Twenty seven (27) isolates of Alternaria porri were isolated from diseased leaf samples collected from different onion growing regions of Bangladesh and characterized for cultural, morphological and pathogenic variabilities. A. porri colonies colony colour ranged between light to dark olivacious and grayish white with irregular, regular with concentric ring and regular without concentric ring shape. Margin of colonies were entire, irregular and wavy with effuse, fluffy and velvety texture. Isolates impregnated media with colour ranged between grey to brown on the reverse of the plates. Growth rate of isolates ranged between 2.433 and 3.950 mm/day with fast growth in isolate DSTR 02 and least in MMBH. Morphological variation in conidia production was between 7.720×10 3 to 47.02×10 3 per mm 2 with sporulation time 3.33 to 11.00 days. The conidial shape was straight to curve with light to deep brown colour. The number of horizontal and vertical separation in the conidia ranged from 3.00 to 6.00 × 1.00 to 2.00 with size from 11.20 to 39.20 × 4.76 to 11.43 µm. In pathogenicity test isolates also exhibited variations in size of the lesions (2.77 to 7.55 mm) produced on onion leaves. The results demonstrate existence of considerable variation in cultural, morphological, and pathogenic characters of A. porri isolates prevalent in Bangladesh environment.

Abstract

Twenty seven (27) isolates of Alternaria porri were isolated from diseased leaf samples collected from different onion growing regions of Bangladesh and characterized for cultural, morphological and pathogenic variabilities. A. porri colonies colony colour ranged between light to dark olivacious and grayish white with irregular, regular with concentric ring and regular without concentric ring shape. Margin of colonies were entire, irregular and wavy with effuse, fluffy and velvety texture. Isolates impregnated media with colour ranged between grey to brown on the reverse of the plates. Growth rate of isolates ranged between 2.433 and 3.950 mm/day with fast growth in isolate DSTR 02 and least in MMBH. Morphological variation in conidia production was between 7.720×10 3 to 47.02×10 3 per mm 2 with sporulation time 3.33 to 11.00 days. The conidial shape was straight to curve with light to deep brown colour. The number of horizontal and vertical separation in the conidia ranged from 3.00 to 6.00 × 1.00 to 2.00 with size from 11.20 to 39.20 × 4.76 to 11.43 µm. In pathogenicity test isolates also exhibited variations in size of the lesions (2.77 to 7.55 mm) produced on onion leaves. The results demonstrate existence of considerable variation in cultural, morphological, and pathogenic characters of A. porri isolates prevalent in Bangladesh environment.

Field research was conducted in the central research farm of Sher-e-Bangla Agricultural University (SAU), during the period from October 2020 to March 2021. The experiment was a layout with RCBD design to evaluate the effect of selected fungicides (Indofil, Mancer, and Ridomil Gold), micronutrients (Zinc, Boron, and Magnesium), and botanicals (neem and eucalyptus leave extract) on purple blotch complex of onion. Data were collected on percent disease incidence and percent disease index, the number of leaves per plant, leaf length, bulb diameter, bulb weight (fresh and dry), and yield. The local cultivar Faridpuri was used as a tested variety. Among the selected fungicides, Ridomil Gold gave a better result than other fungicides in controlling the percent disease incidence (52.67%) and percent disease index (49.00%) with the highest number of yield qualities per plant. In addition to this, the highest bulb diameter (18.23 cm), single bulb weight (47.98 g), fresh weight (3.16 Kg/plot), and dry weight (2.88Kg/plot) were also obtained in Ridomil Gold treated plots. This treatment also gave better yield performance (9.60 t ha-1) followed by Indofil (8.20 t ha-1) and Mancer (7.90 t ha-1), and increased yield by 54.34% over the control treatment. From the study, it was also found that among the micronutrients, boron gave more satisfactory results than botanicals.