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Detecting visible disease symptoms in plants

Infection and response • Plant disease (biology only)

Key concepts

What you'll likely be quizzed about

  • Plants infected by pathogens or suffering from nutrient deficiencies exhibit visible changes in form and color.
  • Observation of external signs provides the first indication of disease since infections often alter leaf color, growth rate, or tissue integrity.
  • Identification based on external symptoms typically narrows the cause to a group of agents-viruses, bacteria, fungi, protists, or nematodes-or to nutrient deficiencies.
  • Diagnosis solely based on symptoms has limitations, as various causes can produce similar signs; for example, spots may arise from both fungal infections and abiotic damage.

Flashcards

Test your knowledge with interactive flashcards

Give one visible sign that suggests a fungal infection.

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Dark or colored spots on leaves, like black spot on roses, typically indicate a fungal infection.

Key notes

Important points to keep in mind

Visible symptoms include stunting, spots, rot, growths, malformation, discolouration, and pests; observe patterns and distributions to identify causes.

Mosaic or mottled leaves often indicate viral infections, such as the tobacco mosaic virus.

Dark round spots typically signal fungal pathogens, like rose black spot; warm, wet weather accelerates fungal proliferation.

Stunted growth with pale leaves often indicates nitrate deficiency, not infection; soil nutrients should be checked.

Rot typically follows tissue breakdown by fungi or bacteria, often beginning at wounds or waterlogged tissue; smell and texture can aid cause identification.

Galls or abnormal growths suggest hormonal manipulation by parasites or pathogens; species-specific galls can serve as diagnostic indicators.

Aphids' presence indicates immediate pest management needs as they cause curling and discolouration while transmitting viruses.

Symptom overlap necessitates confirmatory tests (e.g., lab culture, antibody tests) for precise diagnosis.

Compare suspected infected plants with healthy controls of the same age and variety to identify abnormal development.

Record environmental conditions (temperature, humidity, soil moisture) when symptoms first appear to assist in cause identification.

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