Study On Technique Of Watermelon Seed Quarantine Disinfection For Bacterial Fruit Blotch

Bacterial fruit blotch(BFB) of watermelon [Acidovorax citrulli (Schaad et al.1978) Schaad et al.2009] is a worldwide disease on melon crop, causing serious losses. Taking into account of its potential risk, difficulty to control, long-distance transmission mainly by seed transport, China has treated A. citrulli as a domestic and entry quarantine organism. Seed disinfection calls for zero tolerance on viable bacterium. Several seed disinfection methods have been reported, but we need an effective evaluation system to make sure the effectiveness of methods. Authors built a reliable evaluation system for different disinfection methods, and screened for100%disinfection with safety for practical application. Disinfection programs that have been reported were evaluated as not completely effective. By increasing pressure on the disinfection,3seed disinfection programs were obtained that completely killed bacteria on seed, and proved the security.The core of the evaluation system is the use of a streptomycin-resistant(SR) bacteria strain. Firstly, LB medium containing streptomycin (LBS) was used to screen SR colonies of A. citrulli, secondly watermelon seed mixing with SR bacteria or seed from artificially inoculated watermelon fruit were treated with chemical or physical methods respectively at different times, thirdly bacteria were recovered using LBS, and finally recovered bacteria were confirmed by PCR with specific primer pair through based on a pilus protein gene (patent ZL201110452871.7).At next step, this evaluation system with seed mixing with SR bacteria was used to test seed sterilization methods previously reported. The results showed that those disinfection programs were not enough to completely kill the bacteria on the seeds.After we increased disinfection pressure, i.e. higher concentration of disinfection agent and prolonged heat treatment time,3satisfied schedules were screened with seed from noculated fruit,4%hydrochloric acid soaking for20min,2%formaldehyde soaking for30min, at75℃sterilization for48h. Those methods completely killed fruit blotch bacteria on seeds and had no adverse effect on seed germination and vigor. The evaluation also showed sodium hypochlorite, calcium hypochlorite and hydrogen peroxide at the recommended concentrations and time could not completely kill the bacteria, even at higher concentration in the cost of low seed germination (Hunan Agricultural Sciences20135). On the other hand, the authors also tried the irradiation sterilization for bulk seed sterilization. The test results show that the radiation intensity at8K and10K can completely kill bacteria, but, unfortunately, significantly inhibited seed germination. Radiation intensity6K and below proved less effective.The evaluation system above for seed disinfection methods of fruit blotch pathogen can be used as one standard for effect evaluation of disinfection methods of bacteria. Those3disinfection solutions proved feasible by the evaluation system can promote the application in production.