Abstract: Sixteen hybrid varieties/lines viz. Sonar Bangla-1, Sonar Bangla-5, Sonar Bangla-6, Sonar Bangla-12, 262H, 283H, 284H, 287H, 291H, 305H, 306H, 311H, 312H, 317H, 321H, 352H and check variety BR28 were evaluated under natural epiphytotics against Bipolaris oryzae to find out the effect of brown spot on their yield and yield contributing characters. The hybrid line 321H showed highest disease incidence (80%) and severity (77%) followed by 352H, 305H, 291H and Sonar Bangla-5. The lowest incidence (40.50%) and severity (45%) were recorded in case of hybrid line 312H followed by 287H, 284H and 283H. Eventually, 312H performed the highest yield (6.88 t ha-1) and the lowest (3.54 t ha-1) was in case of 321H. Yield contributing character like number of effective tillers/hill, panicle length, number of grains/panicle were significantly higher in 312H while number of spotted grains and unfilled grains/panicle were significantly higher in 321H. Disease incidence and severity were correlated significantly and negatively with the yield.
Introduction
Rice is the staple food constitute 92% of the food grain production in Bangladesh and 72% of the total cropped land is devoted to rice production (Anonymous, 1999). The production of food grain is increasing in the same rate as increasing the population of the country leaving an annual food deficit of 1-2 million tons. With the present yield level and area under rice cultivation it will be rather difficult to produce sufficient production of rice in the near future to fulfil the national demand. Besides, there is a little scope to increase the area under rice cultivation, as the situation rather demands to release some land for other crops.
One of the best options available to the plant breeder is the hybrid rice. For its yield potential, hybrid rice has brought a great hope and aspiration to meet the challenging demand of food deficits of the 21st century (Fakir, 1998). To overcome the present yield ceiling of existing rice varieties, hybrid rice seems to be highly attractive and a viable alternative.
Since, 1998 some private company has been importing hybrid rice seed from India and China. Concerned Research Institutes also exploring some hybrid lines/varieties for its probable utility in the country. But the performances of these varieties/lines were not satisfactory in the farmers' field because of pest susceptibility including brown spot and other diseases.
Among the rice diseases brown spot is an important seed borne disease (Fakir, 1998). It is the cause of historical Bengal famine in 1942-43 and also yet been considered to be one of the major disease of rice in Bangladesh causing 40-60% yield reduction (Mia, 1998). Bipolaris oryzae (Drechlera oryzae), the pathogen of brown spot causes severe infection during the reproductive phase of the plant that cause both quantitative and qualitative losses (Klomp, 1977). It causes germination failure, disease of emerging seedlings or growing plant resulting spotted, discolored and unfilled grains and ultimately yields reduction. Therefore, before the release of hybrid rice, it is of course, to be considered that those varieties/lines must be resistant/tolerant to field diseases with other desirable agronomic qualities. Thus, screening and selection of disease resistant variety/lines before mass production of seed is vital.
In view of the above facts this experiment has been conducted to estimate the incidence and severity of brown spot disease of different hybrid varieties/lines and to determine the effect of brown spot on the yield and yield contributing characters of rice.
Materials and Methods
The experiment was conducted in the field Laboratory of Department of Genetics and Plant Breeding, Bangladesh Agricultural University, Mymensingh on boro season in 2001. The hybrid varieties/lines used in the experiment have been given in Table 1.
The soil of the experimental plot was characterized by sandy loam texture with pH around 6.5, organic matter 1.14%, total nitrogen 0.085% and C/N ratio 13.5 (Anonymous, 1979). Seedlings were raised in 15th January 2001 and were transplanted by 15th February' 2001. Urea @180 kg ha-1, TSP @ 100 kg ha-1, MP @ 70 kg ha-1, gypsum @ 60 kg ha-1 and cowdung @ 6000 kg ha-1 were used. One third of the urea and entire TSP, MP, gypsum and cowdung were applied during the final land preparation. Remaining two-third of the urea was applied in two splits, one at 21st day after transplanting and the other before panicle initiation. Overall crop management was performed as per proper agronomic practices. Randomized complete block design was used as experimental design with (2 x 2.5 m2) unit plot size.
Assessment of incidence and severity of brown spot: The incidence and severity of brown spot was recorded under natural epiphytotic condition at 50 and 70 days after transplanting. Ten hills were randomly selected from each plot and data were collected on number of tillers/hill, number of diseased tillers/hill and percent leaf area diseased (LAD).
Disease incidence was calculated by the following formula (Rajput and Bartaria, 1995).
Disease severity was calculated by the Anonymous (1980) recommended grading scale as follows (Rajput and Bartaria, 1995).
The crops were harvested at its full maturity and selected 10 tagged hills of each plot were separated and data were recorded on number of effective tillers/hill, panicle length (cm), number of healthy grains/panicle, number of spotted grains/panicle, number of unfilled grains/panicle and yield (t ha-1). Data were analyzed using computer MSTAT package program and comparisons of the means were done by Duncan's multiple range test (DMRT) with LSD value.
Results and Discussion
The incidence and severity of brown spot of different hybrid varieties/lines of boro rice differ significantly from one another in comparison to check variety (Table 2). The incidence and severity range from 40.50 to 80 and 45 to 77% respectively among the varieties/lines. The maximum incidence (80%) was observed in 321H followed by 352H, 305H, 291H, Sonar Bangla-5 and 317H. The lowest incidence (40.50%) was observed in 312H followed by 284H, 287H, 283H and Sonar Bangla-6. The check variety - BR28 scored 62.50 % diseases incidence.
Similarly the maximum severity (77%) was noticed in 321H followed by 352H, 305H, 291H and Sonar Bangla-5. The minimum severity (45%) was noticed in 312H followed by 287H/284H, 283H and Sonar Bangla-6. The check variety scored 61.80% diseases severity.
The consequence effect of disease incidence and severity was observed on the yield and yield contributing characters of the hybrid varieties/lines. The number of effective tillers/hill, panicle length, number of healthy grains/panicle, number of spotted grains/panicle and number of unfilled grains/panicle were significantly differ from one another in comparison to check variety (Table 3). The number of effective tillers/hill, panicle length and number of healthy grains/panicle were found significantly the highest in 312 H followed by 284H, 287H and 283H. The number of spotted grains/panicle and the number of unfilled grains/panicle were significantly the highest in 321H followed by 352H, 305H, 291H, Sonar Bangla-5 and Sonar Bangla-1. Ultimately the maximum yield (6.88 t ha-1) was obtained by 312H followed by 284H, 287H, 283H and Sonar Bangla-6 while the minimum yield (3.54 t ha-1) was obtained by 321H followed by 352H, 305H, 291H and Sonar Bangla-5. The check variety BR28 yielded 4.88 t ha-1.
Correlation between % incidence and yield (t ha-1) was Y = 110.23– 9.6448X (r = –0.808) and between % severity and yield (t ha-1) was Y = 108.2 –9.501X (r = –0.984).
| Table 1: | Detailed information of the used hybrid materials |
| A = Cytoplasmic male sterile line (Female), R = Restorer line (Male) | |
| Table 2: | Incidence and severity of brown spot caused by Bipolaris oryzae in different hybrid varieties/lines of boro rice |
| Table 3: | Effect of brown spot caused by Bipolaris oryzae on the yield and yield contributing characters of different hybrid varieties/lines of boro rice |
The correlation indicates that the entries of hybrid varieties/lines were effective in increasing the yield by suppressing the fungal activities as evident with lower incidence and severity.
Experimental findings revealed that some hybrid lines to be attributed tolerance character against Bipolaris oryzae in comparison to check variety. Similar trend of results were reported by Rajput and Bartaria, (1995), Shukla et al. (1995), Prabhu et al. (1996), Prasad et al. (1998), Jha et al. (1999), Chuahan et al. (2000) and Payasi and Singh (2001). Payasi and Singh (2001) found resistant character in accession lines JR75, RWR92-3 and RWR54 against brown spot of rice. Chuahan et al. (2000) screened the accession line Lohana 1, Kalamkata and Rangi as highly resistant against brown spot. Jha et al. (1999) found six highly resistant and three resistant among fifty rice accession lines in India.