You notice one of the plants in your glasshouse trial shows severe necrosis, dieback and chlorosis on the leaves as shown in the photo below. You suspect a bacterial infection of the leaves. What will be your next step to confirm your diagnosis?

Question

BlackTom AI · Accepted Answer

An ooze test quickly detects bacterial exudation from lesions, and Nutrient Agar (NA) is the appropriate general medium to isolate bacterial leaf pathogens, so performing an ooze test and then isolating on NA is the correct approach.

1) Relevant knowledge points
- Bacterial leaf pathogens often produce a visible sticky or milky exudate (ooze) from lesions; an ooze/streaming test is a rapid field/lab screen.  
- Nutrient Agar is a non-selective, bacteriological medium suited to grow many plant-pathogenic bacteria.  
- Potato Dextrose Agar (PDA) is optimized for fungi and yeasts, not routine bacterial isolation.  
- Gram stain and other tests require a pure bacterial isolate; they do not by themselves confirm pathogenic infection.

2) Step-by-step reasoning/process
- Inspect lesion for signs of bacterial ooze. Perform an ooze test: place a fresh lesion in a drop of sterile water on a slide or press the leaf surface onto a clean glass slide; look for milky droplets or streaming of bacterial cells under low magnification or by eye.  
- If ooze is observed (positive), aseptically excise lesion tissue and streak onto Nutrient Agar plates to obtain bacterial colonies. Incubate (e.g., 24–48 h at appropriate temperature) and examine colony morphology.  
- From distinct colonies, pick isolates for further identification (Gram stain, biochemical/ molecular tests, pathogenicity/Koch’s postulates) to confirm the causal agent.

3) Intermediate steps (practical details)
- Collect lesion, avoid older desiccated tissue; do not surface-sterilize before an ooze test. Streak directly from lesion margins to NA after positive ooze. Incubate and subculture to purity.

4) Why the correct answer is correct
- The ooze test is the fastest indicator of a bacterial infection; NA is the correct medium to culture and recover bacterial pathogens for confirmation.

5) Why other options are incorrect
- Surface-sterilising and plating immediately risks losing surface ooze and may not reveal streaming bacteria; plating on PDA favors fungi, not bacteria. Gram staining alone won’t confirm disease without isolation; doing an ooze test then culturing on PDA is inappropriate because PDA is suboptimal for bacteria.

Conclusion: therefore the correct next step is to perform an ooze test and, if positive, isolate and culture the bacteria on Nutrient Agar (the provided answer).

You notice one of the plants in your glasshouse trial shows severe necrosis, dieback and chlorosis on the leaves as shown in the photo below. You suspect a bacterial infection of the leaves. What will be your next step to confirm your diagnosis?单项选择题

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