Seed Health and

Seed Transmitted Diseases

Ineke Wijkamp, Seed2Plant

Outline of the presentation

• General remarks; Production of healthy seeds

and disease management

• Pathogens and pest; Soil-borne and seed-borne

diseases; some examples

• Seed treatments

• Seed health testing; Methods for testing,

organizations, legislation and certification

Introduction

Production of healthy seeds

Disease management: Integral approach aimed

at the entire production system

Before sowing; reduction/eradication of inoculum

Culture measures and sanitation practices in the field

Postharvest: seed treatment, removal of pathogens

Legislation, seed tests and certification of seed

• Seed movement globally

• Quarantine diseases

Disease management

Cultural practices in (seed) crops

Crop rotation

Elimination of alternative hosts

Destruction of inoculum in the field

Control of insect vectors

Irrigation practices

Planting practices

Ventilation of seed crops

Fertilizer programs

Transplanting

Harvesting

Selection of seed production area‟s

Disease management

Use of resistant cultivars for “quality diseases”

Use of tolerant cultivars is less desirable for

quarantine pathogens

Disease management

Seed health management during harvest and post

harvest

Avoid cross-contamination

Reduce infection levels (sorting, upgrading, and seed

treatments)

Optimise storage conditions to suppress the progress of

seed infections

Pathogens and pests

Knowledge about pathogens,

pests and epidemiology is

important

More specific:

Seed-borne diseases

Pathogens and pests

Types of pests and pathogens

Organisms that cause infectious disease (fungi,

oomycetes, bacteria, viruses, viroids, virus-like

organisms, phytoplasmas, protozoa, nematodes and

parasitic plants)

Ectoparasites (insects, mites, vertebrates) or other

pests that affect plant health by consumption of plant

tissues

Epidemiology (multi-disciplinary approach)

“DISEASE TRIANGLE”

Other criteria

Seed transmission, Q-organisms

disease

Conductive environment

Pathogen Susceptible host

Conductive environment

Pathogen Susceptible host

Disease triangle

Seed-borne pathogens

Seed-borne pathogens are defined as any

infectious agent carried on the seeds, internally

or externally, that has the potential to cause

disease in either seeds or the developing plants

• Infected seed is the primary inoculum. If seed

infection is controlled, the disease is controlled

• Important pathogen, but infected seed is a

minor source of inoculum

• Seed-borne microorganisms, but never

demonstrated to cause disease

• Pathogens that infect seed in fields or in

storage, & reduce seed quality

Seed-borne pathogens

Important characteristics

Type (e.g. Oomycetes, Fungi, Bacteria, Viruses,

Viroids, Nematodes)

Localization on/in the seed

Transmission routes/Classes of seedborne organisms

• Role of seed infection (are there other sources of survival and infection routes?)

Quarantaine organisms (NL, vegetables)

• Clavibacter michiganensis subsp. michiganensis • Potato spindle tuber viroid (PSTVd) • Ditylenchus dipsaci (on Allium spp.)

Impact of disease (e.g. yield, infection of seed,

germination & vigour, quality issues)

Seed-borne pathogens

Seed-borne pathogens

List of important pathogens on vegetable seeds (from ISHI); part 1

Bean Pseudomonas savastanoi pv. phaseolicola;

Xanthomonas axonopodis pv. phaseoli

Brassica Phoma lingam; Xanthomonas campestris pv. campestris

Carrot Alternaria dauci and Alternaria radicina;

Xanthomonas hortorum pv. carotae

Celery Septoria apiicola

Corn Salad Acidovorax valerianellae

Cucurbit Acidovorax avenae subsp. citrulli;

Squash mosaic, Cucumber green mottle mosaic, and

Melon necrotic spot viruses

Seed-borne pathogens

List of important pathogens on vegetable seeds (from ISHI); part 2

Lettuce Lettuce mosaic virus

Pea Pea seed-borne mosaic virus and Pea early browning

virus; Pseudomonas syringae pv. pisi

Pepper Tobamoviruses; Xanthomonas spp.

Tomato Clavibacter michiganensis subsp. michiganensis;

Pepino mosaic virus; Tobamoviruses; Xanthomonas spp.

Seed-borne pathogens

• Seed-borne vs. soil-borne diseases

• Localisation on/in the seed

• Active process of infection (internal) vs. passive

access (seed surface, contaminant in seed lot)

• Interactions:

Internally seed-borne; pathogen is present in seed

tissues

Externally seed-borne; pathogens exist as external

carry-over (contamination) on the seed surface

Fungal structures/ fruiting bodies

Small side-step: Soil-borne diseases

Soil-borne diseases

Soil-borne diseases

• Many fungal pathogens persist (survive) in the

soil or in residues on the soil surface

• The soil acts as a reservoir for inoculum of

these pathogens; widely distributed

• Pythium, Phytophthora, Rhizoctonia,

Sclerotium, Fusarium spp., …, ….

• Seedling is infected during germination, pre-

emergence or post-emergence phases of

seedling establishment

Disease cycle of damping-off and seed decay

Soil-borne diseases

Seed-borne diseases

Interactions of seed-borne pathogens with seed:

Some examples of pathogens and disease cycles

• carried on seed

Phoma lingam (Blackleg disease)

Tobacco mosaic virus as surface contaminant on

seed

• more active seed infection (internally)

Bacterial Fruit Blotch in watermelon

Clavibacter in tomato

Lettuce mosaic virus

Blackleg Disease Cycle (From: http://www.canolacouncil.org/)

Seed-borne diseases

Pycnidia (fruiting structures); on rapeseed (blotter test), close-up and drawing

Seed-borne diseases

Phoma lingam – Black Leg Disease

Seed-borne diseases

Routes of active seed infection

Systemic infection via vascular system

• Many viruses, e.g. Lettuce mosaic virus;

• Vascular wilt of fungi, e.g. Verticillium dahliae,

Fusarium oxysporum

• Some bacteria, e.g. X. campestris pv. campestris

Penetration through floral parts (stigma ->embryo)

• Ustilago nuda (grains), pollen borne viruses, e.g.

Cucumber mosaic virus, LMV

Penetration through ovary wall (infection usually

outside embryo)

• Cladosporium variabile (spinach), Botrytis spp. (onion)

Ascochyta pisi, Alternaria brassicicola

1. Systemic infection via vascular system 2. Indirect systemic infection via stigma to embryo 3. Indirect infection via ovary wall

Seed-borne diseases

1

3

2

Active seed infection

Seed-borne diseases

Disease cycle Bacterial Fruit Blotch in watermelon

Lesions on bacterial fruit blotch infected watermelon fruit that have developed cracks in the rind. (Courtesy R. Walcott)

Seed-borne diseases

Bacterial Fruit Blotch caused by Acidovorax avenae subsp. citrulli

Seed-borne diseases

Relationship between blossom inoculum and seed

infestation by Acidovorax avenae ssp. citrulli

From: Lessl et al. (2007). J. Phytopathology 155 (2): 114-121

Tomato Fruit and Seed Colonization by Clavibacter michiganensis subsp. michiganensis through External and Internal Routes Tancos et al., 2013. Applied and Environmental Microbiology 79(22): 6948-6957

Seed-borne diseases

• Experiments show the ability of Cmm to invade

tomato fruits and seeds through multiple entry

routes

ripening fruits: active movement and expansion of

bacteria into the fruit mesocarp and nearby xylem

access of seeds systemically through the xylem

Management of seed borne pathogens:

Seed treatments

• Physical (pathogen is killed, not the seed)

• Chemical (protection against pathogens)

• Biological (protect seed/seedling against

pathogens + induce systemic

resistance/improve plant growth

Seed treatments

• Physical treatment; example hot water

treatment

Treatment: duration x temperature

Not selective

lethal dosages (time/temperature) vary by organism

Optimum (high enough to inactivate the pathogen,

not the seed)

Treatment window should be large enough

Seed treatments

Seed treatments

treatment duration

effe

ct

Typical surface plot hot water treatment

Seed treatments

Treatment window; Optimum for treatment

Treatment temperatures in interval [Min-Max]give 100% eradication of the infection and seeds have full germinability

germination Infection rate germination Infection rate

Physical seed treatments

Hot water

UV

Ozone

Electron beam

Hot dry air

Hot humid air

Aerated steam

Microwaves

Others, e.g. cathode rays, ultrasound

Seed treatments

Chemical seed treatments

Fungicides. Most common; Thiram, Metalaxyl-M,

Iprodion

Bactericides

Insecticides (vector control)

Chlorine treatment (surface sterilisation); e.g.

Xanthomonas campestris pv. carotae

Seed treatments

Seed treatments

Evaluation of fungicide seed treatments for control of seed borne Stemphylium botryosum (du Toit et al., 2007. Plant Dis. Mgmt Reports 1:ST003)

Biological seed treatments

Few available

Biological control agents patented by early 1999;

84% were bacteria and 16% were fungi

Modes of action: antagonism, antibiosis,

competition and mycoparasitism

Usually any single mode of action gives activity

against a very narrow spectrum of pathogens

Proper loading of the appropriate number of cells

on each seed is difficult

Stability and viability

Consistency

Regulatory and registration issues

Seed treatments

Seed health testing

Reasons for seed health testing

To determine whether infection is below threshold

For quarantine or phytosanitary certification

To determine plant stand/health

Became increasingly more important with the

global movement of seed and the need for

international standards

Seed health testing

Methods for seed health testing

field inspections

direct visual examination

incubation

grow-out

indicator tests

Serological (ELISA, immunofluorescence)

DNA/RNA based tests

Seed health testing

Direct tests; essentially 3 steps

1. isolation of the pathogen from seeds

2. detection and identification of the pathogen

3. confirmation of viability and pathogenicity of the isolate by inoculation of assay plants

With such a direct test, the presence of a pathogen on and/or in the seed is demonstrated

Seed health testing

Indirect tests

Faster, simpler and less expensive

IF, DAS-ELISA, PCR

An inherent drawback of indirect tests is the fact that the presence of viable pathogens is not demonstrated

Seed health testing

Organizations (Europe) that are involved in seed health

ISTA; International Seed Testing Association

ISHI; International Seed Health Initiative

ISF; International Seed Federation

ESA; European Seed Association

Seed health testing

International Seed Testing Association, ISTA

Develop and publish standard procedures

in the field of seed testing

Global network: Member laboratories in over

70 countries/distinct economies world wide

Vision: Uniformity in seed quality evaluation worldwide

ISTA Technical Committees (18)

ISTA Seed Health Committee (SHC) consists of 14 members

from 13 countries.

http://seedtest.org/en/home.html

Seed health testing

International Seed Health Initiative, ISHI (1)

International Seed Health Initiative for Vegetable crops

(ISHI-Veg)

Initative of the vegetable seed industry (1994)

ISHI-Veg represents the production of over 75% of the

world‟s vegetable seed supply

„generalised‟ test method developed by ISHI (within the

International Technical Groups)

Seed health testing

International Seed Health Initiative, ISHI (2)

It is the mission of the International Seed Health Initiative to

secure the delivery of sufficiently healthy seed to customers

on a worldwide basis

work in collaboration with ISTA for the validation of their

seed health testing methods, so as to ensure methods are

scientifically sound, reliable and robust. They co-operate with

national and international regulatory and accreditation

authorities.

http://www.worldseed.org/isf/ishi.html

Seed health testing

International Seed Federation, ISF

The International Seed Federation represents

the interests of the mainstream of the seed

industry at a global level through interaction

and dialogue with public and private institutions

that have an impact on international seed trade.

ISF is a non-political, non-profit organization resulting from

the merger of two highly respected international

organizations: FIS and ASSINSEL.

http://www.worldseed.org/isf/home.html

Seed health testing

European Seed Association

Represents more than 30 national seed

associations from EU Members States

Companies (individual and associate members)

International involvement and observer status (ISF, CPVO,

UPOV, OECD, ISTA, IPPC, FAO etc.)

All seed related topics, including seed health

http://www.euroseeds.org/Topics

Seed health testing

Other Standards, accreditations

• NAL; Naktuinbouw accredited laboratories http://www.naktuinbouw.nl/en/topic/nal-accreditation

• GSPP; Good Seed and Plant Practices http://www.gspp.eu/

The purpose of Good Seed and Plant Practices (GSPP) is to prevent tomato seed and plant lots from being infected by Clavibacter michiganensis subsp. michiganensis (Cmm).

Seed health testing

The International Movement of Seed

• Professional seed companies are increasingly becoming global players and have very international operations. The business today is characterized by:

• International teams working on product development in multiple R&D and screening sites to test variety adaptation to various climatological and ecological zones

• Seed produced in countries in the Northern and Southern hemispheres in different climatic zones for reasons of quality and logistics

• Seed processing, treatment, enhancement, quality control and packing done frequently at a limited number of central sites for reasons of quality and efficiency

• Marketing and distribution of seeds done globally from a central logistic facility. Seed can be re-exported to multiple destination countries in small shipments over many years.

Concluding remarks