The disease triangle and plant disease epidemiology
Why plant disease needs host, pathogen and environment together, how epidemics build up, how to calculate the infection rate, and how control works.
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A plant disease happens only when three things coincide: a susceptible host, a virulent pathogen and an environment favourable to infection. Remove any one, and there is no disease. This simple idea, the disease triangle, is the first principle of plant pathology and the logic behind every control method.
Epidemiology, the study of how disease spreads through a crop over time and space, builds on it. Both are core to plant pathology courses and exams, from the triangle itself to the calculation of an epidemic's infection rate.
The triangle
The host
The plant must be susceptible, and at a susceptible stage. Many fungi infect young tissue; others infect flowers, ripening fruit or senescing leaves. Large areas of a single, genetically uniform variety make the host side of the triangle very large, which is why monocultures of one variety suffer the worst epidemics.
The pathogen
The pathogen must be present, in sufficient quantity (the inoculum), and virulent on that host variety. Pathogens of plants include:
- Fungi and oomycetes (water moulds, such as late blight of potato and downy mildews), which cause most plant diseases;
- bacteria and phytoplasmas;
- viruses and viroids, mostly carried by insect vectors;
- nematodes and parasitic plants.
Pathogens vary: new races or strains that overcome a resistance gene arise by mutation and selection, and they can spread across continents with trade or on the wind.
The environment
Most foliar diseases need free water on leaves (dew, rain or irrigation) for a minimum number of hours, at a temperature in the pathogen's favoured range. Humidity, temperature, rainfall, wind, soil moisture and soil pH all matter. Weather is why the same field can be healthy one year and devastated the next.
The fourth dimension: time and people
Disease also needs time: enough hours of leaf wetness for spores to infect, enough days for cycles to repeat. Adding time turns the triangle into a disease pyramid (or tetrahedron). People are now often added as a fifth element, because humans choose varieties, move pathogens around the world, change the environment with irrigation and fertiliser, and decide on control.
How disease develops: the disease cycle
Each infection follows a cycle:
- Inoculation: the pathogen (spore, bacterial cell, virus particle) reaches the plant.
- Penetration: through natural openings (stomata), wounds, or directly through the cuticle.
- Infection and colonisation: the pathogen establishes and grows in the tissue. The time from infection to visible symptoms is the incubation period; to new spore production, the latent period.
- Reproduction and dissemination of new inoculum by wind, rain splash, water, insects, seed, soil, tools or people.
- Survival between seasons: in crop residues, soil, seed, alternative or alternate hosts, or as resting structures.
Primary inoculum starts the epidemic each season; secondary inoculum, produced on newly infected plants, spreads it within the season.
Monocyclic and polycyclic diseases
The epidemiologist J. E. Vanderplank, whose 1963 book founded quantitative plant epidemiology, divided diseases by how many cycles they complete in a season.
| Type | Cycles per season | Increase | Examples |
|---|---|---|---|
| Monocyclic ("simple interest") | one; no secondary spread within the season | grows with the amount of primary inoculum | many soil-borne wilts and root rots, smuts |
| Polycyclic ("compound interest") | many, each a few days to weeks | explosive, roughly exponential early, then levelling off | rusts, late blight, powdery mildews, many leaf spots |
| Polyetic | slow, over several years | builds year to year | many tree diseases and some virus diseases of perennials |
The distinction decides control. Monocyclic diseases are controlled by reducing initial inoculum: clean seed, rotation, soil treatment, sanitation. Polycyclic diseases are controlled mainly by reducing the rate of spread: resistant varieties, protective fungicides timed to weather, and conditions less favourable to infection.
Measuring an epidemic: the apparent infection rate
The proportion of tissue diseased, x, plotted against time, gives the disease progress curve. For polycyclic diseases it is S-shaped (logistic): slow at first, then rapid, then levelling off as healthy tissue runs out. Vanderplank's apparent infection rate, r, summarises the speed:
The term ln(x ⁄ (1 − x)) is the logit of disease. Because it turns the S-shaped curve into a straight line, r is its slope, in units per day.
Calculating the apparent infection rate
Leaf rust covers 1 per cent of the leaf area of a wheat crop on day 0 and 20 per cent on day 10.
- logit(0.01) = ln(0.01 ⁄ 0.99) = ln(0.0101) = −4.595.
- logit(0.20) = ln(0.20 ⁄ 0.80) = ln(0.25) = −1.386.
- r = (−1.386 − (−4.595)) ⁄ 10 = 3.209 ⁄ 10 ≈ 0.32 per day.
A resistant variety that halves r to 0.16 per day would take about 20 days instead of 10 to go from 1 to 20 per cent: (4.595 − 1.386) ⁄ 0.16 ≈ 20 days, often long enough for the crop to mature first. A fungicide that cuts initial disease from 1 to 0.1 per cent at the same r = 0.32 delays the epidemic by (ln(0.01 ⁄ 0.99) − ln(0.001 ⁄ 0.999)) ⁄ 0.32 ≈ 7 days. Vanderplank's point: for polycyclic diseases, cutting the rate usually achieves more than cutting the starting inoculum.
The area under the disease progress curve (AUDPC), adding up disease over all assessment dates, is the common single figure for comparing varieties or treatments in trials.
Control, side by side
Every control method shrinks one side of the triangle.
Against the host side
- Resistant varieties, the cheapest and most widely used control. Single major genes (vertical, race-specific resistance) give strong protection but can be overcome by new races; many minor genes (horizontal, partial resistance) slow the epidemic and last longer.
- Variety mixtures and diversification across fields.
- Avoiding the susceptible stage, for example by sowing dates that let crops escape peak infection periods.
Against the pathogen side
- Exclusion: quarantine, phytosanitary certification, and certified disease-free seed and planting material.
- Eradication and sanitation: removing infected residues, volunteer plants and alternate hosts; rogueing diseased plants; crop rotation for soil-borne pathogens.
- Protection with fungicides and bactericides, grouped by mode of action using the FRAC classification, and rotated to avoid resistance. Product registrations differ by country.
- Biological control with antagonistic microbes.
Against the environment side
- Spacing, pruning and canopy management to dry leaves faster; drip rather than overhead irrigation; drainage for root diseases; balanced fertiliser (too much nitrogen favours many leaf diseases).
- Forecasting systems that use weather data to warn when infection conditions occur, so sprays are timed rather than routine.
In practice these are combined into integrated disease management, the plant-pathology side of integrated pest management.
Key points
- Disease requires a susceptible host, a virulent pathogen and a favourable environment together; time and people make the disease pyramid.
- The disease cycle: inoculation, penetration, infection and colonisation, reproduction and dissemination, survival.
- Monocyclic diseases (one cycle) are controlled by reducing initial inoculum; polycyclic diseases (many cycles) mainly by reducing the infection rate.
- Apparent infection rate r = change in logit(x) ÷ time; resistance that lowers r delays epidemics strongly.
- Control targets each side: resistance and diversity (host), exclusion, sanitation, rotation and fungicides (pathogen), canopy, water and forecasting (environment).
What the exams ask
- School-leaving and entrance papers: the disease triangle, groups of pathogens, and examples of methods that target host, pathogen or environment.
- Degree and research-fellowship entrances: the disease cycle and its terms, monocyclic versus polycyclic diseases, Vanderplank's simple and compound interest, the apparent infection rate and AUDPC, vertical and horizontal resistance.
- Professional examinations for plant protection officers: forecasting, quarantine, fungicide resistance management and designing integrated disease management.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why do epidemics often follow wet weather?
Most fungal and bacterial leaf pathogens need hours of leaf wetness to germinate and infect, and rain splashes spores from plant to plant. A wet spell opens the environment side of the triangle.
Is a resistant variety resistant forever?
Race-specific resistance from one major gene can be overcome when a new pathogen race appears, sometimes within a few years. Combining genes, using partial resistance and diversifying varieties make resistance last longer.
What is the difference between incubation and latent period?
Incubation runs from infection to visible symptoms; the latent period runs from infection to the production of new infectious spores. The latent period controls how fast a polycyclic epidemic can grow.
Sources
- Agrios, G. N. Plant Pathology, 5th edition. Elsevier Academic Press, 2005. Chapters 2 and 7.
- Vanderplank, J. E. Plant Diseases: Epidemics and Control. Academic Press, 1963.
- Madden, L. V., Hughes, G. and van den Bosch, F. The Study of Plant Disease Epidemics. APS Press, 2007.
- Francl, L. J. "The disease triangle: a plant pathological paradigm revisited." The Plant Health Instructor, American Phytopathological Society, 2001.
- FRAC. FRAC Code List: Fungal control agents sorted by cross-resistance pattern and mode of action. Fungicide Resistance Action Committee, updated annually.
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