Seed Pathology Overview Agron 338, Seed Science and Technology

• Dr. Gary Munkvold

• Professor of Plant Pathology

munkvold@iastate.edu

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