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The use of direct and indirect methods in seed health testing

Gerbert Hiddink: Enza Zaden Seed Operations B.V.

<div>The use of disease-free seeds is an effective measure to prevent the introduction and spread of seedborne pathogens into new production areas. Plant protection authorities and the seed industry pursue the same goal: the free movement of healthy seed lots. Seed health tests should be able to detect the target seedborne pathogen at an appropriate threshold, defined as the amount of infection that could potentially lead to disease outbreaks. The viability and pathogenicity of target organisms are determined by tests such as bioassays, grow outs or pathogen isolation by classical microbiological methods followed by pathogenicity assays, otherwise known as “direct” tests. More recently, “indirect” tests based on PCR and sequencing (NGS) have been developed as pre-screening methods. When pre-screen and direct methods are combined they can improve the speed and effectiveness of seed health tests. Defining the limit of detection and selectivity of a pre-screen test is crucial since its purpose is to detect seed lots that are <em>not</em> infected with the target pathogen(s). Consequently, any positive result with a pre-screen test should be classified as a “suspect” that needs further investigation and verification with a test that confirms the presence of a viable and pathogenic organism. The International Seed Health Initiative for Vegetable Crops considers the development of seed health tests that combine indirect and direct test methods as a necessity to meet Koch’s postulates.</div>