Plants can be infected by bacteria, viruses, fungi and other parasites — just like animals. Recognising disease symptoms is important for farmers, gardeners and conservationists.
Common symptoms of plant disease:
STUNTED GROWTH — plant is smaller or grows more slowly than expected for its age and growing conditions.
SPOTS ON LEAVES — discoloured patches, often black, brown or orange.
AREAS OF DECAY (ROT) — brown, soft, collapsing tissue in stems, roots or fruit.
ABNORMAL GROWTHS — unusual swellings, galls or distorted structures.
MALFORMED LEAVES OR STEMS — twisted, curled or distorted plant parts.
Look up symptoms in a plant disease identification guide or database.
Send a sample to a laboratory for microscopic examination.
Test using specific diagnostic kits (e.g. lateral flow tests for specific pathogens).
Observe when and where the disease appears — conditions often give clues (e.g. fungal diseases favour wet, warm weather).
Physical Defences in Plants
Plants have evolved a range of physical defences that act as barriers to prevent pathogens and pests from entering:
CELL WALLS made of cellulose — provide a rigid physical barrier around each cell. Much harder for pathogens to penetrate than a soft cell membrane alone.
WAXY CUTICLE on leaves and stems — a waterproof, waxy layer secreted by epidermal cells. Prevents pathogens from landing and colonising the leaf surface. Also reduces water loss.
BARK on woody stems — tough outer layers protect living tissue (phloem and cambium) underneath from physical damage and pathogen entry.
THORNS and SPINES — physical deterrents to herbivores, reducing the chances of wounds through which pathogens can enter.
CLOSING OF STOMATA — when plants detect pathogen attack, they can close stomata to prevent pathogen entry through these openings.
Chemical Defences in Plants
Plants also produce a wide range of chemicals that deter herbivores and fight pathogens:
ANTIBACTERIAL COMPOUNDS — some plants produce substances with antimicrobial properties:
Allicin in garlic — shown to have antibacterial and antifungal activity.
Tannins in oak bark and tea leaves — bitter astringent compounds that deter herbivores and have some antimicrobial properties.
POISONS (ALKALOIDS) — plants produce toxic secondary metabolites to deter animals from eating them:
Nicotine in tobacco plants — highly toxic to insects (used as an insecticide).
Caffeine in coffee and tea plants — deters insects from feeding.
Quinine in cinchona trees — bitter taste deters herbivores; also has antimicrobial properties.
Digitalis (foxglove) — toxic to vertebrates including humans if eaten in large quantities.
INSECT-REPELLING COMPOUNDS — volatile chemicals released from leaves deter insects from landing and laying eggs.
STICKY RESINS and LATEX — trap insects on the surface or clog their mouthparts if they try to feed.
Responding to Attack
Plants can also mount active responses when they detect pathogen invasion:
HYPERSENSITIVE RESPONSE — when a pathogen is detected, the plant rapidly KILLS the cells immediately surrounding the infection site, creating a zone of dead cells.
This dead zone acts as a firebreak — the pathogen cannot spread through dead cells, containing the infection.
SYSTEMIC ACQUIRED RESISTANCE (SAR) — after a localised infection, the whole plant can become more resistant to future attacks.
The plant produces signalling molecules (like salicylic acid) that travel to uninfected parts of the plant, 'warning' them to prepare chemical defences.
Many of the chemicals plants produce for defence have also been used by humans:
Aspirin derived from salicylic acid.
Quinine — antimalarial drug.
Morphine and codeine — painkillers from poppies.
Many modern medicines originated as plant defence chemicals.
⚠️ Common Mistake
Plants do NOT have an immune system like animals — no antibodies, no lymphocytes, no phagocytes. Their defences are physical (walls, wax, bark) and chemical (poisons, antimicrobials). They are passive or triggered responses, not a learned adaptive immune system.