عزل وتشخيص البكتريا Pectobacterium carotovorum المسببة لمرض التعفن الطري على البصل في محافظة كربلاء ومكافحتهُ احيائياً وكيميائياً

Isolation and Identification of Pectobacterium carotovorum Causing Soft Rot Disease on Onion in Kerbala Province and Its Biological and Chemical Control

Abstract
Bacterial soft rot disease represents a major challenge for onion cultivation in Iraq. This study aimed to isolate and identify the bacteria responsible for soft rot disease on onions collected from fields, storage facilities, and markets in Karbala province, as well as to evaluate their pathogenicity on onion plants. Additionally, the study investigated the susceptibility of several onion cultivars commonly grown in Iraq (Local Red, Texas Grano, Local White, Giza Egyptian, and White Grano) to infection, assessed the host range of the identified bacteria (potato, turnip, radish, beet, garlic, and Local Red onion), and evaluated the effectiveness of several biocontrol agents, chemical pesticides, and biological inducers, individually and in integration, against the disease under laboratory and greenhouse conditions. The experiments were conducted in the Plant Protection Department, College of Agriculture, University of Karbala, during the 2024–2025 growing season.
Isolation results yielded 12 bacterial isolates from onion plants showing soft rot symptoms collected from various sites. Pathogenicity tests on onion bulbs demonstrated that all isolates were capable of inducing soft rot symptoms, with isolate Pcc7 exhibiting the highest virulence in terms of lesion size and speed of symptom development, reaching 62.62 mm of rotted tissue within 72 hours, which was significantly higher than isolates Pcc5 and Pcc2 (40.1 and 58.22 mm, respectively). Thus, Pcc7 was selected for subsequent experiments.
Morphological, microscopic, and biochemical characterizations identified the bacterium as Pectobacterium carotovorum subsp. carotovorum, further confirmed by polymerase chain reaction (PCR). Sequence analysis of the PCR-amplified products, using the BLAST tool, revealed that isolate Pcc7 belongs to P. carotovorum subsp. carotovorum, showing genetic variation compared with previously registered isolates of the same species in the National Center for Biotechnology Information (NCBI). Therefore, it was deposited under the researcher’s name with the accession number PV390657.
Susceptibility tests of onion cultivars to infection with isolate Pcc7 revealed that the Local Red cultivar was significantly more susceptible than the others, with the highest lesion diameter (21.2 mm).
The host range assay confirmed that isolate Pcc7 possesses a wide pathogenic spectrum (polyphagous), inducing soft rot symptoms on all tested plant species, though with varying severity levels.
Evaluation of the effectiveness of the bio-preparation EM-1 against P. carotovorum subsp. carotovorum (Pcc7) on Nutrient Agar medium showed significant inhibitory effects on bacterial colony formation, particularly at the concentration of 8 ml/L (6.66 colonies). Moreover, melatonin and glutathione displayed strong antibacterial activity, achieving up to 100% inhibition at a concentration of 1.0 mg/L.
Chemical pesticide efficacy tests against Pcc7 demonstrated that Beltanol and Ganger achieved complete inhibition (100%) at 1.0 ml/L, while Oxyride exhibited 96.04% inhibition, significantly outperforming Nordox (93.61%) at the same concentration.
Greenhouse experiments confirmed that the integrated treatment, combining biological and chemical agents, provided a synergistic effect by reducing disease incidence to 0.00% while simultaneously enhancing plant defense responses and growth indicators. This highlights the effectiveness of integrated management strategies in sustainable soft rot control.
These findings emphasize the potential of adopting an integrated and sustainable approach for managing bacterial soft rot in onions, offering an effective alternative to reliance solely on chemical pesticides.