® SC, Badge® X2, or Previsto® at the heading stage significantly reduced BPB severity, with the reductions ranging from 42 to 96% [91–93]. However, except Previsto® with a relatively lower level of copper-active ingredient, all other copper products produced varying degrees of phytotoxicity on sprayed leaves and panicles and under certain environmental conditions reduced yields [86, 91–93]. These copper products have been registered as bactericides and fungicides for control of various bacterial and fungal diseases in citrus, tree crops, vegetables, vines, and field crop (soybeans, wheat, oats, and barley) in the USA. Probably due to their potential phytotoxicity and yield reduction, all these copper products have

not been registered for management of the BPB disease on rice in the USA.

than the bactericide ipconazole/copper (II) hydroxide.

In addition to oxolinic acid and copper-based bactericides, other bactericides such as kasugamycin, probenazole, and pyroquilon are used for management of rice seedling rot and grain rot in Japan [16] and Honduras (Lex Ceamer, personal

Several studies have been conducted to develop biological control methods as a strategy for management of BPB of rice. In Japan, Tsushima and Torigoe [94] conducted the first research on the use of bacterial antagonists for control of BPB under field conditions. An antagonistic *Pseudomonas* sp. strain was found to be effective to suppress seedling rot when pretreated onto rice seeds prior to planting [60]. Furuya et al. [95] also found that rice seedling rot was reduced following seed treatment with avirulent strains of *B. glumae*. Miyagawa and Takaya [96] found that an avirulent strain of *B. gladioli* when applied onto rice panicles was very effective to reduce BPB severity. In the USA, five *Bacillus amyloliquefaciens* strains were found to be antagonistic against *B. glumae* in vitro and reduce BPB severity when applied at the heading stage in the field trials conducted in Louisiana [97]. When applied at the flowering stage, two strains of *Bacillus* sp., with antibacterial activities toward *B. glumae*, were demonstrated to reduce BPB severity by as much as 50% and increase grain yield by more than 11% in the field trials conducted in Texas [87, 88]. In a separate BPB-spread field trial study, one of the strains also showed its ability to significantly limit the spatial spread of BPB from a focal point of inoculum [55]. In addition to bacterial biocontrol agents, bacteriophages (also known as phages) have been demonstrated to be effective for management of rice seedling rot in Japan. Adachi et al. [98] found that two bacteriophages were able to lyse *B. glumae* and were highly effective to control seeding rot when rice seeds were pretreated with them. One of the bacteriophages evaluated was even more effective in reducing seeding rot

Few studies have been conducted to understand and develop cultural practices that could reduce the incidence and severity of BPB in rice. High levels of nitrogen fertility tend to increase the susceptibility of rice plants to the BPB disease. Avoiding excessive nitrogen rates can help reduce the damage caused by BPB. In an Arkansas study evaluating the effects of nitrogen on BPB severity, it was demonstrated that the severity of BPB at the high nitrogen rate (247 kg/ha) was 1.6 times higher than at the low rate (168 kg/ha) applied during a cropping season [99]. Under the Southern

**78**

BPB has been reported in more than 18 countries and has become a global rice disease. Currently, BPB is one of the major diseases in rice in many countries, including Japan, the USA, and Latin America. The disease is highly destructive, which can cause almost complete losses in yield and milling quality under the most favorable conditions. The outbreaks of BPB are triggered by conditions of high temperatures. With predicted global warming, the disease is likely to be more prevalent on a global scale and to cause more damage in epidemic regions in the future [20, 74]. The global land and ocean surface temperature has been increased by as much as 0.85°C over the period of 1880–2012 based on the 2014 IPCC report [100]. Under the 1°C warming scenario, it is estimated that the increased damage caused by this disease in the Southern USA would result in a \$103 million USD annual decrease in consumer surplus and a loss of rice production equivalent to feeding 1.9 million people (Aaron Shew, personal communication).

Effective management of this bacterial disease is challenging. Unlike most of other rice diseases, The BPB disease often develops after the heading stage, and typically no symptoms and signs can be observed before heading. Therefore, no scouting methods are currently available to detect and predict the development of the disease. No standardized seed treatment methods have been developed and commercialized specifically to eradicate or reduce the pathogen populations in rice seeds. No chemical control agents are labeled for management of the BPB disease in most countries, including the USA. The efficacy and increasing use of oxolinic acid have been affected by the development of oxolinic acid resistance in the populations of *B. glumae* in Japan and other countries. No commercially available biocontrol agents have been developed. Most of commercially available rice cultivars are susceptible or very susceptible to BPB.

Therefore, effective and sustainable control of the BPB disease largely depends on integrated use of available management options. Plant quarantine is the first defense to exclude the BPB pathogens from disease-free countries and regions. The use of pathogen-free seed or certified seed is another effective measure to control this disease. Planting with cultivars having a resistant level as high as possible is always an effective recommendation to reduce the damage caused by the disease. A limited number of rice cultivars, including hybrids, with partial resistance to BPB are available for commercial use in many countries. Since no source of complete resistance has been discovered so far, more research is needed to look for new sources of resistance through screening a greater number of germplasm lines, including those from other countries and the wild species of *Oryza*. Continued studies are needed to further characterize, fine map, or even clone the QTLs associated with BPB resistance that have been identified. More investigations are desired to understand the genetic control of BPB resistance in available resistant rice cultivars and lines, especially hybrids. These studies may lead to the development of molecular makers linked to BPB resistance that can help breeders facilitate the selection of BPB resistance in early breeding generations with more confidence. Recent advances in rice genomics and newly developed genome editing tools like CRISPR may provide new and powerful tools to better understand the mechanisms associated with BPB resistance and develop new rice cultivars with a higher level of resistance to BPB in the future. Developing and use of resistant cultivars is the best strategy to minimize the damage caused by BPB and maximize rice production in the long term.
