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Exotic seed as (potential) source of risk to plant, animal and public health: A review
Foreign germplasm will continue to be an important component of turf grass breeding programmes. Hundred of overseas pathogens are not found in the USA, and some have been responsible for major, economically devastating losses. Processing turf grass germplasm through quarantine will continue to be an important component of the US federal programme of safeguarding exotic germplasm introduction and eventual introgression.
Among the different consequences likely to be harmful as a result of the development of mould in grain and seeds, the contamination by certain mycotoxins represents a danger to animal and human health. An exporting country must be able to provide constructive and reasoned arguments in order to propose new legislation concerning these toxins.
Ten seed-borne pathogenic fungi of five vegetable seeds and one fruit seed, and the diseases caused by them on their respective hosts, have been recorded.
Mycotoxins have been found in four samples of seeds from 12 samples studied. In two of them, the toxin level exceeded the allowable concentration: Aflatoxin B-I - 15 mg/kg, and Zearalenon - 2,000 mkg/kg. Similarly, in three of five soya samples imported from Spain, Aflatoxin B-1 20, 30 mkg/kg was found, respectively, while in one, sterigmatosistin 150 mkg/kg was discovered.
Health hazards in man and animals posed by seed-borne and storage fungi could be in the form of allergic diseases and Aspergillus infections. Hazards due to toxins produced by field fungi include ergotism, fusaritoxicoses, pink rot dermatitis (Sclerotinia sclerotiorum) and lupinosis (Phomopsis leptostromiformis and Diaporthe woodii), while mycotoxicoses originating from storage fungi include aflatoxicoses, nephrotoxicoses and neurotoxicoses.
The discovery of mox virulent c morphotype of S. sapinea could have serious implications for the management of this pathogen as well as for quarantine practices in countries that import pine seeds.
Contamination of seed
Twenty weed species were found contaminating rice (Oryza Sativa L.). Farmers thought that Echinochloa Spp. spread via contaminated rice seed and that I. schaemum rugosum seed was largely soil-borne.
Over the past 400 years, plant immigrants have arrived in the US in huge number, as a result of accidental and deliberate introduction of a new range of acute and chronic environmental hazards for immigrants.
Before 1900, many new naturalised species were introduced repeatedly and widely into the US as seed contaminants, through extensive international commerce in crop seeds.
Analysis of soya seeds imported by Armenia, supposedly genetically modified, showed 11 species of fungi with contaminants from the genera Aspergillus, Penicillium, Stemphylium and Fusartium. The most specific diversity of contaminants has been exhibited in the samples from the US and Canada (seven species) and from Spain (five species).
Managing seed-related SPS (TBT) issues
It is in the long-term interests of the seed industry to proactively manage SPS/TBT-related seed industry issues that will help win markets with certainty and confidence. In this context, some relevant examples from the literature are reviewed below.
Management strategies used to minimise seed infection in the seed production field include cultural, chemical and disease resistance. Harvesting, drying, processing and storage operations can be used in minimising the spread, and in the eradication, of seed-bone pathogens.
Methods of controlling seed-borne pathogens methods include general management practices, biocontrol agents, fungicide applied to seeds and foliage, and thermotherapy.
Important factors that determine seed quality, expressed as a percentage, are the uniformity of germination as well as the presence of certain diseases in a seed lot. Although the physiological aspects of germination are of the utmost importance, the impact of seed-borne pathogens should not be ignored.
The amount of chlorophyll was related to the germination performance and the health status of seeds - the percentage of seed contaminated with Fusarium Spp. and Cochlio bows sativus, both known to affect germination, showed a relation with CF value - the sub-sample with the highest CF signals was always the most heavily infected. CF sorting of barley seeds improved their germination quality, not only on the basis of removing less mature grains, but also due to the removal of the seeds with the heaviest fungal infection levels (14).
Powdery mildew and leaf rust occur yearly in wheat growing areas of southeastern US. When seed was treated with triadimenol, the optimum time of foliar fungicide application for maximum yield response was delayed.
In laboratory and packing house experiments, treatment of potato tubers with steam or organic mercury reduced the incidence of the seed-borne pathogens. These results demonstrate that steam treatment can be an efficient method for disinfecting potato tubers, easily applied by packing houses to large volumes.
Treating planting materials with heat (thermotherapy) is a century-old method of disease control that has proved to be effective against various pathogenic microorganisms. When no efficient chemicals are known to control a disease, treating seeds by heat may be of great interest.
The precipitation, high temperature and humidity that accompany harvests of first and second seasons greatly reduced seed quality.
Wheat seed infection by Pyronophora triticirepentis occurred primarily after the early dough stage and was positively correlated with tan spot severity on the flag leaf shortly after anthesis.
A specific, highly sensitive and rapid PCR assay for the detection of Curtobacterium flaccum faciens Pv. Flaccum faciens (Cff) in bean seeds was achieved. Cff is a seed-borne bacterium on the EPPO K2 quarantine list. This procedure may be useful for routine diagnosis of Cff, overcoming the problems of conventional techniques .
The sensitivity of the Alternaria dauci - radicina selective medium (ADRSM) in detecting a radicina on carrot seeds was similar to other methods currently in use, but ADRSM was more sensitive than the other methods in detecting A. duci on infested carrot seeds.
Impact on sustainable seed trading
If SPS/TBT-related seed industry issues are not managed they can have an adverse impact on the sustainability of trade in seed in Asia in the context of the SPS regime under WTO. Some examples of possible impact in this regard are reviewed below.
Cotton is the main agricultural export commodity from Tanzania. The most significant disease of the crop is Fusarium wilt caused by F. oxysporum F. sp. Vasin fectum. Phytosanitary measures instituted at cotton ginners to prevent the distribution, for planting, of seed infected with the wilt fungus have become difficult to apply.
At the buying posts visited, there was no system for separating cotton varieties or for identifying seed cotton purchased from villages infected with fusarium wilt. As a result, seed subsequently distributed for planting is likely to be a source of infection for the spread of disease. The implication of economic liberalisation in the cotton sector is discussed with respect to seed distribution and management of fusarium wilt.
Phaeropsis sapinea is an opportunist pathogen of the pinus species and other conifers. Infection occurs when host trees are predisposed by adverse environmental conditions or mechanical damage. Breeders market oil palm seeds that are intended to provide users with high-yielding, quality planting material. Only seeds marked by breeders can secure initial investments involved in setting up or renewing an oil palm plantation.
Quarantine regulations can help manage SPS issues
It is through effective quarantine that Asian countries can manage the problem of exotic pest introduction. Some relevant examples are reviewed below.
Seeds of several crop from other countries that might be registered are in the prohibited quarantine category and can be introduced only under permit. The number of seeds needed for quarantined grow outs will vary depending on the class of registration.
When seed potatoes have been inspected and meet the requirements of the regulations under the Seeds Act, they can be certified and receive an official tag. For international movement, seed is re-inspected to ensure that it meets the import requirements of the importing country. At that point, a phytosanitary certificate is issued. Phytosanitary import restrictions are applied to prevent the introduction and spread of pests requiring quarantine.
Frequently, phytosanitary measures are viewed as technical barriers to trade, as zero risk means zero trade, and attempts are being made to apply more standardised criteria for pest-risk assessment. Countries are working through regional plant protection organisations such as NAPPO, EPPO and FAO to develop international standards.
Strict quarantine and inspection regulations ensure that bean seed crops are free from seed-borne diseases such as Halo blight (Pseudomonas syringae, p.v. phaseoli), Common Bacterial Blight (Xanthmonos campestris, p.v. phaseloi), Bacterial Brown Spot (Pseudomonas syringae, p.v.syringae) and Anthracnose (Colletotrichum lindimuthianum), and they allow a minimum infection with the bean mosaic virus.
Recommendations
- In the face of the upcoming WTO regime, access to international seed markets will have to rely heavily on the maintenance of seed standards in terms of quality, safety and genetic purity of seed stocks. Proactive engagement with the International Seed Testing Association (ISTA) will thus help to facilitate the seed trade in Asia, as it will help in selling seed in international markets with confidence and certainty.
- Asian countries can learn from experience of the Organisation for Economic Co-operation and Development (OECD) Seeds Scheme. The Scheme was commissioned in 1999 by the Seeds Industry Association of Australia and the Grains Council of Australia. It prescribes common rules for certifying seed for movement in international trade. This modality, if replicated in Asia, would place the regional seed industry in an enabling environment.
- Strategic planning to regulate, in a harmonised way, the regulatory framework, development of quality infrastructure and its accreditation, and human resource development at the Asian regional level, with a view to establishing a network in this region.
- Realising the importance of SPS/TBT standards, Asian countries and the Asia-Pacific Seed Association (APSA) need to engage proactively in the standard setting process and introduce health/quarantine security.
- The establishment of an Asian Seed Council (ASA) to manage seed certification and accreditation in the region is needed. For such an undertaking, regional governments, the seed industry and APSA may jointly provide initial funding.
- There is a need for capacity-building initiatives in the area of risk assessment, implying risk identification and characterisation, risk communication and risk management. Such a capacity at the regional level, inter alia, in the area of risk assessment of GM seeds, for example, would be rewarding as, for individual countries, it would be a costly proposition.
- The adoption of Hazard Analysis and Critical Control Points (HACCP) at the regional level. A standardised HACCP protocol for countries of the region would also help in securing markets once it has been adopted by trading partners.
Such initiatives at the regional level, together with similar initiatives for other sectors will help set the stage for building up Asia as a Regional Trading Area (RTA). This would be to the advantage of the countries of the region, keeping in view their socio-economic and technological conditions, which are complementary and supportive, in achieving the goal of RTA.
(concluded)
(The author is Director/Coordinator WTO, Pakistan Agricultural Research Council (PARC), Islamabad)
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