The sampling of wood from diseased grapevine trunks is usually a destructive process that involves cutting the arms or even total uprooting. As an alternative, an increment borer (Pressler borer) could allow the study of disease evolution over time for individual vines. A borer was evaluated on vines with and without Eutypa dieback, and covariance analyses conducted to determine the correlation between foliar symptoms and the relative incidence of Eutypa lata and Diplodia seriata. The variation of isolation frequencies was similar within and between vines. D. seriata was more frequently isolated than E. lata, but the two fungi followed different patterns in relation to foliar symptom intensity. E. lata was rare in asymptomatic material and was more frequently isolated as the foliar symptoms increased, stabilizing at the highest symptom intensities. For D. seriata, the isolation frequency was highest from asymptomatic and highly symptomatic vines, which could agree with the endophytic character of this fungus. The experimental error was high, probably due to the blind nature of the sampling.