The best tests for Pierce's Disease (PD) of grape vines are the classic symptoms on the vine; leaf burn, leaf petiole retention, green islands at shoot nodes, cluster raisining while most clusters on the same vine are normal, and vine death.

The ELISA or PCR tests are good only if the test results are positive. If the results are negative, the sample could be a false negative, and the sample is actually positive. Growers should not wait for a positive lab test before attempting to prune PD out of the vines, symptoms are enough.

The best test is vine symptoms. Late July, August and September are the best times to observe the symptoms. If at least three of the five symptoms are identified, and if the vines are growing in a PD zone, the vines need to be marked with a flag or paint, so they can be pruned back to the trunk or above the graft union during winter pruning.

Leaf Burn is non-uniform slow marginal necrosis of the leaf blade. The green leaf blade will turn brown at one or two points on the edge of the leaf. This is different from brown all the way around the leaf edge, as with drought or salt burn. A second leaf sign of PD is a yellow front between the brown and green on white grape varieties, and a maroon front for reds.

Petiole Retention occurs when leaf blades on a shoot fall off with a number of petioles attached to the shoot. Normally a leaf falls off with the petiole attached to the leaf blade and not the shoot.

Green Nodes as islands on a brown shoot are a symptom of PD. Normally the shoot internodes and nodes mature at the same speed, thus turning from green to brown together. However, with PD there can be distinctively green and brown areas in the internode area.

Limited Cluster Raisining occurs when one or two clusters on a vine will dehydrate and turn black while all of the berries and other clusters ripen normally.

Vine Death can occur from PD when dormant canes appear normal at winter pruning. A near death infected vine can have limited or no bud break in the spring.

Symptomatic vines can occur as individuals or in a group, they can be at the perimeter or within the vineyard. Young and old vines can have PD. Very few grape varieties are tolerant to PD.