Plant Disease¶

Part of 4.3 Infection and Response.

Plant disease is easier to understand when you treat a plant as a living system facing pathogens, pests and nutrient problems. The challenge is to recognise the cause and then explain the response.

Learning Objectives¶

Detection and Identification¶

Plants can be affected by viruses, bacteria, fungi and insect pests, as well as by mineral deficiencies. Telling these apart is an important skill.

Visual Symptoms¶

When observing plants in the field, the following signs can point to disease or deficiency:

• Stunted growth — growth is slower than expected; often caused by reduced photosynthesis, pest damage, or nitrogen deficiency.
• Spots on leaves — for example, black spot is a fungal disease that leaves dark marks on rose leaves.
• Areas of decay — parts of leaves or stems rot and break down.
• Unusual growths — lumps or galls on stems and roots (e.g. crown gall disease).
• Malformed stems or leaves — distorted development, as seen after infection by the ash dieback fungus.
• Presence of pests — seeing aphids or mealybugs on a plant strongly suggests infection or damage is occurring.

Further Investigation Methods¶

When visual inspection is not sufficient, other approaches are used:

• Gardening manuals and databases — consulting reference materials listing symptoms for specific plant species helps narrow down the likely cause.
• Eliminating environmental causes — supplying the plant with missing nutrients can confirm or rule out deficiency as the cause of symptoms.
• Laboratory tests — scientists can detect the DNA or antigens of a suspected pathogen, or use a microscope to examine microorganisms in detail.
• Monoclonal antibody testing kits — diagnostic kits use monoclonal antibodies to identify specific pathogens, in a similar way to how pregnancy tests detect the hormone hCG.

Specific Plant Diseases¶

Mineral Ion Deficiencies¶

Plants absorb ions from the soil through their roots. Deficiency of key ions produces characteristic symptoms:

• Nitrate deficiency — nitrates are essential for making amino acids, which build proteins. Without enough nitrate, the plant cannot make enough protein for growth, leading to stunted growth. Leaves may appear pale or yellowed.
• Magnesium deficiency — magnesium is a core component of chlorophyll. Without it, the plant cannot synthesise enough chlorophyll, causing chlorosis — a yellowing of the leaves. This reduces the plant's ability to photosynthesise.

Plant Defence Responses¶

Plants have no mobile immune system, but they have evolved a range of physical, chemical and mechanical defences against pathogens and herbivores.

Physical Defences¶

• Cellulose cell walls — these rigid walls surround every plant cell. They form a structural barrier that pathogens must penetrate to infect the cell. (Animal cells have only a cell surface membrane, not a cell wall.)
• Waxy cuticle — leaves are covered by a thick, waxy cuticle. This prevents most pathogens from entering the leaf, and also reduces water loss.
• Bark — stems are protected by layers of dead cells that form bark. This acts as a physical shield around the living tissue beneath.

Chemical Defences¶

• Some plants secrete antibacterial chemicals that kill bacterial pathogens before they can establish an infection. This is analogous to the antimicrobial chemicals found in human tears.
• Certain plant-produced chemicals have value as medicines. For example, the compound in willow bark and leaves is related to aspirin, which relieves pain and fever. Quinine, extracted from cinchona tree bark, was used to treat malaria for many years.
• Some plants produce poisons that make them unpalatable or toxic to herbivores, reducing the risk of being eaten.

Mechanical Defences¶

• Thorns and hairs — sharp thorns deter large herbivores (e.g. cacti), and surface hairs can trap or deter smaller insects.

Common Confusions¶

• Disease vs deficiency: A student might assume that yellow leaves always mean disease. In fact, yellowing (chlorosis) is also a classic symptom of magnesium deficiency, which is a nutrient problem — not an infection. Similarly, stunted growth can result from either nitrate deficiency or disease. Always consider both possibilities.
• Physical vs chemical defences: The cell wall and cuticle are physical barriers; antibacterial compounds and poisons are chemical defences. Thorns and hairs are sometimes called mechanical defences. Make sure you categorise each correctly.
• Nitrate vs magnesium deficiency: Nitrate deficiency causes stunted growth (proteins cannot be made); magnesium deficiency causes chlorosis/yellow leaves (chlorophyll cannot be made). These are distinct and commonly confused.

Key Terms¶

• Pathogen: a disease-causing microorganism or agent.
• Deficiency symptom: a sign caused by shortage of a mineral ion rather than infection.
• Cuticle: a waxy layer on the leaf surface that helps reduce water loss and pathogen entry.
• Cellulose cell wall: a rigid outer layer that strengthens plant cells and helps resist damage.
• Testing kit: a diagnostic tool used to help identify the cause of plant disease.
• Chlorosis: yellowing of plant leaves caused by insufficient chlorophyll, often the result of magnesium deficiency.
• Nitrate deficiency: a shortage of nitrate ions in the soil, leading to stunted growth because the plant cannot make enough protein.
• Magnesium deficiency: a shortage of magnesium ions, leading to chlorosis because the plant cannot synthesise chlorophyll.
• Crown gall disease: a bacterial infection that causes tumour-like growths on stems and roots, restricting water flow through the plant.
• Tobacco mosaic virus (TMV): a viral disease of plants that creates a mosaic pattern of discolouration on leaves and reduces photosynthesis.
• Black spot: a fungal disease, commonly affecting rose plants, visible as dark spots on leaves.
• Antibacterial chemical: a substance produced by some plants that kills or inhibits bacterial pathogens.
• Thorn: a sharp outgrowth from a plant stem that acts as a mechanical defence against herbivores.

Connected Pages¶

• 4.3.2 Monoclonal Antibodies
• GCSE Biology Key Terms Glossary