Methods of Preventing Introduction of Microbial Pathogens

Methods of Preventing Introduction of Microbial Pathogens Methods of excluding pathogens form the primary step in crop disease management. Crop cultivation in a disease-free environment as long as possible would be the ideal situation, since application of chemicals and consequent residues in food materials and environmental pollution could be avoided. Methods of exclusion are essential for keeping the pathogen level under check and this may result in minimal loss in yield. The effects of exclusion may not be clearly perceptible as they are applied elsewhere and also generally before the incidence of diseases in a location. These methods may be divided into two groups, namely i) use of disease-free seeds and planting materials and ii) regulatory methods involving quarantine and inspection

8.1 USE OF DISEASE-FREE SEEDS AND PLANTING MATERIALS

Infected seeds and planting materials are the primary sources of infection and the plants growing from them serve as potential sources of infection from the early crop growth stage, especially in the case of viral diseases. Young infected seedlings are better sources of viral infection since a greater percentage of vectors can acquire the virus from such infected seedlings than from infected older plants (Narayanasamy, 1972). The seeds infected by fungal and bacterial pathogens introduce them into the soil where they may be absent, thus spreading the disease in new locations. The Plant Disease Committee (PDC) of the International Seed Testing Assocation (ISTA) has prepared Working Sheets, describing seed health testing methods for individual pathogens separately for each host plant species. Tolerance limits for the presence of pathogens in the seeds are specified for quarantine seedborne diseases. Seed consignments may be rejected based on 'zero-tolerance' enforced on them to prevent their introduction into new areas. Certification programs, on the other hand, aim to restrict pathogen populations in the propagating materials below specified tolerance limits. The methods employed for indexing seeds and planting materials to assess pathogen population levels have been discussed in Chapter 5. These methods help to detect infection by fungal, bacterial, viral, phytoplasmal and viroid pathogens and to eliminate infected seeds and seed materials.