Antagonist bacteria in action: A strategy for managing sugarcane white leaf disease

Sugarcane white leaf disease (SCWL) is one of the most threatening diseases affecting sugarcane cultivation in Thailand. Current disease elimination techniques remain ineffective as the pathogen is seed-borne. Interestingly, sugarcane infected with SCWL phytoplasmas exhibits fewer symptoms when cultivated under favourable conditions compared to adverse environments, highlighting a relationship between plant health and stress tolerance. This study aimed to investigate integrated management practices to enhance the growth and survival of SCWL-infected sugarcane. Antagonistic bacteria were isolated from soil samples collected from sugarcane fields in Khon Kaen, Thailand. Experiments using Streptomyces spp. No. 8 and Bacillus subtilis No. 15 as soil drenches in sugarcane infected with phytoplasmas below a critical threshold showed statistically significant improvements in plant height, root fresh weight, and brix levels compared to untreated controls. Field evaluations using disease- free sugarcane cuttings treated with hot water and B. subtilis No. 15 confirmed the effectiveness of this management method. Seedlings were grown in a greenhouse with PGPR-3 biofertilizer and received weekly foliar sprays of B. subtilisNo. 15 before being transplanted into fields enriched with organic and PGPR-3 biofertilizers and treated bimonthly with B. subtilis No. 15. The results showed that planting disease-free sugarcane increased the survival rates to 100%, stalks per plant by 55%, height by 28%, and brix levels by 11% compared to disease-free and latent-infected plants. Ten months after planting, no white leaf symptoms were observed in cuttings with phytoplasma content at 9 copies/μl. In infected cuttings managed with this method, no symptoms appeared, despite phytoplasma levels reaching 174 copies/μl. In contrast, infected cuttings without management exhibited 58% white leaf symptoms, with phytoplasma content exceeding 23,800 copies/μl. These findings support the conclusion that biological control of phytoplasma diseases in sugarcane using antagonistic bacteria is a promising and sustainable alternative to chemical control.