2. 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
3. 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
4. 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
5. Disease management
Use of resistant cultivars for “quality diseases”
Use of tolerant cultivars is less desirable for
quarantine pathogens
6. 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
7. Pathogens and pests
Knowledge about pathogens,
pests and epidemiology is
important
More specific:
Seed-borne diseases
8. 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
10. 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
11. • 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
12. 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
13. 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
14. 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.
15. 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
17. 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
19. 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
21. Pycnidia (fruiting structures); on rapeseed (blotter test),
close-up and drawing
Seed-borne diseases
Phoma lingam – Black Leg Disease
22. 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
23. 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
25. 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
26. 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
27. 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
28. 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
29. • 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
31. Seed treatments
Treatment window; Optimum for treatment
Treatment temperatures in interval [Min-Max]give 100%
eradication of the infection and seeds have full germinability
germinationInfection rate germinationInfection rate
32. 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
33. 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
34. 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)
35. 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
37. 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
38. 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
39. 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
40. 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
41. 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
42. 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
43. 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
44. 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
45. 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
46. 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
47. 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
48. 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
