Diagnosis is a critical first step in controlling the spread of viruses, especially when dealing with GLD because it is so complex. However, as explained in the symptoms section, GLD is extremely difficult to identify based only on visual effects. GLD and GLRaV diagnosis in grape cultivars is therefore based on more reliable scientific methods.
The field-based or biological method called field indexing is a standard protocol used to diagnose GLD. In this method, bud wood from a test vine is grafted on to a susceptible indicator host and then planted in the field for symptoms to develop. V. vinifera cv. Cabernet franc is a good diagnostic host because this variety is sensitive to GLD and expresses characteristic symptoms in the field. However, field indexing is labor-intensive, time-consuming, and requires a large area of land. In addition, it takes 2–3 seasons of evaluating the symptoms on an indicator host to determine if GLD is even present. And no amount of biological indexing can determine the type of GLRaV present in a test material.
Two laboratory-based methods, namely, enzyme-linked immunosorbent assay (ELISA) and reverse transcription-polymerase chain reaction (RT-PCR) are widely used for the routine diagnosis of different GLRaVs. These tests are versatile, provide results within a couple of days, and a large number of samples can be tested in a relatively short period of time (a few days to a few weeks depending on the number of test samples). Although ELISA is a popular choice for virus detection due to its simplicity and cost-effectiveness, it cannot detect all currently known GLRaVs due to lack of antibodies to some of the GLRaVs. In contrast, RT-PCR assays discriminate between different GLRaVs and are more sensitive than ELISA at detecting viruses in much lower concentrations. The major limitation of RT-PCR is its expense.
It should be noted that all 3 methods of GLD diagnosis are complementary. Depending on the requirement, a combination of these tests is performed to determine the sanitary status of a given grapevine sample.
Contact the grape virologist at WSU for advice on testing grapevine samples for virus and virus-like diseases.