Causal agent of Bakanae and symptoms: Bakanae is associated with several species of Fusarium but Fusarium fujikuroi Nirenberg (Syn: F. moniliforme (Sheld.) has been observed as the main causal pathogen (Wulff et al., 2010). Besides, F. proliferatum was also reported for causing bakanae disease in rice (Quazi et al., 2013). Seed contamination with this fungus provides initial foci for primary infection. F. fujikuroi may infect rice plants from pre-emergence to mature stage, with severe infection of rice seeds resulting in poor germination or withering (Iqbal et al., 2011). Bakanae is a seedling disease, but it can be observed throughout the growing season. Infected seedlings are appeared to be elongated, thinner and slightly chlorotic when compared with healthy seedlings. Symptom development is influenced by the amount of inoculum present, fungus strain and the relative quantities of gibberellin and fusaric acid. Besides, symptom development is affected by temperature, 35ºC is optimum for disease symptoms development while it does not appear below 20ºC (Hino and Furuta, 1968; Takeuchi, 1972). The incidence and severity of the Bakanae disease symptoms vary with regions and isolate.
Bakanae in Bangladesh: In Bangladesh, farmers cultivate rice almost in all three cropping seasons viz., Aus (April-June), Amon (July-October) and Boro (December-March) with hybrid, high yielding variety (HYV) and local rice verities. Previously rice Bakanae disease was mostly observed in Aus season but nowadays the disease has been found commonly in Boro season. It is thought that excessive use of nitrogen fertilizer and favorable weather conditions (30 ºC -35 ºC) are responsible for causing Bakanae in Bangladesh. Almost all cultivated rice varieties in Bangladesh are more or less susceptible to Bakanae disease (Hossain et al., 2011). Among them, BRRI Dhan 29, BRRI Dhan 58 were found as the most susceptible to Bakanae which has been cultivated in Boro season.
In recent years, Bakanae disease has been found to occur commonly with high incidence in the Haor areas of Bangladesh compared to other areas. Geographically, seven districts of North-East Bangladesh like- Sunamganj, Kishoreganj, Netrokona, Sylhet, Habiganj, Maulavibazar and B. Baria comprise most of the haor areas. In these seven districts, total cultivated area is about 1.26 million hectares which produce more than 5.25 million tons of paddy per year (Alam et al., 2010; Huda, 2004). Rice is the main cultivated crop in haor areas. About 88% of the total area under the haor districts falls under the rice cropping area (DAE, 2008). It is important to mention that Haor areas contribute 18% to the national rice production (Huq, 2012). In most of the haor areas, BRRI Dhan 58, BRRI Dhan 29, BRRI Dhan 28, BR-19, BR-14, Gochi, Rata has been cultivated as the prominent Boro varieties. Among them, BRRI dhan 29 ranked first in respect of percent of cultivated farmers (92.20%) and areas (58.30%) (Muttaleb et al., 2008). Significant disease incidence was recorded in the most cultivated variety BRRI Dhan 29. In our recent survey, BRRI Dhan 58 was also found as most susceptible to this disease. In Bangladesh, a virulent pathogen with a plentiful amount of susceptible host exists and therefore, there is a possibility for this disease to reach at an epidemic level in the near future if a favorable environment exists.
Regarding Bakanae disease, a limited number of research has been conducted to find Bakanae resistance varieties. Accordingly, 87 rice varieties (46 BRRI released varieties, 40 landraces of BRRI germplasm and a Chinese variety “Purbachi”) were tested where BRRI varieties was found comparatively higher susceptible than the landraces (Hossain et al., 2015). Among them, BRRI Dhan 40, 30, 44, 32, 28, 29, 24, 25, 26, BR 8, 9, 10 were found as the most susceptible varieties while BR 23 and BR 11 were found as lower susceptible variety (Hossain et al., 2015). Interestingly, most of the farmers used BRRI released varieties though they are susceptible (Hossain et al., 2011). Besides, farmers in Bangladesh, especially in the haor area, has been used the rice seeds for planting from their previous yield rather than purchased new seeds. For sowing purposes, every year they kept some rice seeds for the next season. Moreover, they used the seeds for sowing without proper seed treatment. Though there are a lot of hybrids and HYV varieties, farmers are comfortable to cultivate with BRRI Dhan 28, BRRI Dhan 29, BRRI Dhan 58 etc. to get more yield. Even they are not willing to cultivate new hybrid varieties as they believe that other varieties are not high yielding. Thus, Bakanae pathogen is remained in the same land year after year and transmits from seed to soil or soil to seed. Management of Bakanae: To minimize the yield loss, Bakanae need to manage. According to Gupta et al. (2015), use of clean non-infected seed, hot water treatment, proper selection of geographical area, time and method of nursery sowing and transplanting, removal and destruction of infected plants from the field and balanced fertilization help in reducing the incidence of bakanae disease by minimizing the inoculum and seed-borne infection of the pathogen. However, the most effective and profitable strategy is to follow an integrated approach that uses several techniques like crops rotation, using resistant varieties, planting in warm soil, use balanced fertilizer, seed treatment with appropriate fungicides etc.