Characteristics of Soil Transmission of Ralstonia syzygii subsp. syzygii, the Cause of Sumatra Disease of Clove in Indonesia

Background and Objective: Transmission of Sumatra disease of cloves is known to occur through the insect vectors from sick plants to healthy plants. This research studied the transmission of Ralstonia syzygii subsp. syzygii, the cause of Sumatra disease through the soil. Materials and Methods: Observations were made on two-year-old seedling plants planted in Kendal, Central Java, Indonesia infected with Sumatra disease. Detection of rhizosphere soil, infected plant residues and symptomatic plants was carried out using PCR method. The transmission experiment of Sumatra disease in clove seedlings was applied by artificial inoculation with the method of shoot titration, root immersion, root watering and root wounding conducted at greenhouse. Symptom appearance, incubation period, disease severity and Area Under Disease Progress Curve (AUDPC) were recorded periodically. Results: Ralstonia syzygii subsp. syzygii was detected in symptomatic replanting plants, rhizosphere soil layers and infected plant debris in the field. In the greenhouse experiment, all inoculation methods were able to cause Sumatra disease symptoms. The incubation period for the root immersion, root watering and root wounding treatment was shorter (12 DAI) than the titration treatment (17 DAI). The root wounding treatment gave the highest value of the disease severity (37.04) and AUDPC (201.85) compared to other treatments. Conclusion: Ralstonia syzygii subsp. syzygii can be detected in infected soil and plant debris and can be transmitted through the soil.