It has long been recognized that diseases cause production losses in perennial crops such as fruit trees and grapevines. To screen for detrimental diseases in these crops, plant quarantine and protection programs first made use of biological indicators. These indicators are specific plant species or varieties that are known to display symptoms when exposed to certain disease agents. Herbaceous indicators are used by spreading the sap of the plant being tested across the leaves of the indicator plant. This process may be completed in days or weeks. Woody indicators are used by grafting the material of the subject plant to a variety known to display symptoms if infected. After the grafting is complete, disease agents present in the subject plant are expected to infect the indicator, which would display symptoms. Indicators are only known to display symptoms for certain agents; it is possible that an indicator may show no symptoms, but a disease agent is still present. It can take many years for woody indicators to establish and show symptoms. Diagnostic technology evolved to ELISA and PCR molecular methods, often in conjunction with indicator plants. ELISA and PCR are advancements over biological indexing because they allow the determination of specific disease agents, and typically require less time to complete testing.

Download