Seed Testing: Principles and Practices

Chapter 10. Seed Health Testing

237 Seed Health Testing Seedborne pathogens have been recognized as a major means of dissemination of plant pathogens since prehistoric times. Baker (1972) cites the following historically factual reports of associations between plant pathogens and seeds. Claviceps purpurea on rye (Hellig, 1699). Orobanche minor seeds mixed in Vicia spp. seeds (Michelli, 1723). Anguina tritici in wheat seeds (Needham, 1743). Dust of bunt balls in wheat was the pathogen Tilletia spp. (Tillet, 1755). Seed pathology, however, did not emerge as a sub-discipline of plant pathology until the early part of the 20th century when seed analysts began to notice relationships between seedborne fungi that developed during laboratory germination tests and poor seed quality (Neergaard, 1977). These findings initiated a worldwide process of cataloging seedborne microflora that has recorded associations between approximately 2400 microorganisms and seeds of 383 genera of plants (Richardson, 1990). Concurrent with these activities, epidemiological studies were carried out of the seedborne phase of economically important diseases , such as bacterial blights of beans, smuts of cereals, and Stewart’s wilt of corn (Neergaard, 1977). Baker (1972) was the first to identify these studies with seed pathology. He described three environments in which seeds exist: the seed production field; the period covering harvest, conditioning, and storing; and the planted field. He also defined categories of associations between pathogens and seeds within the environments and indicated how these related to control strategies. McGee (1981) integrated the life cycle of a plant pathogen into the three environments and suggested that the role of a seed pathologist should be to study the seed aspects of the life cycle of the pathogen and their interactions with environmental, cultural and genetic factors that influence the cycle. The model in Figure 10.1 combines the concepts proposed by Baker (1972) and McGee (1981) and provides an outline of where various strategies may be deployed in the management of seed diseases. Seed health testing is an important management practice ensuring seedling establishment and minimizing transmission of the pathogen to the plants grown from the seeds. The Application of Seed Health Testing Seed health testing is primarily applied in management of plant diseases in three ways: to detect inoculum thresholds of seedborne pathogens that can be transmitted to the plant grown from the seed; to determine potential impact of seedborne inoculum on stand establishments in the planted field; and to meet requirements for phytosanitary certification of seed lots going into export markets. Chapter contributed by Denis McGee, Professor Emeritus Seed Science Center, Iowa State University. 10 238 Chapter 10 Management of Seed Transmission of Plant Pathogens Infected seeds are the major inoculum source for numerous plant diseases, and when seed infection is controlled, the disease is controlled (McGee, 1995). This method of control requires knowledge of the incidence of seedborne infection above which there is substantial risk of crop damage from the pathogen transmitted by the seed. These inoculum thresholds need to be established in well designed experiments that must include suitable seed health assays. Although the literature is replete with descriptions of seed health test methods, very few are thoroughly researched to determine that they are specific, accurate, reproducible, and practical. Effective methods also should have a degree of sensitivity that relates to the application of the test results. The next step in establishing inoculum thresholds is to plant seeds with different infection levels in the field and establish a correlation between seed and plant infection. For diseases that have no repeating cycles of infection, such as seedling infecting smuts, strong correlations between seed infection and field disease can usually be expected. Rennie and Seaton (1975) showed that the loose smut embryo test of barley seeds was highly correlated with field disease across a range of environments and in different cultivars of barley. It is much more difficult to establish inoculum thresholds for diseases for which secondary infections occur from other inoculum sources. The influence of secondary infection by aphids is the primary reason why lettuce mosaic virus is controlled with an inoculum threshold of 0 infected seeds in 30,000 in California, while in the Netherlands, the value is 0 in 2,000. The Netherlands has a cooler climate and thus a lower aphid population than California. Furthermore, the growers break the disease cycle with other crops, while lettuce is grown under continuous rotation in California (Kuan, 1988). The final step in establishing an inoculum threshold is to apply appropriate statistical analysis to results. This often has been accomplished by interpolating the value directly from...