Learning objectives
- Classify pathogens and give an example of each
- Draw and explain the disease triangle
- State Koch's postulates in order
1. Classes of plant pathogen
Fungi and oomycetes cause the majority of plant diseases (late blight, rusts, mildews, wilts). Bacteria cause soft rots, wilts and blights. Viruses cause mosaics, mottles and leaf curl and are usually spread by insect vectors. Nematodes cause root galls and decline. Non-infectious (abiotic) disorders come from nutrients, water, temperature or chemicals.
- Fungus — Phytophthora infestans (late blight of potato/tomato)
- Bacterium — Ralstonia solanacearum (bacterial wilt)
- Virus — Tobacco mosaic virus (mosaic patterning)
- Nematode — Meloidogyne spp. (root-knot)
2. The disease triangle (and tetrahedron)
Disease develops only where a susceptible host, a virulent pathogen and a favourable environment overlap. Remove any one side and disease does not develop — this is the logic behind every control measure you will ever write about.
Adding time as a fourth factor gives the disease tetrahedron; adding human activity gives the disease pyramid used in epidemiology.
3. Symptoms versus signs
A symptom is the plant's reaction — wilting, chlorosis, necrosis, galls, cankers. A sign is the physical evidence of the pathogen itself — fungal mycelium, spore masses, bacterial ooze. Confusing the two is one of the most commonly penalised errors.
4. Koch's postulates
1) The suspected pathogen must be consistently associated with the diseased plant. 2) It must be isolated and grown in pure culture. 3) When inoculated into a healthy plant it must produce the same disease. 4) It must be re-isolated from that plant and match the original culture.
Note the exception: obligate parasites such as rusts and viruses cannot be grown in pure culture, so the postulates must be modified for them.
Key definitions
- Pathogen
- An organism capable of causing disease in a host plant.
- Inoculum
- Any part of a pathogen that can initiate infection, e.g. a spore or a sclerotium.
- Virulence
- The degree of damage a pathogen can cause to a susceptible host.
Exam tips
- Draw the disease triangle whenever you are asked why a disease appeared in one season and not another.
- Give the full binomial in italics and spell it correctly — marks are awarded for scientific names.
Practice exam questions & model answers
Q1.Using the disease triangle, explain why late blight was severe in one season but absent the next.
The host and pathogen may have been present in both seasons, but disease requires a favourable environment. Late blight needs cool, wet, humid conditions; in a dry season that third side of the triangle is missing, so the epidemic does not develop.
Q2.Differentiate between a symptom and a sign, giving one example of each.
A symptom is the plant's visible reaction to infection, e.g. wilting. A sign is direct evidence of the pathogen itself, e.g. white fungal mycelium on the leaf underside.
Q3.Why can Koch's postulates not be applied directly to a rust fungus?
Rusts are obligate parasites and cannot be grown in pure culture on artificial media, so the second postulate cannot be fulfilled without modification such as inoculation from infected host tissue.
Continue revising
Plant Cell Structure and Function
Plant BiologyPhotosynthesis: Process, Equation and Limiting Factors
BotanyLeaf Structure, Stomata and Transpiration
Agricultural SciencePlant Nutrition, NPK and Soil Fertility
Put the theory to work — diagnose a real leaf photo or browse the disease library.