Background and Objective
Bacterial leaf blight of rice, caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the most destructive and economically significant diseases of this globally strategic crop. The disease is widely distributed in major rice-growing regions and can cause severe yield losses, especially under warm and humid environmental conditions that favor rapid pathogen multiplication and spread. Given the essential role of rice in global food security, developing sustainable and effective management strategies for bacterial leaf blight remains a key priority in modern plant protection. Even though chemical control measures have traditionally been used to lower disease incidence, their repeated application is associated with limitations such as inconsistent efficacy, emergence of resistant pathogen strains, environmental contamination, and negative impacts on non-target beneficial microorganisms. Thus, increasing attention has been directed toward alternative approaches that boost plant defense capacity and promote biological suppression of pathogens. In this context, the present study aimed to ascertain the efficacy of natural growth stimulants and biological agents in the management of bacterial leaf blight under field conditions.
Materials and Methods
In this study, the effects of various treatments, including humic acid, effective microorganisms (EM), and the antagonistic bacterium Peribacillus frigoritolerans, were appraised on four rice cultivars (Fajr, Hashemi, Setayesh, and Shirudi) under natural field conditions. These cultivars represent commonly cultivated varieties with different levels of susceptibility and agronomic characteristics. The experiment was undertaken as a factorial arrangement based on a randomized complete block design with four replications to ensure reliable statistical comparison of treatment effects. The treatments were applied ten days after transplanting the seedlings to allow proper establishment of the plants. The pathogenic bacterium was inoculated seven days after treatment application to ascertain the preventive and protective effects of the applied treatments against subsequent pathogen challenge. The population dynamics of the bacterium within leaf tissues were measured on days 1, 5, 10, 15, and 20 post-inoculation to monitor bacterial colonization, multiplication, and persistence over time. Disease severity was assessed 30 days following inoculation based on the mean length of necrotic lesions on infected leaves, providing a quantitative indicator of disease development as well as host response.
Results
The results revealed that treatment efficacy varied depending on both cultivar and assessment time. The combined treatment of humic acid and EM consistently and significantly (p≤0.01) diminished the Xoo population and necrotic lesion length compared to the pathogen-inoculated control in the Fajr, Hashemi, and Setayesh cultivars. This reduction was sustained across multiple sampling intervals, suggesting a stable suppressive influence on bacterial growth within plant tissues. The combined application was therefore identified as the most effective treatment strategy among those appraised in this study. Humic acid treatment alone also exhibited significant efficacy in the Fajr and Hashemi cultivars, reflecting its potential role in stimulating plant growth, improving nutrient uptake, and enhancing defense-related physiological responses. EM treatment alone presented an inhibitory effect only at specific time points and primarily in the Setayesh cultivar, suggesting that its performance may depend on the host genotype or environmental conditions. The P. frigoritolerans treatment failed to control the disease significantly in any of the cultivars under the conditions of this experiment, as no meaningful reduction was observed in bacterial population or lesion length when compared to the inoculated control.
Discussion
The integrated application of humic acid and EM, as a sustainable and environmentally friendly strategy, presents high potential for managing bacterial leaf blight in susceptible rice cultivars. The enhanced effectiveness of the combined treatment may be attributed to ameliorated plant vigor, stimulation of host defense mechanisms, as well as favorable modulation of the rhizosphere microbial community, which together may limit pathogen establishment and disease progression under field conditions. Further, the synergistic interaction between these components may boost physiological resilience and improve overall plant performance during pathogen stress. This approach, by enhancing overall plant and rhizosphere health, can serve as a suitable alternative for lowering reliance on chemical methods in rice disease management programs and may contribute to more resilient as well as sustainable production systems in intensive rice-growing regions.