Phytopathogenic mollicutes

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.