Seed Pathology Overview Agron 338, Seed Science and Technology
• Dr. Gary Munkvold
• Professor of Plant Pathology
515-294-7560
• Seed pathology (Agarwal & Sinclair, 1997) –
the study of seedborne diseases and
pathogens (includes other propagative materials)
• Seed pathology (Munkvold, 2008) –
the study and management of
diseases affecting seed production
and utilization; disease management
practices applied to seeds
Seed pathology - terms
• Seed health refers primarily to the presence or absence of disease-causing organisms, such as fungi, bacteria and viruses, and animal pests, such as eelworms and insects, but physiological conditions such as trace element deficiency may be involved. Seed health testing deals with detection and identification of seed-borne microorganisms.
• Seed quality is a concept made up of different attributes, namely purity, moisture content, and germination.
Seed pathology - terms
Source: Danish Seed Health Centre: http://www.dshc.life.ku.dk/
Seed Quality Characteristics
• Germination ability
• Vigor
• Analytical (mechanical) purity
• ‘Pure live seed’ = (%purity × %germination)/100
• Varietal (genetic) purity
• Seed health (absence of seedborne/seed-
transmitted pathogens)
Encyclopedia of Seeds, 2006
Seed Quality Characteristics cont.
• Moisture content
• Uniformity
Additional seed quality parameters:
• Seed size
• Visual appearance
• Seed treatment quality (dose and uniformity)
Encyclopedia of Seeds, 2006
1. Plant pathology review
• What is plant disease?
• Biotic vs. abiotic disease vs. injury
• Types of pathogens
• Disease triangle
• Effects of diseases on plants
What is plant disease?
“…when the ability of the cells or part of a plant to carry out one or more essential functions is interfered with by a pathogenic organism or adverse environmental factor, the activities of the cell are disrupted, etc.,… the cells malfunction or die…”
Agrios, 2005
What is plant disease?
• Normal cell or plant functions are disrupted (efficiency is reduced)
• Caused by a prolonged interaction with another organism or abiotic factor
• A process, not a physical entity
Disease triangle
Temporal, quantitative
Plant pathogens
• Fungi & Oomycetes
• Bacteria
• Viruses and viroids
• Phytoplasmas
• Nematodes
• Parasitic plants
Agrios, 2005
Fungi
• Kingdom Mycota
• Microscopic, filamentous or single-celled, eukaryotic
• Cell walls composed of chitin & other compounds
• No chlorophyll
• Spore-producing
Bacteria
• Kingdom Procaryotae
• Microscopic, single-celled,
prokaryotic
• Cell membrane and cell walls
• Reproduction by fission
• Phytoplasmas
• Lack cell walls
Viruses
• Nucleic acid strands + coat
protein
• Obligate parasites
• Reproduce by utilizing host
biosynthesis processes
Nematodes
• Kingdom Animalia
• Microscopic, unsegmented
worm-like organisms
Lance Nematode
Stunt Nematode
Nematodes
Agrios, 2005
Agrios, 2005
Agrios, 2005 Stem rust disease cycle
Pythium disease cycle
Agrios, 2005
2. Seed Pathology
• Seeds implicated in plant disease for millennia
• Seed treatment suggested in 1733
• “Seed pathology” initially consisted of detecting and identifying seedborne organisms
• Paul Neergaard – first seed pathology publications and textbook
2. Seed Pathology overview
• Disease management in seed production
• Seed conditioning
• Seed infection processes
• Seed storage
• Seed health testing
• Seed transmission
• Seed treatment
• Seedling diseases
Environments for Seed – Pathogen
Interactions
SEED PRODUCTION FIELD
PLANTED FIELD
HARVESTING
PROCESSING
STORAGE
INFECTION DISEASE DEVELOPMENT
TRANSMISSION
SPREAD SURVIVAL
Cultural practices
Disease resistance
Chemical control
Inoculum thresholds
Cultural practices
Disease resistance
Seed health tests
Seed treatment
Seed conditioning
Environmental control From McGee, 1995
Disease management in seed
production – why is it different? • Crop value
• Seed quality –Performance
–Appearance
• Quarantine issues
• No flexibility in genotype
choice
• Limited location flexibility
•Limited crop rotation
•Environment
Association of pathogens
with seeds • Seedborne
• Pathogen – accompanying, external, or internal
• Seed-transmitted • Pathogen passes from seed to plant • Transmitted vs. transported or
disseminated
• Seed as a pathway • Seedborne or seed-transmitted • Pathogen can be introduced and
established through seed
Classes of Seedborne
Microorganisms 1. Pathogens for which seed is the major
inoculum source, and when seed infection is
controlled, the disease is controlled.
2. Important pathogens of a crop, but the infected
seed is a minor inoculum source.
3. Seedborne microorganisms that have never
been shown to cause disease as a result of
their presence on seeds.
4. Pathogens that can infect seed either in the
field or in storage, and reduce quality.
1
3
2
4
• Hand sorting – Discard insect-damaged and diseased
ears
• Drying and shelling – Arrest fungal development; discard
glumes and damaged kernels
• Air screen cleaning – Discard glumes and damaged kernels
• Size grading – Eliminate kernels physically altered
because of disease
Disease Control During Seed
Corn Conditioning
• Gravity separation
− Removal of diseased seeds and
other potentially contaminated, low-
density material
• Seed treatment – Reduces contamination / impact of
seedborne pathogens
• Germination tests – Detect seed lots of questionable vigor
Disease Control During Seed
Corn Conditioning
• Removal of sclerotia of Sclerotinia
sclerotiorum from soybeans
• Removal of ergot sclerotia from barley
• Removal of small/low density seeds infected
by Phomopsis spp and S. sclerotiorum of
soybean and Fusarium graminearum of
wheat.
• Removal of purple soybeans infected to
detect Cercospora kikuchii by optical sorting.
Examples - Disease Control
During Seed Conditioning
Sclerotinia sclerotiorum
contamination of soybean
seeds Ergot sclerotia mixed with
barley seeds
Cleaner Output Example –
Corn
Alan Gaul
Seed health testing
• Phytosanitary certification
• Quality control
• Disease management through
application of tolerances /
thresholds
• Estimate stand establishment
impacts
• Sampling!!!
Mechanisms of seed
transmission (Neergaard)
• Intraembryal → systemic infection
• Ustilago nuda
• Intraembryal → local infection
• Ascochyta pisi
• Extraembryal → systemic infection
• Drechslera graminea
• Extraembryal → local infection
• Alternaria brassicicola
• Seed contamination → systemic infection
• Ustilago hordei
Mechanisms of seed
transmission (Neergaard)
• Seed contamination → saprophytism and
local infection
• Sclerotinia sclerotiorum
• Seed contamination → saprophytism and
systemic infection
• Fusarium oxysporum
• Seed contamination by structures from
organ- specific seed infection followed by
an extramatrical non-parasitic phase and
later by direct organ- specific seed infection
• Claviceps purpurea
Factors influencing seedling
disease
•Seedborne pathogens
•Soilborne pathogens
•Seed structure
•Seed age
•Seed vigor
•Seed exudates
•Insect interactions
•Climatic conditions
•Cultural factors
Seedling disease
management • Seed quality
• Crop rotation, tillage, seedbed
preparation, drainage •Soilborne inoculum
•Soil moisture & temperature
•Seed-soil contact
• Fertilizer and herbicide practices •Avoid injury, promote plant vigor
• Planting date •Escape inoculum due to unfavorable conditions for disease
• Resistance •Available for some seedborne and soilborne pathogens
• Seed treatment, soil fungicides
Chemical seed treatments Fungicides
To protect against seed & seedling pathogens; ensure stand establishment
Primarily for soilborne pathogens
Also for some seedborne pathogens (e.g., Phomopsis, Sclerotinia)
Prevent seed transmission (smut fungi)
Control of storage fungi
Systemic activity to protect wheat seedlings from leaf infection
Insecticides To protect against insects feeding on seeds & seedlings
Systemic activity to reduce feeding on roots or above-ground
plant parts
Polymer coatings Planting & germination characteristics
SAR inducers
Seed Treatment Trends
Insecticide seed treatments now standard
on corn
6 active ingredients at once may become
common
Seed treatments will become standard for
soybeans
Target organisms number in the dozens for
a single crop (microorganisms, insects, nematodes,
birds, mammals)
Low cost (relatively), improving efficacy,
convenience, low environmental impact
Plant Pathology 494 / 594, Seed Pathology
• Instructors: • Gary Munkvold, ISU Seed Science
• Derrick Mayfield, ISU Seed Science
• Nancy Gonzalez, ISU Seed Science
• Lindsey DuToit, Washington St. Univ.
Seed Technology & Business / Pl P 592,
Seed Health Management (on-line)
• Gary Munkvold, ISU Seed Science
• Denis McGee, ISU Seed Science (ret.)
Pl P 494/594 Outline
• Disease management in seed production
• Seed conditioning
• Seed infection processes
• Seed storage
• Seed health testing
• Seed transmission
• Seed treatment
• Seedling diseases
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