Symptoms The BPB symptoms include seedling blight, sheath rot and panicle blight causing an enormous yield loss of rice in each year in the world (Nandakumar et al. 2009, Zhou and Jo 2014). Toxoflavin a toxin produced by the bacterium is an important factor to induce symptoms development on rice seedlings and grains (Jeong et al. 2003, Matsuda and Sato 1988). BPB symptoms can be observed on plantlets, leaf sheath and panicles (Figure 1). The BPB infected panicles bear light to dark brown, moderately or completely discolored glumes. It may cause unfilled or aborted grains under severe infection (Ham et al. 2011). The bacteria make damage by inhibiting seed germination or producing panicle blight or sheath rotting or flower sterility or grain abortion at severe infection (Wamishe 1914, Ham et al. 2011). The rachis or panicle branches remain green at early infection and at later stage, heavily infected panicles remain upright due to empty glumes (Wamishe 1914). A dark brown lesion may be observed on the flag leaf sheath of certain tillers resulting severe panicle damage.
Epidemiology Both B. glumae and B. gladioli have been recognized as the causal agents of BPB. However, the earlier one distributed widely in the world (Table 1) as well as more virulent, causing more economic losses than the latter one. Bacterial pathogens causing BPB are frequently observed in the air, water, and soil. Survival of these pathogens in soil usually affected by soil type, soil pH, and weather conditions (Tsushima 1996). Host vulnerability, inoculum density, and climatic factors play the vital roles in these bacterial infection process (Tsushima and Naito 1991, Tsushima 1996). BPB is frequently observed at the heading stage of rice when the night temperature is high and rainfall occurs frequently. With an appropriate environmental conditions (30–35°C temperature and above 80 % relative humidity), BPB can be increased rapidly and may cause serious epidemics (Cha et al. 2001, Syahri et al. 2019). However, the thermal death point for the causal agents of BPB is at 70°C (Kurita et al. 1964). The flowering and heading time of the variety may also affect plant susceptibility. Rice plants are vulnerable to BPB infection within 1–3 days of flowering (Tsushima, 2011). Plants are also susceptible to BPB after 4–5 days of heading to subsequent 11 days (Syahri et al. 2019). Both the bacterial species were widely observed in rice seed lots in China, Japan, Philippines, and USA (Cui et al. 2016, Cottyn et al. 2001, Sayler et al. 2006) and these infected seeds serve as the primary inoculum source (Nandakumar et al. 2009). Upon seed germination bacterial pathogens initiate infection that occupies the roots and lower sheaths and then moves up as an epiphytic way (Tsushima 1996, Hikichi 1993). Primary infection occurs once B. glumae or B. gladioli contaminated seeds are sown and then transplanted to the main fields (Nandakumar et al. 2009). Secondary infection of nearby plants occurs at heading stage (Mizobuchi et al. 2018). Recently, Li (2016) observed that B. glumae can infect the rice plant directly by colonizing the vascular bundle of lateral roots and then disseminated to the upper part of the plants through vascular system. The bacterium colonizes and multiplies in spikelets immediately after invasion through stomata or wound in the glume epidermis by using storage sugars in developing grains (Hikichi 1993, Hikichi et al. 1994). Jeong et al. (2003) further reported that B. glumae could also infect some other crops including eggplant, tomato, perilla, sesame and hot pepper. The bacteria can survive on both host plants and soils under varied environmental conditions (Compant 2008, Nandakumar et al. 2009).
As BPB incidence and severity is highly influence by the weather conditions, the relationship between the BPB occurrence and pathogens survival with the climatic factors, such as temperature, relative humidity and rainfall, need to be studied in order to manage BPB effectively.
Management Use of BPB-free rice seed is the key constant to reduce yield loss caused by BPB. Besides, farmers could use partially resistant rice cultivars or may apply available chemicals or biocontrol agents, together with proper cultural practice to reduce BPB infection. For effective and sustainable control of BPB, these available management strategies should be used integrative. Integrated practice of the existing management strategies can be an effective and sustainable way to manage the BPB of rice.