To diagnose plant disease effectively, tissue should be taken from which zone?

• A. The smallest leaf possible
• B. Plant tissue that has been dead for a long time
• C. Plant tissue from the transitional area between healthy and damaged areas
• D. Dry plant material

Answer: (C)

Sampling tissue at the margin where healthy tissue meets diseased tissue is most informative because that border is where the disease process is actively taking hold. The invading pathogen and the plant’s responses are most evident there, so you’re likely to find signs of infection, early symptoms, or the organism itself. Tissue from this transitional zone provides a representative snapshot of the current infection, helping you identify the cause more accurately.

Tissue from dead areas or long-dead material isn’t useful for diagnosing an active disease because the pathogen may no longer be present, and the tissue is degraded, making it hard to detect causal agents. Dry plant material likewise isn’t ideal for culture or molecular tests and won’t reflect the active disease state. Sampling a very small or non-border area might miss the pathogen or the symptom development, whereas the border area captures the ongoing interaction between host and pathogen.