Technical Abstract: Squash vein yellowing virus (SqVYV) causes watermelon vine decline in watermelon (Citrullus lanatus var. lanatus). Current methods for identification of SqVYV-infected plants are based on the polymerase chain reaction (RT-PCR), tissue blot nucleic acid hybridization assays (NAHA), and visual symptoms. Latent class analysis was used to estimate the sensitivities and specificities of RT-PCR, NAHA, and visual symptoms as diagnostics for SqVYV and to determine whether their performances varied among tissue type (crown vs. vine tissue), where samples were taken along the vine relative to the crown, genus, and habitat (field- versus greenhouse-grown plants). Results showed that RT-PCR had the highest sensitivity (0.94) and specificity (0.98) of the three tests. NAHA had better sensitivity than symptoms for SqVYV detection (0.70 vs. 0.32), while symptoms was more specific than NAHA and a better indicator of non-infection (0.98 vs. 0.65). For the grouping variables, RT-PCR and NAHA had better sensitivity but poorer specificity for diagnosing SqVYV in crown tissue than vine tissue, whereas symptoms had very poor sensitivity but excellent specificity in both tissues. Test performance also varied with habitat and genus, but not with distance from the crown. The results given here provide quantitative measurements of test performance for a range of conditions, and provide the information needed to interpret test results when tests are used in combination for a diagnosis.