DEVELOPMENT OF ALTERNARIA BLIGHT IN SOME ADVANCED MUTANTS OF BRASSICA NAPUS UNDER FIELD CONDITIONS
M. Jalaluddin and M. A. Kashem
The natural development of Alternaria blight caused by Alternaria brassicae (Bent) Sacc. on three advanced induced mutants of Brassica napus (Mut. 19, Mut. 36 and Mut. 43) along with their mother cultivar, Nap-3 was studied under field conditions during winter seasons of 1994-95 and 1995-96. The number of lesions that caused severe blights on leaves and pods and leaf defoliation increased significantly with advanced stages of crop growth. The pattern of development and the rate of progress of disease were almost similar in all the tested materials. Severe Alternaria blight development was initiated 40 days after sprouting which may be considered as critical stage of development of the disease in the field. To avoid yield loss fungicidal spray programme should be initiated during this critical stage of disease development. All the induced mutants produced higher seed yield than their mother cultivar, Nap-3. However, the induced mutant-43 produced the highest seed yield (191.4g/m2) followed by mutant-19 in both the growing seasons.
KEYWORDS : Oil-seed rape/mustard, Alternaria brassicae, Disease management, Disease
progress, Seed yield.
The amount of edible oil produced from oil-seed mustard and rapeseeds does not meet the current requirements of the 126 million people of Bangladesh. Most of the common people of this country are traditionally fond of mustard oils. The country is deficit of about 90,000 metric tons of edible oil annually (Lutfur Rahman et al. 1993) which is imported from other countries. Among the prevailing diseases, Alternaria blight caused by the fungus Alternaria brassicae (Bent) Sacc is the most destructive which limit oil-seed production. The disease may cause significant losses in both temperate and tropical oil-seed crops (Verma et al. 1994). About 30-48% yield losses have been reported in different countries of the Indian subcontinent (Rai et al. 1976; Fakir, 1980 and Das Gupta, 1989). The disease is seed-borne ( Meah et al. 1985) and it can some extent be managed through the use of fungicides (Meah et al. 1988, Howlider et al. 1985). The degree of success for management of any plant disease depends on the acquired knowledge of the development of the disease at various stages of crop growth. Very little work has been done on the development of the disease under field conditions in Bangladesh. Therefore, the natural development of Alternaria blight was studied to identify the critical stage of development of the disease in the field in order to suggest for economic management practices to increase seed quality and yield in rapeseed mutants of Brassicae napus developed at the Bangladesh Institute of Nuclear Agriculture, Mymensingh.
MATERIAL AND METHODS
The experiments were conducted in randomized complete block design with four replications at the Bangladesh Institute of Nuclear Agriculture farm, Mymensingh and its substation farms at Jamalpur and Ishurdi during the winter seasons of 1994-95 and 1995-96. The development of Alternaria blight on the three advanced mutants (viz. Mut. 19, Mut. 36 and Mut. 43) of the Brassica [BINA1]napus along with their mother variety Nap-3 were studied. The unit plot size was 5mx5m. The seeds were sown in continuous rows with spacing 20cm apart. The complete doses of fertilizers were applied @ 90kg N, 80kg P2[BINA2]O5, 40kg K2O and 40kg S ha-1 (Das and Rahman,1994). Intercultural operations were done when required.
The development of Alternaria blight on leaves and pods and the percent leaf defoliation/plant at various stages of plant growth were recorded in all the three locations. After initiation of the disease, the severity was recorded on 15 randomly selected rapeseed plants from each plot. The severity on leaves was assessed 20, 40, 60 and 80 days after sprouting and on pods 40, 60 and 85 days after sprouting. The severity was recorded by counting the number of lesions per leaf and the percent blighted areas on leaves and pods. The crops of each year were harvested at full maturity. Seed yield from 1m2 plots was recorded for each tested material. The combined analyses were done among the different locations. Analyses of variance of different parameters for disease development at different growth stages and locations during 1994-95 and 1995-96 were made.
RESULTS AND DISCUSSION
The characteristic symptoms of the disease were visible on leaves and pods 35 and 55 days after sprouting, respectively and then the number of lesions and percent blighted areas on leaves and pods and leaf defoliations were significantly increased with increased plant growth in all the locations during 1994-95 and 1995-96 cropping seasons (Fig.1, 2 and 3). The pattern of the disease
Fig.1 Mean lesions of Alternaria brassicae developed on leaves (A1 and A2) and pods (B1 and B2) in induced mutants and varieties of Brassica napus at different stages of crop growth.
Fig. 2 Mean areas of leaf (A1 and A2) and pod (B1 and B2) blighted due to Alternaria brassicae in induced mutants and varieties of B. napus at different stages of crop growth.
Fig. 3 Percent leaf defoliations recorded on the induced mutants and varieties of Brassica napus at different stages of crop growth.
development and or progress were almost similar in all the tested material of rapeseed (Fig. 1 and 2). The results indicate that there were no variations among the cultivars with respect to the development of the disease. The disease initiated on the youngest plants after emergence and was visible on leaves 35 days after sprouting and then the inoculum of the pathogen become established and multiplied on the growing plants and caused severe blights on leaves of the growing crops in the subsequent stages of crop growth. Before pod formation the inocula of the pathogen become established on the growing plants for infection to the newly developed pods. To avoid any yield loss fungicidal spray programme should be commenced at the initial stage of disease establishment in the growing plants or developing pods. Tripathi et al. (1987) reported that a spray schedule involving 4 sprays of captafal starting 30 d after sowing at 15-d intervals was the best combination for maintaining a disease free crop.
The development of the disease was highly significant among the years, locations and the stages of crop growth. These results confirmed the results reported by Dang et al. (1995) who reported that the development of the disease was influenced by different environment at factors prevailed in different years and locations i. e. agro-ecological zones of the country.
In field studies during 1994-95 and 1995-96 severe disease was appeared with low temperature (10 - 150 C min. 24 -270 C max.) and high RH (80-90%). The result of disease intensity and the pattern of their development were almost similar to the result obtained by Sinha et al.(1992).
A significant positive correlation (r = 0.98) was found between lesion number and disease severity which confirms, the result reported by Dingar and Singh (1985) and Suhag et al. (1985).
Though there were no significant differences of the Alternaria blight severities among the induced mutants and the mother variety Nap-3 of B. napus but the induced mutant 43 showed the highest seed yield followed by Mutant 19 (Fig. 4). This result confirms the results obtained in different
Fig. 4 Yield performance of the induced mutants and their mother cultivar of B. napus during 1994-95 and 1995-96
experiments conducted by the scientists of Plant Breeding Division of the Bangladesh Institute of Nuclear Agriculture in different ago-ecological zones of Bangladesh (Anonymous, 1997 and 1998). Considering the yield and the level of tolerance to Alternaria blight the induced mutant 43 and 19 can be recommend as commercial cultivars for growing by the farmers of the country.
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