Baseline sensitivity of South African Fusarium circinatum to tebuconazole

Abstract

Pine pitch canker, caused by Fusarium circinatum , is among the most destructive forest diseases worldwide, leading to growth suppression, wilting, and root disease in pine seedlings. Infections result in canker formation, contributing to significant losses in wood quality and yield in pine plantations. In South Africa, the sensitivity of F. circinatum to fungicides has not been investigated, representing a critical knowledge gap, particularly because fungicides are widely used to control other Fusarium species. This study aimed to establish the baseline sensitivity of a South African F. circinatum population to tebuconazole (TBZ), a sterol biosynthesis inhibitor. A total of 86 isolates, collected from various pine species across different geographic regions and years, were tested in vitro for their response to increasing concentrations of TBZ. Sensitivity was assessed by measuring mycelial growth inhibition and calculating the mean effective concentration (EC 50 ). The population-wide EC 50 was 0.039 ± 0.012 µg mL − 1 , establishing a baseline sensitivity to TBZ. No significant differences in EC 50 values were observed across years of isolation, provinces, host species, or mating types. These results provide an important reference for future resistance monitoring and inform the strategic use of TBZ in South African forestry. Interestingly, less sensitive isolates observed in a subset of older isolates may reflect historical exposure to fungicides in nursery environments or the introduction of less sensitive strains later on through trade, infected planting material, or other propagative sources.

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