SN/T 5333-2020 in English

This standard has established a three-level identification system integrating symptomatology, Philippine downy mildew, Sugarcane downy mildew and Sorghum downy mildew for four quarantine pathogens.

Key points for morphological identification

Key observations: number of sporangiophore branches (2-4 times), conidia size (15-41μm) and oospore characteristics. For example, Java downy mildew fungus has no oospores, while the oospore wall thickness of sugarcane downy mildew fungus is 3.8-5μm.

Molecular detection process

1. DNA extraction: CTAB method was used to treat diseased tissues
2. Universal primers ITS5/ITS4 were used to amplify a 442bp fragment
3. Specific primers Pssm-2/Pssm-3 were used to amplify a 120bp fragment
4. Sequencing was compared with the NCBI database (similarity ≥ 98%)

Quarantine operation specifications

• Seed samples need to be tested for moisture content (<14% is not disease-infectious)
• Focus on checking the oospores in the sorghum husk
• Positive samples must be stored at 4℃ + sterilized by high pressure

Technology evolution analysis

Compared with the 2000 version, this standard adds:
1) Requirements for molecular equipment such as PCR instruments
2) Specific primer detection system
3) The sequence comparison judgment threshold is increased from 95% to 98%