For the first time in Brazil, Fundecitrus has identified evidence of partial resistance of the fungus Phyllosticta citricarpa, which causes citrus black spot, to fungicides of the strobilurin group. This finding corroborates warnings issued by growers and technicians during the 2024–25 season regarding failures in controlling the disease in areas of the São Paulo Citrus Belt.
SAMPLE STUDY
A study leading to the finding was conducted by collecting samples of P. citricarpa from commercial orchards in different regions of São Paulo. Forty samples collected in the 2024–25 harvest were analyzed and compared to other samples of the fungus sensitive to strobilurins collected between 1998 and 2012.
According to Fundecitrus researcher Geraldo Silva Junior, strobilurins act on the respiration of fungi. Silva added that resistance of fungi to strobilurins can occur through simple mutations in three different regions of cytochrome b, at codons 129, 137 or 143. “If the mutation occurs at codons 129 and 137, the resistance is considered partial, but if the mutation is at codon 143, the mutation is complete, and the fungicides cease to be effective in controlling the fungus,” he said.
Molecular analyses conducted at Fundecitrus identified the presence of a mutation in codon 137, associated with a partial and variable reduction in sensitivity to strobilurins.
“This alteration compromises the action of strobilurins on the fungus, resulting in a decrease in the effectiveness of fungal inhibition,” said Fundecitrus postdoctoral researcher Thiago Carraro. “The mutation that characterizes the partial resistance of the fungus has been detected so far in the southwest, south, north and central regions of the state, at varying levels of reduction in the fungicide’s efficiency against the fungus.”
In the case of P. citricarpa, the chances of complete resistance occurring are very low, because if the mutation occurs at codon 143, it tends to be lethal to the fungus. However, the mutation at codon 137 has occurred and appears to be responsible for the reduced effectiveness of black spot control in São Paulo.
GROWER RECOMMENDATIONS
“This partial resistance means that strobilurins may continue to control the fungus; however, at reduced levels in some areas,” Silva said. “Therefore, the recommendation is to reduce the number of strobilurin applications, avoid using these fungicides alone, and adopt rotation, mixing or alternation with copper-based products, triazoles and carboxamides.”
He said other control measures include cultural control such as pruning of dry branches, management of fallen leaves and early harvesting.
ONGOING EFFORTS
Fundecitrus continues to monitor the sensitivity of the fungus in new areas to estimate the frequency of resistant fungi throughout the Citrus Belt and has already developed a rapid protocol for identifying resistance.
Experiments with different chemical and biological fungicides are underway to verify which products are effective in controlling resistant fungi. The information obtained will serve as a basis for adjustments to technical recommendations and for strengthening anti-resistance strategies.
Learn more about citrus black spot here.
Source: Fundecitrus
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