A study by Fundecitrus has confirmed partial resistance to strobilurin fungicides in Colletotrichum gloeosporioides, the fungus responsible for citrus black spot, in orange groves across São Paulo state. The findings, based on analysis of 40 fungal samples collected from commercial orchards, reveal a genetic mutation impacting the effectiveness of these widely used treatments.
Researchers compared current fungal samples with strains collected between 1998 and 2012, identifying a mutation in codon 137 linked to reduced fungicide sensitivity. “Strobilurines act on the respiration of fungi. We identified a mutation in codon 137 associated with a partial and variable reduction in the sensitivity of the agent,” explained Geraldo Silva Junior, a researcher at Fundecitrus.
Crucially, the study did not detect a mutation in codon 143, which would confer complete resistance, potentially rendering strobilurines entirely ineffective. According to Fundecitrus, a complete mutation in this location is considered lethal to the fungus itself, reducing the likelihood of its occurrence. However, the observed partial resistance is sufficient to diminish the efficacy of current treatment protocols, particularly in the southwestern, southern, northern, and central regions of São Paulo state.
The confirmation of resistance follows warnings from producers and technicians during the 2024/25 harvest regarding difficulties in controlling the disease in various areas of the São Paulo citrus belt. The 2024/25 orange crop in the São Paulo and Triângulo/Southwest Minas Gerais region was initially estimated at 230.87 million 40.8 kg boxes, making it the second smallest in 37 years, according to data published in June 2025. The Fundecitrus re-estimated the 2024/25 crop at 215.78 million boxes, a 7% reduction from the May estimate, citing climate conditions and smaller fruit size.
In response to the findings, Fundecitrus is recommending citrus growers adjust their management strategies to mitigate the spread of resistance. These recommendations include reducing the number of strobilurin applications per season, avoiding the sole use of these products, rotating and mixing strobilurines with other fungicides such as triazoles, carboxamides, or copper-based products, and strengthening cultural control practices, including pruning dry branches, removing fallen leaves, and harvesting fruit early.
The 2024/25 citrus season presented significant challenges for Brazilian growers, including historic droughts and pest pressures. The overall production for the season reached 230.9 million boxes, according to data audited as of June 30, 2025.
