5.11B - Identifying plant disease

5.11B · Higher: identifying plant disease

Higher tier, separate Biology (5.11B). A plant with yellow leaves or poor growth is not automatically infected: shortage of a mineral ion, too little or too much water, unsuitable soil pH, frost, heat or chemical damage can produce similar changes. Identification therefore combines several kinds of evidence.

1. Observe visible symptoms. Record exactly what has changed and where, such as leaf spots, discolouration, wilting, stunted growth or rotting tissue. A symptom shows that the plant is unhealthy, but it rarely identifies a pathogen on its own.
2. Eliminate possible environmental causes. Check relevant conditions rather than assuming infection. Soil moisture or drainage can test a water-stress explanation; a soil test can check pH or mineral availability; temperature records can reveal frost or heat; and growing records can reveal herbicide or other chemical exposure.
3. Analyse the distribution. Map which plants and plant species are affected and how the pattern changes over time. Sudden, fairly uniform damage across several species often supports an environmental cause. A small group of affected plants that expands over time can support a pathogen hypothesis because infection can spread. These are clues, not proof, so exceptions must be considered.

Field patterns guide the next investigation. They cannot by themselves identify a pathogen: an expanding patch could still reflect a spreading environmental problem. Compare affected and unaffected plants before deciding which explanation the evidence supports.

5.11B · From field clues to a diagnosis

Use diagnostic testing. Send a sample of affected tissue to a laboratory when field evidence is not specific enough. Microscopy can reveal characteristic fungal structures; some fungi or bacteria can be isolated and cultured; antibody-based test strips can detect a particular pathogen; and DNA tests such as PCR can detect a matching pathogen sequence. A suitable positive test identifies the pathogen more specifically than symptoms alone.