Fungal, bacterial and viral plant diseases: telling them apart
How to tell fungal, bacterial, viral and other plant diseases apart from signs and symptoms, how Koch's postulates prove a cause, and why it matters.
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A sick plant does not say what is wrong with it. Yellow leaves, spots, wilting and rot can be caused by fungi, oomycetes, bacteria, viruses, nematodes, insects, nutrient shortages, herbicides or the weather. Getting the cause right matters because the remedies are completely different: a fungicide does nothing against a virus, and no spray cures a waterlogged root.
Diagnosis is a skill built on a few principles: the difference between signs and symptoms, the patterns each kind of pathogen leaves, and, when certainty is needed, laboratory tests and Koch's postulates. These are core plant pathology exam topics everywhere.
Signs and symptoms
- A symptom is the plant's response to disease: yellowing (chlorosis), dead tissue (necrosis), wilting, stunting, galls, distortion, rots, cankers, mosaic.
- A sign is the pathogen itself, or something it produces, visible on the plant: fungal threads (mycelium), spores, fruiting bodies, rust pustules, the white powder of powdery mildew, the downy growth of downy mildew, bacterial ooze, nematode cysts.
Symptoms are often similar for different causes; signs are the strongest clue. A hand lens and a moist chamber (a sealed bag with damp paper, left overnight to encourage fungal growth on a suspect leaf) are the diagnostician's first tools.
Fungal and oomycete diseases
Fungi cause the majority of plant diseases. Oomycetes (water moulds) look and behave like fungi but are a separate group, more closely related to brown algae; they include the agents of late blight of potato and tomato and of many downy mildews and root rots.
Typical clues
- Visible signs: mycelium, spores in powdery, fuzzy, or rusty masses, black dots of fruiting bodies (pycnidia, perithecia) inside leaf spots or on stems, sclerotia (hard resting bodies).
- Leaf spots that are often roughly circular, frequently with concentric rings ("target spots") and a darker margin, because the fungus grows outward from the infection point.
- Rusts: raised orange, brown or black pustules that rub off as dust.
- Powdery mildews: white powder on the upper leaf surface. Downy mildews (oomycetes): grey or purplish growth on the lower surface under yellow patches.
- Wilts from vascular fungi, with brown streaks in the stem's water-conducting tissue.
- Damping-off of seedlings in wet, cold soil.
Bacterial diseases
Plant-pathogenic bacteria enter through natural openings and wounds; they cannot penetrate intact cuticle, and they need water to spread and infect.
Typical clues
- Water-soaking: young lesions look dark, greasy and translucent when held up to the light.
- Angular leaf spots: the spots are bounded by veins, giving straight edges, because bacteria spread through the spaces between cells and are stopped by veins.
- A yellow halo around the lesion, from toxins.
- Ooze: sticky droplets on lesions in humid weather, drying to a crust.
- Soft rots with a foul smell in fleshy organs (potato tubers, onions, cabbages).
- Vascular wilts: a quick field test is to cut a wilting stem and dip it in clear water; milky threads of bacteria streaming from the cut end point to bacterial wilt.
- Cankers, galls (crown gall) and blights (fire blight of apple and pear).
Other causes to rule out
- Nematodes: root galls (root-knot nematodes), cysts on roots, stunted and yellow plants in patches that enlarge year by year, wilting in the heat of the day.
- Phytoplasmas: yellowing, proliferation of shoots ("witches' broom"), green flowers (phyllody); spread by leafhoppers.
- Insect and mite damage: chewing, mining, stippling, honeydew and sooty mould, the pests themselves.
- Abiotic disorders (non-infectious): nutrient deficiency or toxicity, frost, heat, sun scald, drought, waterlogging, salt, air pollutants, herbicide injury.
A useful rule: infectious diseases spread over time and are patchy; abiotic disorders often appear suddenly over large areas or in patterns that follow something people did, such as spray swaths, irrigation lines, field edges or a low spot in the field, and do not spread from plant to plant.
| Clue | Fungus or oomycete | Bacterium | Virus | Abiotic disorder |
|---|---|---|---|---|
| Visible sign of the pathogen | often (spores, mycelium) | sometimes (ooze) | never | none |
| Typical lesion shape | round, ringed | angular, water-soaked, haloed | mosaic, rings, curl | uniform, sharp patterns |
| Distribution in plant | discrete lesions | discrete lesions or wilt | systemic, new growth | often whole plant or one side |
| Pattern in field | spreading foci | spreading, after rain | scattered, near vectors | sudden, follows operations |
| Controlled by fungicides | yes | no | no | no |
Proving the cause: Koch's postulates
Robert Koch's rules, set out for animal diseases in the 1880s and adapted for plants, establish that a particular organism causes a disease:
- The organism is associated with the disease in every case examined.
- It is isolated and grown in pure culture, and described.
- When the pure culture is inoculated into healthy plants of the same kind, it produces the same disease.
- The same organism is re-isolated from those plants.
The postulates cannot be applied directly to obligate parasites that will not grow on artificial media, such as rust fungi, powdery mildews, viruses and phytoplasmas. For these, the organism is maintained on living plants, and proof relies on transmission from purified material, electron microscopy, serology and molecular methods.
Laboratory diagnosis
- Microscopy of spores and fruiting bodies.
- Culturing on selective media.
- Serological tests, especially ELISA and lateral-flow strips, widely used for viruses and some bacteria.
- Molecular tests: PCR and quantitative PCR, LAMP (isothermal tests usable in the field), and sequencing.
- Indicator plants that develop characteristic symptoms when inoculated with a virus.
Working through a diagnosis
A grower reports tomato plants with yellowing leaves and scattered dark spots after a week of rain.
- Pattern: plants are affected in patches that have grown over a week, worst in a low, wet corner. It is spreading: infectious, not a spray accident.
- Lesions: the spots are small, dark and angular, with yellow halos; held to the light, young spots look water-soaked. No mycelium or spores appear after a night in a moist chamber.
- Likely cause: a bacterial leaf spot, favoured by rain splash.
- Confirmation: send samples for culturing or a molecular test, especially if seed contamination is suspected.
- Consequence for control: a fungicide would be wasted. Remove badly affected plants, avoid overhead irrigation and working in wet crops, use clean seed and rotation, and where locally approved, protectant copper-based products can slow spread.
Had the spots been round and ringed with dark fruiting bodies in their centres, a fungal disease such as early blight would be the lead suspect, and the control plan would differ.
Why the cause decides control
- Fungal and oomycete diseases: resistant varieties, rotation, sanitation, canopy and water management, and fungicides chosen by FRAC mode of action and timed to infection periods.
- Bacterial diseases: clean seed and planting material, avoiding wounds and wet-field work, resistant varieties, and copper or antibiotic products only where permitted. Chemical control is limited.
- Viral diseases: there is no cure in the plant. Control relies on virus-free seed and planting material, resistant or tolerant varieties, managing vectors, removing infected plants and weed hosts, and barrier crops or sowing dates that avoid vector peaks.
- Abiotic disorders: correct the cause: nutrition, drainage, irrigation, spray practice.
Key points
- Symptoms are the plant's response; signs are the pathogen itself. Signs give the surest field diagnosis.
- Fungi: round, often ringed spots with spores or fruiting bodies; rusts, mildews, wilts and damping-off.
- Bacteria: angular, water-soaked spots bounded by veins with yellow halos; ooze; smelly soft rots; bacterial streaming from wilting stems.
- Viruses: mosaic, mottle, ring spots, curl and stunting; no signs; mostly vector-borne; no curative treatment.
- Abiotic disorders appear suddenly, in patterns linked to operations or terrain, and do not spread.
- Koch's postulates: association, isolation, reproduction of disease, re-isolation; obligate parasites need modified proof.
What the exams ask
- School-leaving and entrance papers: definitions of sign and symptom, examples of fungal, bacterial and viral diseases, and how each is spread.
- Degree and research-fellowship entrances: diagnostic features of each pathogen group, Koch's postulates and their limitations, obligate parasites, serological and molecular diagnosis.
- Professional examinations for plant protection and extension officers: field diagnosis and distinguishing biotic from abiotic disorders, and matching control to the cause.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Can one plant have more than one disease at once?
Yes, and often does. A plant weakened by a virus or by nematodes is more open to fungal infection, and wounds from insects let bacteria in. Diagnose the main cause, but look for others.
Why do viruses not have a cure in plants?
Viruses replicate inside the plant's own cells using its machinery, and plants have no immune system that drugs can boost as in animals. Prevention, resistance and vector management are the only practical tools.
Is a fungal disease always worse in wet weather?
Most leaf-infecting fungi need wet leaves, so yes. But powdery mildews often thrive in dry weather with high humidity at night, and some soil-borne fungi favour dry, warm soils.
Sources
- Agrios, G. N. Plant Pathology, 5th edition. Elsevier Academic Press, 2005.
- Schumann, G. L. and D'Arcy, C. J. Essential Plant Pathology, 2nd edition. APS Press, 2010.
- Riley, M. B., Williamson, M. R. and Maloy, O. "Plant disease diagnosis." The Plant Health Instructor, American Phytopathological Society, 2002.
- Hull, R. Plant Virology, 5th edition. Academic Press, 2014.
- EPPO. Diagnostic protocols for regulated pests (PM 7 series). European and Mediterranean Plant Protection Organization, ongoing.
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