Plant Disease Diagnosis Combines Evidence
Definition
Pathogen
A microorganism that causes disease. Pathogens include some bacteria, viruses, fungi and protists.
- Visible symptoms, environmental measurements, the distribution of affected plants and diagnostic tests are combined to identify a cause.
- One symptom is not conclusive because infection and environmental stress can produce similar effects.
- A random pattern affecting one species may suggest infection, while similar symptoms across several species may suggest an environmental cause.
Key Idea
Diagnosis begins with observations and ends with evidence that distinguishes between possible causes.
Observe Symptoms in the Field
Definition
Chlorosis
The yellowing of a plant's leaves caused by a lack of chlorophyll, often a sign of magnesium deficiency.
- Possible symptoms include spots, lesions, abnormal growth, decay, discolouration, wilting and leaf loss.
- Chlorosis is yellowing caused by reduced chlorophyll and may result from infection or mineral deficiency.
- Record photographs, dates, plant species and comparisons with healthy plants.
- Nitrate deficiency causes stunted growth because nitrate ions are needed to make amino acids and proteins.
- Magnesium deficiency causes chlorosis because magnesium ions are needed to make chlorophyll.
Eliminate Environmental Causes
Definition
Mineral deficiency
A problem in a plant caused by a lack of a mineral ion, such as stunted growth from too little nitrate.
- Measure soil moisture, pH and mineral-ion availability before concluding that a pathogen caused the symptoms.
- Uniform symptoms across one area may suggest waterlogging, drought or mineral deficiency.
Example
yellow leaves across every plant in one low-pH bed suggest an environmental cause, while clusters spreading outwards from one plant suggest transmission.
Map the Distribution
- Map affected plants and compare the pattern with wind direction, water flow, insect activity or newly introduced plants.
- A spreading cluster supports a transmissible cause but does not identify the pathogen.
Confirm the Pathogen
- Laboratory identification may use microscopy, culturing, antibody tests or analysis of pathogen genetic material.
- A positive specific test provides stronger evidence than symptoms alone.
- Microscopy may reveal fungal hyphae or spores, culture can grow a pathogen, antibody tests detect specific antigens and genetic tests identify pathogen DNA or RNA.
- Healthy controls show the normal result, while repeated samples reduce the effect of individual variation and sampling error.
Exam technique
- Describe a sequence: observe symptoms, rule out environmental causes, analyse distribution, then use a diagnostic test.
- To improve confidence, repeat samples and include healthy comparison plants.
Reach a Supported Conclusion
- The conclusion must fit all the evidence and acknowledge anomalous or conflicting results.
- Correlation between a symptom and a site condition does not prove causation.
Self review
- Name three visible signs of plant disease.
- How can environmental causes be eliminated?
- What can a distribution pattern show?
- Why is a diagnostic test needed?
