Forensic Plant Pathology

plant pathology

interdisciplinary science that includes knowledge of botany, microbiology, crop science, soil science, ecology, genetics, biochemistry, molecular biology, and physiology

biosecurity concerns

• reduce crop yield and quality

• decrease consumer confidence

• economical and international concerns

• undermine public confidence

agoterrorism indicators

• new geographical location (disease not seen in this area before)

• absence of insect vector required for natural introduction

• unusual pattern of disease in the field

• weather history nonconductive to pathogen survival or disease development

• disease occurring at odd time of year

• disease present in one field but not others

• physical evidence of inoculation (equipment)

• motivation

symptoms for plant disease outbreak

• stunting: plant disease that results in dwarfing and loss of vigor

• chlorosis: yellowing of leaf tissue due to lack of chlorophyll

• soft rot: diseases caused by pathogens that secrete enzymes capable of decomposing cell wall

• tissue proliferation (PT): abnormal tissue-like growth

Koch’s postulates

criteria required for accessing if a microorganism causes a disease

1. microorganism must be found in diseased but not healthy not individuals

2. microorganism must be cultured from the diseased individual

3. introducing the cultured microorganism into a healthy, susceptible organism must reproduce the disease

4. the microorganism must be recovered from the newly infected experimental host and proven identical to the original agent

presumptive testing

identifies presence of disease but may not provide proof of cause

analytical tools depends on infected plant + pathogen

• ELISA, PCR, HTS

• HPLC, GC/MS, LC/MS, trained dogs - ideal for small molecule + toxin identification

satellite imaging

track the spread of a disease outbreak and compare it to known special patterns of natural occurring outbreaks

forensic investigation of plant disease outbreaks

• Sample Collection
  • Pathogen identification
  • Identification of pathogen sources
  • Attribution or exclusion of causative agents

What might be a forensically relevant
sample?

• Whole plant
• Plant parts
• Plant swabs
• Soil
• Insects
• Water
• Air
• Alternate hosts

Short timeline & response (1989–1990)

Mid–late 1989: Trap counts spike; multiple urban clusters
detected (Los Angeles basin and nearby counties).
• State/Federal response: repeated aerial malathion + bait
spraying; expanded ground treatments; intensified trapping.
• Dec 1989: Letter from a group calling itself “The Breeders”
claims deliberate releases; attribution remains unresolved.
• 1990: Shift away from aerial malathion; expanded Sterile
Insect Technique releases; new legal restrictions on medfly
importation.

Dec 1989: “The Breeders” letter Attribution status

• An anonymous group asserted responsibility for deliberate
medfly releases during the crisis.
• Authorities investigated; no definitive, publicly
documented attribution or prosecutions followed.
• Why it was hard (then): limited genetic resolution,
incomplete provenance tracking, confounding effects of
spraying on field signals.
• What would be done today: preserve chain of custody for
trap/larval specimens; WGS for origin assignment;
supply-chain audits; space-time modeling; integrated,
likelihood-based attribution

What the data looked like (then)

• High adult-catch numbers in traps over a short period;
comparatively low larval finds in some treated areas.
• Spatiotemporal clustering near transportation corridors and
urban landscapes; signal confounded by repeated spraying.
• Entomological context: medfly can establish from infested
fruit; multiple introductions possible via produce supply
chains.
• Evidence gaps (1989): limited population-genetic
resolution; incomplete provenance tracking; public
resistance to spraying.

Competing hypotheses (what patterns to expect)

H1 - Natural incursion via infested fruit: gradual spread from
points of entry; genetic diversity linked to source region;
larvae in untreated hotspots.
H2 - Repeated deliberate releases: abrupt, multi-point adult
detections without larval build-up; unusual temporal
synchrony; evidence of handling/distribution.
H3 - Multiple accidental introductions: several small,
genetically distinct clusters tied to different supply chains;
partial overlap in time/space

Forensic toolkit: 1989 vs. today

• 1989: trap counts, limited larval sampling, coarse mtDNA
markers, shipping inspections, epidemiologic mapping.
• Today: whole-genome/SNP panels for region-of-origin
inference; rapid metabarcoding of larval guts; digital
supply-chain records, camera feeds.
• Modern analytics: Bayesian space-time models; network
analysis of produce flows; likelihood-based attribution
integrating genetics + movement + traps

Building an attribution case TODAY (court-defensible)

• Chain-of-custody for traps, baits, suspect fruit: labels, timestamps,
storage temps; preserve raw trap cards and lures.
• Genetic workflow: reference panel of global medfly populations →
WGS/SNP calling → assignment tests (with error estimates).
• Spatiotemporal analysis: first detections, kernel density, wind/traffic
overlays; test for synchrony inconsistent with single natural
incursion.
• Supply-chain tracing: distributor/retailer lot genealogy; import
manifests; cold-chain gaps; witness interviews.
• Alternative explanations documented; limitations stated (e.g.,
detection bias during spraying, incomplete genotypes)

1989–1991 Outcome & After-Action

• Attribution: No conclusive law-enforcement determination
publicly established.
• Operations: Aerial malathion spraying curtailed in 1990; pivot
to large-scale Sterile Insect Technique (SIT) releases.
• Policy: Tightened quarantine/produce inspections; new
penalties for importing/possessing medflies.
• Programs: Enhanced trapping density, data management, and
community engagement for future incursions.
• Lesson: Strategy shifted under scientific uncertainty + public
resistance and evidence standards and communications matter.

Depending on the scenario, what are some agencies that
would be involved in the investigation?

FBI (Lead authority if domestic terrorism)
• USDA (Lead authority for non-terrorist but criminal act)
• DHS CBP/Immigrations
• Customs Enforcement
• Coast Guard

Frye standard

admit only methods “generally accepted” by the
relevant scientific community

Daubert standard

judge as gatekeeper; considers testability,
peer review, known error rate, standards/controls, and general
acceptance

Federal Rule of Evidence 702 (amended 2023)

must show that
the expert’s methods and opinions are reliable and properly
applied

Molecular (PCR/qPCR/WGS)

Validation & controls: documented LoD/LoQ; negative/positive, extraction blanks, spike-ins;
inhibition checks.
• Error sources & mitigation: cross-contamination, inhibitors (polyphenols), index-hopping;
batch acceptance criteria.
• Standards & reproducibility: published/validated protocols (e.g., NPDN/APHIS), SOPs, inter-
lab comparisons

Remote sensing (UAS/satellite/hyperspectral)

Reliability: sensor specs, calibration, ground truthing (field plots), documented processing
chain (radiometric/atmospheric corrections).
• Authentication: who acquired data, date/time/coordinates, metadata hashes; continuity of
custody from provider to analyst.
• Error characterization: confusion matrices; known limitations (clouds, mixed pixels

Common pitfalls

Commingling/compositing hides source attribution (e.g., seed lots, soil cores).
• Biological change post-collection (RNA decay, overgrowth on swabs/leaf tissue) alters analyte
profile.
• Matrix inhibitors (tannins, polysaccharides) cause false negatives without inhibition controls.
• Improper permits/packaging (regulated pests/soils) risk evidence exclusion or violations.
• Ambiguous labeling (host vs. pathogen ID, cultivar, GPS precision) breaks provenance

Mitigations

Discrete units + replicate strategy; avoid unnecessary compositing; document sampling design.
• Cold chain, RNAlater/DNA shield as appropriate; time-to-freeze targets; cooler temp logs.
• Include field blanks, extraction blanks, spike-in recovery and inhibition controls; pre-registered
acceptance criteria.
• Use APHIS/NPDN SOPs; proper permits & shipping (e.g., UN3373/P650 where applicable).
• Tamper-evident seals, barcoded IDs; chain-of-custody forms with every transfer; photo logs and
site maps

What to bring to court (checklist)

SOPs and validation reports (LoD/LoQ, error rates, controls), with
versioning and authorship.
• Full chain-of-custody packet: labels, seals, transfer logs, cooler
temps, photos, GPS, permits/shipping docs.
• Raw data and metadata: Files with QC summaries; instrument logs;
remote-sensing tasking/processing metadata.
• Independent replication or inter-lab comparison results; proficiency
test performance.
• Expert report mapping Daubert factors to your methods;
conservative, plain-language limitations section