Soil Microsclerotial Density Is Strongly Associated with Olive Wilt Severity in Tunisian Orchards

Verticillium wilt, caused by Verticillium dahliae, is a persistent soilborne constraint to Mediterranean olive production. This cross-sectional study evaluated the association between culturable soil microsclerotial density and ordinal wilt stage across five Tunisian orchards and separately documented the analytical provenance of an IGS-targeted SYBR Green qPCR assay.

The retained dataset comprised one orchard-stage composite soil unit for each of four symptom stages in each orchard (20 units), with three technical plate counts per unit. Archived records do not preserve tree/core-level sampling metadata, which were not imputed. Stage effects were tested with orchard as a block, and an orchard-fixed-effects regression, cluster bootstrap, and influence analyses quantified the ordered trend and robustness. qPCR summaries were treated as ancillary assay information rather than field inoculum measurements.

Mean inoculum increased from 8.4 MS/g at the asymptomatic stage to 168.8 MS/g at the severe/dead stage. The orchard-blocked stage effect was strong (F(3,12) = 79.98, P < 0.0001; partial η² = 0.952), and the adjusted trend was 0.421 log10 units per stage (95% CI, 0.351-0.491; R² = 0.922). Excluding the BH stage-3 value produced a similar slope (0.405; 95% CI, 0.331-0.478).

The association was robust within this dataset, but the cross-sectional design and incomplete archived field-sampling metadata preclude causal or predictive interpretation. Observed stage-specific ranges are descriptive rather than validated thresholds.

Soil microsclerotial density was a strong local correlate of olive wilt stage. Predictive thresholds require prospective, independently replicated and spatially standardized validation.