Download Diagnosing Plant Problems

The Basics of Plant
Diagnostics
USAID IPM Workshop
October 22 - 25
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: www.clemson.edu/plantclinic
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.

Consider the Possible Agents
Biotic Agents
Abiotic Agents
Damage from
Weather
Chemicals
Mechanical
Nutritional problems
Cultural problems
Pathogens or Pests:
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Fungi
Bacteria
Viruses, viroids
Phytoplasmas
Nematodes
Insects and mites
Patterns of Abiotic Damage
Cotton field with chemical damage
Damage occurs suddenly.
Many species of plants
affected.
Symptoms often
distributed in a large
area.
Damage is uniform even if
in small area.
Laboratory tests
Diagnostic tests for abiotic causes
Soil, water
pH testing
Nutrient analysis
Soluble salts analysis
Analysis for chemicals
Plant tissue tests
Nutrient analysis
Analysis for chemicals
Patterns of Biotic Damage

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Fusarium wilt of tomato
R. Billings

Loblolly pine killed by southern pine beetle
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.
Consult literature resources
APS Compendia series
 US ARS Systematic Mycology and
Microbiology website
 Pestnet Diagnostic Service:
www.pestnet.org
 Other Online resources

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.
Localized vs. Systemic Symptoms
Localized Symptoms
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Leaf spots
Stem lesions
Fruit rots
Blights
Galls
Systemic Symptoms
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Virus infections
Leaf yellowing
Stunting, slow growth
Wilting
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.
Identification of Signs
Mycelium

Distinctive appearance
allows id. of some fungi.
Mycelium of Rhizoctonia sp.

Mycelium of Phytophthora sp.
Advantageous if fungus
produces no spores.
Signs for Localized Symptoms
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Cercospora leaf spot
of watermelon,
Cercospoora citrulina
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Southern stem rot,
Sclerotium rolfsii
Downy mildew of veronica,
Peronospora sordida
Fungal fruiting bodies
or spores
Downy mildews
Powdery mildews
Sclerotia
Bacterial ooze
Laboratory tests
Moist Incubation

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Moist chamber
Goal – to induce
sporulation.
Important for obligate
fungal pathogens.
Avoid overly moist
conditions.
Identification of Signs
Fungal signs
Fruiting bodies

Morphology of fruiting
body aids in fungal
identification.
Spore-bearing mycelium

Morphology of spores
provides fungal
identification
Leaf Spot Diagnosis
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Yellow spots on
leaves only.
Spores (sign) form
on leaf undersides.
Microscopic mount
reveals Fulvia fulva
is causal.
Can now diagnose
visually
Leaf Spot Diagnosis
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Sally Miller
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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.
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.
Laboratory tests
Bacterial isolations

Cultures of
Ralstonia
solanacearum
Suspend infected
tissue in sterile water.
 Streak suspension on
bacterial medium.
 Can use selective
media.
 Transfer single,
isolated colony.
Isolate identification
Bacterial identification
 Biochemical tests.
 Carbohydrate utilization (BIOLOG).
 Fatty acid methyl ester analysis
(FAME).
 Polymerase chain reaction (PCR).
 Serological tests, e.g. ELISA.
Melon Vine Wilt Diagnosis
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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
Laboratory tests
Fungal isolation
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Fungal culture.
Nutritive agar
media, can be
selective.
Diagnostician can
often id. to genus
from appearance of
culture
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.
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.
Koch’s Postulates
1.
2.
3.
4.
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.
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.
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.
Pathogen Identification
ELISA : a very
pathogen specific
serological test
Multi-well test for multiple
samples
Individual test ELISA kit
ELISA uses animal
antibodies to
detect pathogens.
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.
Questions??