Unit 11: Diagnosis of Field Diseases and Pathogens - Subjective Questions

Field disease diagnosis is the systematic process of recognizing a plant health problem under field conditions and determining its probable cause through observation, examination, testing, and interpretation of evidence.

Major objectives include:

• Identifying whether the disorder is biotic or abiotic.
• Recognizing the disease from its characteristic symptoms and signs.
• Determining the causal pathogen or pathogen group.
• Assessing disease distribution, incidence, and severity.
• Identifying environmental and agronomic factors that favor disease development.
• Selecting suitable management measures, including biopesticides, biofertilizers, sanitation, and cultural practices.
• Preventing unnecessary pesticide use caused by an incorrect diagnosis.

A reliable diagnosis should combine field history, symptom patterns, pathogen signs, laboratory examination, and confirmatory tests.

A systematic field diagnosis generally involves the following steps:

1. Identify the host: Confirm the crop, cultivar, growth stage, and normal appearance of the plant.
2. Collect field history: Record planting date, previous crops, seed source, irrigation, fertilizer use, pesticide applications, and recent weather.
3. Observe disease distribution: Determine whether affected plants occur randomly, in patches, along rows, near field margins, or in low-lying areas.
4. Examine symptoms: Inspect roots, stems, leaves, flowers, fruits, and vascular tissues for abnormalities.
5. Look for pathogen signs: Search for mycelium, spores, fruiting bodies, bacterial ooze, nematode cysts, or other visible structures.
6. Compare affected and healthy plants: Differences may reveal the site and progression of infection.
7. Form a preliminary diagnosis: Decide whether the cause is fungal, bacterial, viral, nematode-related, or abiotic.
8. Collect representative samples: Preserve suitable diseased and healthy tissues for laboratory analysis.
9. Confirm the cause: Use microscopy, isolation, pathogenicity tests, serology, or molecular methods.
10. Recommend management: Base the recommendation on the confirmed diagnosis and field conditions.

Symptoms are visible or measurable responses of a plant to disease or injury. They are produced by the host rather than being direct structures of the pathogen.

Examples of symptoms:

• Wilting
• Chlorosis and mosaic patterns
• Necrosis and leaf spots
• Cankers
• Galls
• Root rot
• Stunting

Signs are physical structures, products, or parts of the causal pathogen that are visible on or within the diseased plant.

Examples of signs:

• Fungal mycelium on infected tissue
• Rust pustules and spores
• Sclerotia of Sclerotium or Rhizoctonia
• Pycnidia or acervuli
• Bacterial ooze
• Nematode cysts or egg masses

Symptoms may be similar for several causes, whereas signs often provide stronger evidence about the identity of a pathogen. Therefore, both should be examined during diagnosis.

The spatial pattern of affected plants provides important diagnostic evidence.

Biotic diseases generally:

• Begin at one or a few infection points.
• Appear in scattered plants or irregular patches.
• Spread progressively to neighboring plants.
• Show different stages of disease development.
• May follow the distribution of vectors, infected residues, water movement, or soilborne inoculum.

Abiotic disorders generally:

• Appear suddenly over a large area.
• Follow uniform rows, soil types, spray paths, drainage patterns, or field boundaries.
• Affect many plants at a similar developmental stage.
• Do not spread from diseased plants to healthy plants.
• May be associated with frost, drought, flooding, nutrient deficiency, salinity, or chemical injury.

However, distribution alone is not conclusive. Soilborne pathogens can produce patterns resembling nutrient or drainage problems, so field observations must be supported by symptom examination and laboratory tests.

Fungal diseases:

• Often produce distinct leaf spots with defined margins.
• Commonly cause damping-off, rots, blights, mildews, rusts, smuts, and wilts.
• Fungal growth, spores, pustules, or fruiting bodies may be visible.
• Lesions may expand gradually and develop concentric rings.

Bacterial diseases:

• Frequently produce water-soaked lesions.
• May cause soft rots, blights, cankers, galls, and vascular wilts.
• Lesions are sometimes angular because veins restrict their expansion.
• Bacterial ooze or streaming may occur from infected tissue.

Viral diseases:

• Commonly cause mosaic, mottling, vein clearing, ring spots, leaf curling, distortion, and stunting.
• Usually do not produce visible pathogen signs.
• Symptoms may vary with cultivar, temperature, plant age, and mixed infection.
• Distribution may correspond to insect-vector activity or infected planting material.

These characteristics support a preliminary diagnosis, but laboratory confirmation is often required because different agents can produce similar symptoms.

Field history and environmental information help explain why a disease appeared and narrow the range of possible causes.

Important information includes:

• Crop and cultivar grown
• Previous crops and crop rotation
• Source and treatment of seed or planting material
• Planting date and plant age
• Irrigation method and drainage conditions
• Fertilizer and pesticide applications
• Soil type, pH, salinity, and compaction
• Previous disease occurrence
• Recent temperature, rainfall, humidity, frost, drought, or storms
• Presence of weeds, alternate hosts, and insect vectors

For example, prolonged leaf wetness may favor foliar fungal and bacterial diseases, while poor drainage may promote root rots. Herbicide drift may resemble viral distortion, and nutrient imbalance may resemble pathogen-induced chlorosis. Field history therefore connects observed symptoms with likely biological or environmental causes.

Proper sampling is essential because a poor or deteriorated sample may prevent accurate pathogen identification.

Principles of collection:

• Select several representative plants showing early to intermediate symptoms.
• Include the advancing margin between healthy and diseased tissue.
• Collect all relevant organs, including roots and the surrounding soil when root disease is suspected.
• Include a healthy comparison sample where possible.
• Collect insect vectors or visible pathogen structures when relevant.
• Record the host, cultivar, location, date, symptoms, field pattern, and management history.

Precautions:

• Avoid collecting only dead or badly decomposed tissue because secondary organisms may dominate it.
• Use clean tools and separate bags to prevent cross-contamination.
• Place dry foliage in paper rather than sealed wet containers.
• Keep roots and soil in secure plastic bags, separate from foliage.
• Do not add free water to samples.
• Keep samples cool and submit them promptly.
• Clearly label every sample.

Photographs of the whole field, affected plants, and close-up symptoms should accompany the specimen.

Isolation and identification of a fungal pathogen involve the following procedure:

1. Select tissue from the active lesion margin, including both healthy-looking and diseased portions.
2. Wash off adhering soil and debris when necessary.
3. Cut the tissue into small pieces using sterile instruments.
4. Surface-disinfest the pieces with a suitable disinfectant for an appropriate period.
5. Rinse with sterile water and blot dry under aseptic conditions.
6. Place the pieces on a suitable medium, such as potato dextrose agar.
7. Incubate under suitable temperature and light conditions.
8. Transfer emerging fungal growth to fresh medium to obtain a pure culture.
9. Observe colony color, texture, growth rate, pigmentation, and sporulation.
10. Examine hyphae, spores, conidiophores, fruiting bodies, and other structures microscopically.
11. Compare the observations with taxonomic keys or reference cultures.
12. Confirm identity with pathogenicity testing or molecular analysis when needed.

Contamination controls and careful aseptic technique are necessary to distinguish the primary pathogen from secondary colonizers.

Bacterial diagnosis combines symptom examination, isolation, cultural characteristics, biochemical tests, and confirmatory assays.

Detection and isolation:

• Select fresh tissue from the advancing edge of a water-soaked lesion or canker.
• Cut the tissue in sterile water and observe for bacterial streaming.
• Streak the resulting suspension onto a general or selective agar medium.
• Incubate the plates and examine the colonies.
• Re-streak a typical colony to obtain a pure culture.

Identification:

• Record colony color, shape, elevation, margin, texture, and pigment production.
• Examine cell shape and motility.
• Perform Gram staining or a rapid potassium hydroxide test.
• Use biochemical and physiological tests such as oxidase, catalase, carbohydrate utilization, and hypersensitivity reactions where appropriate.
• Apply pathogen-specific serological or molecular assays for confirmation.
• Conduct a pathogenicity test on a susceptible host when required.

Because saprophytic bacteria are common on diseased tissue, isolation from several samples and reproduction of symptoms provide stronger evidence of causality.

Plant viruses are difficult to diagnose visually because:

• Different viruses may produce similar mosaic, curling, chlorosis, or stunting symptoms.
• One virus may produce different symptoms in different hosts or cultivars.
• Symptoms change with temperature, nutrition, plant age, and infection time.
• Nutrient disorders, herbicide injury, and insect feeding can mimic viral symptoms.
• Mixed infections may produce unusual or severe symptoms.
• Some infected plants remain symptomless.

Identification methods include:

• Indicator-host testing: Sap, graft, or vector transmission to plants that develop characteristic symptoms.
• Serological methods: ELISA and immunostrips detect virus-specific antigens.
• Molecular methods: PCR or reverse-transcription PCR detects viral nucleic acid.
• Electron microscopy: Reveals virus particle shape and size.
• Nucleic acid sequencing: Confirms identity and distinguishes related virus strains.

Reliable diagnosis usually combines field observations with at least one specific serological or molecular test.

Field diagnosis:

• Observe patches of stunted, yellow, or wilted plants.
• Compare root systems of affected and healthy plants.
• Look for root galls, lesions, excessive branching, root pruning, cysts, or reduced root volume.
• Note that above-ground symptoms are often nonspecific and may resemble nutrient or water stress.

Sampling:

• Collect soil and feeder roots from the active margin of affected patches.
• Take several subsamples from the root zone and combine them into a representative sample.
• Keep samples cool and prevent drying.

Laboratory diagnosis:

• Extract nematodes using sieving, decanting, centrifugal flotation, mist chambers, or Baermann funnels.
• Examine specimens microscopically.
• Identify nematodes using features such as body shape, stylet, esophagus, tail, vulval region, and perineal pattern.
• Count nematodes to estimate population density.
• Use biochemical or molecular methods when morphological identification is uncertain.

The diagnosis should relate nematode identity and population level to the crop, symptoms, and known damage threshold.

Koch's postulates provide a classical procedure for establishing a causal relationship between a microorganism and a disease.

The postulates are:

1. The suspected pathogen must be consistently associated with diseased plants and absent, or much less frequent, in healthy plants.
2. The pathogen must be isolated from the diseased host and grown in pure culture.
3. The cultured organism must reproduce the same disease when inoculated into a healthy, susceptible host under suitable conditions.
4. The same organism must be re-isolated from the experimentally infected host and shown to be identical to the original isolate.

Importance:

• They distinguish a primary pathogen from secondary or saprophytic organisms.
• They provide experimental evidence of causation.
• They support accurate identification and management recommendations.

Limitations: Some pathogens cannot be cultured on artificial media, viruses require living cells, obligate parasites require host tissue, and symptomless infections may occur. Modified criteria, serology, microscopy, and molecular evidence are therefore used when classical postulates cannot be fully applied.

Disease incidence is the proportion or percentage of plants or plant units that are diseased, regardless of the amount of tissue affected.

It is calculated as:

Incidence is suitable for diseases in which a plant or organ can be classified clearly as diseased or healthy, such as wilt, smut, or damping-off.

Disease severity is the proportion or percentage of plant tissue affected by disease. It can be assessed using visual estimates, standard area diagrams, image analysis, or numerical rating scales.

A disease index based on ratings may be calculated as:

Incidence measures disease frequency, whereas severity measures disease intensity. Both are useful for diagnosis, monitoring epidemics, comparing treatments, and evaluating biopesticide performance.

These symptom categories help organize field observations, although none alone identifies a pathogen.

• Leaf spots: Localized necrotic areas that may have halos, concentric rings, fruiting bodies, or angular margins. They may be caused by fungi, bacteria, or abiotic injury.
• Blights: Rapid and extensive death of leaves, shoots, flowers, or other tissues. Blights are commonly caused by fungi or bacteria.
• Rots: Breakdown and decomposition of tissue. Soft, watery rots may suggest bacterial activity, while dry or firm rots are frequently associated with fungi.
• Wilts: Loss of turgidity caused by impaired water uptake or transport. Possible causes include vascular pathogens, root rots, nematodes, drought, and root injury.
• Cankers: Localized, sunken, cracked, or dead areas on stems and branches, often associated with fungi or bacteria.
• Galls: Abnormal tissue enlargement caused by altered cell division or expansion. Galls may result from bacteria, fungi, nematodes, insects, or mites.

The diagnostician should examine symptom location, progression, distribution, and associated pathogen signs before reaching a conclusion.

Microscopy allows direct examination of pathogen cells and reproductive structures that may be too small to see with the unaided eye.

Uses in fungal diagnosis:

• Observing septate or nonseptate hyphae
• Examining spore shape, size, color, and septation
• Identifying conidiophores, sporangia, asci, basidia, and fruiting bodies
• Detecting fungal structures within diseased tissues

Uses in bacterial diagnosis:

• Examining cell shape and arrangement after staining
• Observing bacterial streaming from infected tissue
• Assessing motility with suitable preparations

Uses in nematode diagnosis:

• Examining the stylet, esophagus, reproductive structures, tail, and other taxonomic characters
• Measuring diagnostic body structures

Uses in viral diagnosis:

• Electron microscopy may reveal virus particle morphology and inclusion bodies.

Microscopy provides rapid preliminary evidence, but closely related pathogens may look similar. Culture characteristics, biochemical tests, serology, or molecular assays may therefore be needed for species-level confirmation.

Serological methods detect pathogen-specific antigens using antibodies. Examples include ELISA, lateral-flow immunostrips, immunofluorescence, and agglutination tests.

Advantages of serological methods:

• Relatively rapid and suitable for testing many samples
• Often economical for routine diagnosis
• Useful for viruses and some bacterial or fungal pathogens
• Immunostrips can be used under field conditions

Limitations:

• Depend on the availability and specificity of antibodies
• Cross-reactions may occur
• Low pathogen concentration can produce false-negative results

Molecular methods detect pathogen DNA or RNA. Examples include PCR, reverse-transcription PCR, quantitative PCR, loop-mediated isothermal amplification, and sequencing.

Advantages of molecular methods:

• High sensitivity and specificity
• Detection of pathogens that are difficult to culture
• Identification of closely related species or strains
• Detection before clear symptoms develop

Limitations:

• Require specialized equipment, reagents, and technical skill
• Contamination may produce false-positive results
• Detection of nucleic acid does not always prove that the pathogen is alive or causing the observed disease

The choice depends on the pathogen, diagnostic purpose, available facilities, and need for confirmation.

Biological products have specific functions and do not control every plant health problem. Diagnosis is therefore required before treatment selection.

• A fungal antagonist such as Trichoderma may suppress certain soilborne fungi but will not cure a viral disease or correct nutrient toxicity.
• A bacterial biocontrol agent must be compatible with the crop, target pathogen, and field environment.
• Biofertilizers improve nutrient availability or plant growth, but they cannot replace direct disease control when a virulent pathogen is established.
• Root colonization, soil moisture, temperature, organic matter, and pesticide residues influence biological product performance.
• Incorrect diagnosis can delay effective action, increase costs, and allow the disease to spread.
• Confirmation of the pathogen helps select the correct organism, formulation, application method, dose, and timing.
• Baseline disease incidence and severity are necessary for evaluating treatment effectiveness.

Thus, accurate diagnosis links the target organism and disease cycle with an appropriate biological management strategy.

Common sources of error include:

• Diagnosing from a single symptom
• Examining only dead or decomposed plants
• Confusing secondary organisms with the primary pathogen
• Ignoring roots, stems, or vascular tissues
• Failing to consider chemical injury, nutrient disorders, or weather damage
• Relying only on photographs
• Collecting unrepresentative or contaminated samples
• Assuming that detection of a microorganism proves causation
• Ignoring mixed infections
• Using an unsuitable diagnostic test or interpreting it without controls

Measures to reduce error:

• Examine several affected and healthy plants across the field.
• Record disease distribution and field history.
• Inspect the entire plant and the surrounding soil.
• Collect samples from active disease margins.
• Use appropriate positive and negative controls in laboratory tests.
• Combine symptomatology, signs, isolation, microscopy, and specific assays.
• Repeat tests when results conflict with field evidence.
• Apply pathogenicity testing where practical.
• Seek specialist identification for unusual or regulated pathogens.

A defensible diagnosis is based on converging evidence rather than one observation or test.

Physiological and nutrient disorders are noninfectious and are associated with environmental, chemical, soil, or nutritional factors.

Features suggesting an abiotic disorder:

• Symptoms appear uniformly or follow soil, irrigation, drainage, or spray patterns.
• Many plants develop symptoms at approximately the same time.
• The problem does not spread from plant to plant.
• Pathogen signs are absent.
• Symptoms may occur mainly on old or young leaves according to nutrient mobility.
• Soil, water, tissue, or chemical analysis reveals an abnormal condition.
• Plants recover when the environmental or nutritional problem is corrected.

Features suggesting an infectious disease:

• Symptoms begin in scattered plants or localized foci and expand over time.
• Several stages of disease are visible.
• Pathogen signs may be present.
• A pathogen can be repeatedly detected in affected tissues.
• Disease development is consistent with a known disease cycle.

Because infectious and abiotic factors may occur together, diagnosis may require tissue analysis, soil testing, pathogen detection, and evaluation of management records.

A suitable diagnostic approach would include the following:

1. Map the pattern: Determine whether patches are associated with low areas, irrigation lines, soil differences, field entrances, or previous crop rows.
2. Review field history: Check crop rotation, cultivar, planting material, fertilizer use, herbicide application, irrigation, rainfall, and previous disease problems.
3. Compare plants: Uproot affected, transitional, and healthy plants without damaging their roots.
4. Examine roots: Look for rot, discoloration, lesions, root pruning, galls, cysts, and reduced feeder roots.
5. Examine stems: Cut stems to check for vascular browning, bacterial ooze, cankers, or insect injury.
6. Assess soil conditions: Evaluate moisture, drainage, compaction, salinity, pH, and nutrient status.
7. Collect samples: Submit whole plants, roots, and root-zone soil from the margins of patches.
8. Conduct laboratory tests: Isolate fungi or bacteria, extract and count nematodes, and perform specific serological or molecular assays when indicated.
9. Interpret all evidence: Possible causes include vascular wilt, root rot, nematode damage, waterlogging, drought, salinity, or nutrient deficiency.
10. Confirm before treatment: Base management recommendations on the identified cause and contributing field conditions.