Molecular identification of Alternaria alternata causing brown leaf spot of potato and its management using novel fungicides

Abstract Brown leaf spot incited by Alternaria alternata has become a major foliar disease that affects potato production and under favorable weather conditions, causes significant yield losses. The objective of current investigation was to determine the causative pathogen and to assess the effectiveness of certain fungicides against A. alternata in both laboratory and field settings. Based on morphological and cultural characteristics, the pathogen isolated from diseased potato leaves was identified as A. alternata . This identification was further supported by molecular characterization using ITS sequence analysis (GenBank accession no. PX062184). Six fungicides comprising contact, systemic and combination formulations were assessed using the Poisoned food technique under in vitro conditions. Mancozeb 75% WP completely suppressed mycelial growth starting from 50 ppm, whereas, chlorothalonil 78.12% WP suppressed mycelial growth from 100 ppm and above, respectively. Among the systemic and combination fungicides, fluxapyroxad 167 g L⁻¹ + pyraclostrobin 333 g L⁻¹ SC totally inhibited fungal growth at all tested concentrations (5–25 ppm), followed by propiconazole 25% EC and tebuconazole 50% + trifloxystrobin 25% WG. Field evaluation of fungicides under natural epiphytotic conditions revealed that chlorothalonil 78.12% WP was the most effective treatment, exhibiting the lowest disease severity (34.75%), highest disease reduction (46.11%) and minimum AUDPC (562.43). Additionally, it yielded the maximum tuber yield (28.06 t ha⁻¹), yield increase (82.56%) and yield gain over control (12.69 t ha⁻¹), all of which had higher yield attributes. Chlorothalonil produced the highest gross (₹2,80,600 ha⁻¹) and net returns (₹1,67,424 ha⁻¹), however, mancozeb 75% WP was less expensive to apply, it had the highest benefit:cost ratio (2.54) and incremental cost-benefit ratio (20.50). Mancozeb was the most cost-effective treatment, whereas chlorothalonil was the most successful fungicide for controlling potato brown leaf spot and increasing productivity.