In-vitro efficacy of systemic, non-systemic and combi-fungicides against Alternaria porri causing purple blotch of onion

Purple blotch of onion, incited by Alternaria porri, poses a serious threat to onion cultivation, significantly impacting both yield and bulb quality. The present study was conducted to evaluate the in vitro efficacy of different categories of fungicides systemic, non-systemic, and combi-fungicides against the most virulent isolate (ChAp1) of A. porri, using the poisoned food technique on PDA medium. Systemic fungicides (seven in total) were tested at 500 and 1000 ppm. Four fungicides—Propiconazole, Hexaconazole, Difenoconazole, and Tebuconazole—completely inhibited fungal mycelial growth (0.00 mm) and achieved 100% inhibition at both concentrations. These treatments were statistically at par and significantly superior over others. Pyraclostrobin exhibited 90.03% and 90.92% inhibition at 500 and 1000 ppm, respectively, while Thiophanate methyl recorded 72.40% and 83.51% inhibition. Azoxystrobin was the least effective among systemic fungicides, showing 62.40% inhibition at 500 ppm and 70.18% at 1000 ppm. Non-systemic fungicides (seven in total) were evaluated at 2000 and 2500 ppm concentrations. The most effective were Copper hydroxide (93.14% and 94.06%), Mancozeb (90.92% and 93.14%), and Dinocap (89.07% and 90.23%). Other effective treatments included Propineb (85.55% and 87.03%) and Copper oxychloride (83.51% and 88.14%). Thiram was the least effective, showing 69.81% inhibition at 2000 ppm and 77.59% at 2500 ppm. Combi-fungicides (seven combinations) were also tested at 2000 and 2500 ppm. Captan 70% + Hexaconazole 5% showed 100% inhibition at both concentrations, outperforming all other treatments. This was followed by Trifloxystrobin 25% + Tebuconazole 50% (92.58% and 93.14%), Hexaconazole 4% + Zineb 68% (91.12% and 91.14%), and Thiophanate methyl 45% + Pyraclostrobin 5% (89.81% and 91.12%). The least effective among the combinations was Metiram 50% + Pyraclostrobin 5%, with inhibition rates of 86.48% and 88.51%, respectively. The study conclusively demonstrated that among all fungicides tested, Captan + Hexaconazole, Propiconazole, Copper hydroxide, and Mancozeb are the most effective in suppressing A. porri under in vitro conditions. These results are in agreement with previous studies and offer a valuable reference for integrated disease management of purple blotch in onion crops.