What is the purpose of indexing in viral diagnosis?

The purpose of indexing in viral diagnosis is primarily to determine the presence of a virus mechanically. Indexing involves using a susceptible indicator plant to test for the presence of a specific virus. When the indicator plant is inoculated with suspected infected tissue, any subsequent symptoms or virus presence can be assessed, confirming the infection status. This method is crucial because it provides a visual and physiological means to verify the presence of the virus, which is essential for managing plant health and diseases.

In this context, the other options do not align with the primary goal of indexing. Quantifying the spread of the virus in a population relates more to epidemiological studies rather than the specific diagnostic function of indexing. Establishing nutritional needs of plants does not pertain to viral analysis but rather focuses on the overall health and requirements of the plant. Lastly, genetically manipulating plants is a biotechnological approach that does not involve the direct detection of viral infections, which is the key aspect of indexing in diagnostic practices.