1. The Basics of Plant Diagnostics
USAID IPM Workshop
October

2. The Clemson University Plant Problem Clinic
An Interdisciplinary Clinic
Diagnosis of plant diseases and other problems
Insect identifications – DPR Entomologist
Plant/Weed identifications – Botanist in Dept. of Biology
Specialists across campus assist with recommendations.
Website:

3. Plant Problem Diagnosis
Diagnosis – process used to identify the cause of problems.
Tools:
Plant sciences, especially plant pathology, mycology and entomology.
Arts of investigation and detective work.

4. Consider the Possible Agents
Biotic Agents
Abiotic Agents
Damage from
Weather
Chemicals
Mechanical
Nutritional problems
Cultural problems
Pathogens or Pests:
Fungi
Bacteria
Viruses, viroids
Phytoplasmas
Nematodes
Insects and mites

5. Patterns of Abiotic Damage
Damage occurs suddenly.
Many species of plants affected.
Symptoms often distributed in a large area.
Damage is uniform even if in small area.
Cotton field with chemical damage

6. Laboratory tests Diagnostic tests for abiotic causes Soil, water
pH testing
Nutrient analysis
Soluble salts analysis
Analysis for chemicals
Plant tissue tests

7. Patterns of Biotic Damage
Symptoms usually localized or scattered.
Pattern is random.
Symptoms develop gradually and get worse over time.
Damage spreads to plants of same or related species.
Fusarium wilt of tomato
R. Billings
Loblolly pine killed by southern pine beetle

8. Consult literature resources
APS Compendia series
US ARS Systematic Mycology and Microbiology website
Pestnet Diagnostic Service:
Other Online resources

9. Identify symptoms and signs
Symptoms - alterations of a plant’s appearance due to a disease, pest or disorder.
Signs - actual pathogen, pest, parts or
by-products seen on a host plant.

10. Localized vs. Systemic Symptoms
Localized Symptoms
Leaf spots
Stem lesions
Fruit rots
Blights
Galls
Systemic Symptoms
Virus infections
Leaf yellowing
Stunting, slow growth
Wilting

11. Systemic Symptoms
Diagnosis is often more difficult First pin point source Vascular disease Root Rots Nematode infestation Isolations or other techniques often needed to determine causal agent.

12. Identification of Signs
Mycelium
Distinctive appearance
allows id. of some fungi.
Advantageous if fungus produces no spores.
Mycelium of Rhizoctonia sp.
Mycelium of Phytophthora sp.

13. Signs for Localized Symptoms
Fungal fruiting bodies or spores
Downy mildews
Powdery mildews
Sclerotia
Bacterial ooze
Cercospora leaf spot
of watermelon,
Cercospoora citrulina
Southern stem rot,
Sclerotium rolfsii
Downy mildew of veronica, Peronospora sordida

14. Laboratory tests Moist Incubation Goal – to induce sporulation.
Important for obligate fungal pathogens.
Avoid overly moist conditions.
Moist chamber

15. Identification of Signs
Fungal signs
Fruiting bodies
Morphology of fruiting body aids in fungal identification.
Spore-bearing mycelium
Morphology of spores provides fungal identification

16. Leaf Spot Diagnosis Yellow spots on leaves only.
Spores (sign) form on leaf undersides.
Microscopic mount reveals Fulvia fulva is causal.
Can now diagnose visually

17. Leaf Spot Diagnosis Leaves defoliating.
Dark, water-soaked spots, more obvious when held up to light
Microscopic mount shows bacterial flow.
Doesn’t id. species, but control is same.
Sally Miller

18. Signs Bacterial signs “Streaming” from freshly cut stem
“Stringing” from cut stems pushed together, then pulled apart.
Bacterial ooze
Bacteria “flow” and morphology seen on compound microscope.
Bacterial streaming from tomato stem infected by Ralstonia solanacearum.

19. Laboratory tests Bacterial isolations
Suspend infected tissue in sterile water.
Streak suspension on bacterial medium.
Can use selective media.
Transfer single, isolated colony.
Cultures of Ralstonia solanacearum

20. Isolate identification
Bacterial identification
Biochemical tests.
Carbohydrate utilization (BIOLOG).
Fatty acid methyl ester analysis (FAME).
Polymerase chain reaction (PCR).
Serological tests, e.g. ELISA.

21. Melon Vine Wilt Diagnosis
Wilt of vine - look for symptoms and signs.
GSB lesions have gumming and pycnidia; if not present may be Fusarium wilt.
Fusarium wilt shows vascular discoloration

22. Laboratory tests Fungal isolation
Nutritive agar media, can be selective.
Diagnostician can often id. to genus from appearance of culture
Fungal culture.

23. Isolate identification
Fungal identification – Other methods
Utilizing keys in literature.
Morphological comparison with drawings in literature.
Literature sources for host diseases and disorders.
Serological, molecular, other tests.

24. Investigate Pathogenicity
If symptoms match, but not a known host,
Kochs Postulates may be next step
A lengthy process, rarely used in routine diagnoses.
Problem arises when organism cannot be cultured.

25. Koch’s Postulates
Note constant association of organism with diseased plants and consistent, observable symptoms.
Isolate and characterize organism in pure culture.
Inoculate healthy plant with organism, and observe the same disease and symptoms.
Re-isolate same organism.

26. Symptoms Viral Symptoms Mosaics
Irregular patches of discolored tissue.
Often with distortion
Ringspots or line patterns
Distinct ring shaped lesions, various line patterns.
Very few can be identified visually.

27. Identification of non-culturable Pathogens
Methods
Serological tests, e.g. ELISA .
Electron microscopy.
Staining for virus-induced inclusion bodies.
Molecular tests, e.g. PCR
DNA Sequencing
Host range testing.

28. Pathogen Identification
ELISA : a very pathogen specific serological test
ELISA uses animal antibodies to detect pathogens.
Multi-well test for multiple samples
Enzyme-linked immunosorbent assay; a sensitive immunoassay that uses an enzyme linked to an antibody or antigen as a marker for the detection of a specific protein, especially an antigen or antibody.
Individual test ELISA kit

29. Plant Disease Diagnosis Steps
8. Final Diagnosis
All information compiled and analyzed.
Organism identified, found to be pathogen of host OR
Lab tests or other information reveal an abiotic cause.
9. Develop control recommendations and present to grower.

30. Questions??