Characterization and fungicidal sensitivity of Phytophthora species causing citrus gummosis in Pakistan

Citrus gummosis, caused by Phytophthora species, is a major constraint on citrus production worldwide, including Pakistan. A survey of 180 orchards across six key citrus-growing districts of Punjab during 2022–2023 assessed disease incidence and severity. Average incidence ranged from 29.8 % to 43.97 %, highest in Sargodha, with severity between 20.0 % and 32.4 %. From 1350 symptomatic samples, 270 were selected for pathogen isolation, yielding 113 Phytophthora isolates. Morphological and multilocus sequencing ( ITS , β-tubulin , EF1-α , Cox-1 ) of 20 representative isolates identified three species: P. nicotianae , P. citrophthora , and P. palmivora . This study reports for the first time P. palmivora as a cause of citrus gummosis in Punjab. Pathogenicity tests on ‘Kinnow’ mandarin confirmed all species as aggressive pathogens. In vitro fungicide sensitivity assays revealed substantial variability in EC₅₀ values among species and fungicides. Folio Gold (mefenoxam + chlorothalonil) showed the highest toxicity, with the lowest EC₅₀ values across all species, most notably 0.21 μg ml⁻¹ for P. nicotianae , followed by Ridomil Gold (metalaxyl-M). Revus exhibited the weakest efficacy, reflected in consistently higher EC₅₀ values. Sigmoidal dose–response modelling demonstrated strong curve fitting and narrow confidence intervals, confirming precise estimates. These trends aligned with growth-inhibition data, where Folio Gold achieved near-complete inhibition below 1 μg ml⁻¹, while other fungicides required higher doses. A heatmap visualization further highlighted distinct sensitivity patterns among species. Greenhouse trials confirmed these results, with Folio Gold reducing disease incidence to 2.2 % (96.6 % control) and improving plant biomass by over 80 %. P. nicotianae remains the most dominant and destructive species, but P. citrophthora and P. palmivora are also prevalent. Folio Gold and Ridomil Gold are the most effective fungicides under local conditions. These findings update pathogen diversity, disease impact, and management options, supporting sustainable citrus production in the region.