Sensitivity of Alternaria alternata, causal agent of Alternaria brown spot on tangerines, to demethylation inhibitor fungicides

• Grant number: 19/26241-0
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• Start date: March 01, 2020
• End date: July 31, 2022
• Field of knowledge: Agronomical Sciences - Agronomy - Plant Health
• Principal Investigator: Lilian Amorim
• Grantee: Mateus Gall Amin
• Host Institution: Escola Superior de Agricultura Luiz de Queiroz (ESALQ). Universidade de São Paulo (USP). Piracicaba , SP, Brazil

Abstract

Brown spot, caused by Alternaria alternata, is the main disease on tangerines at São Paulo State, Brazil. 'Ponkan' mandarin and 'Murcott' tangor, which are the most important varieties at São Paulo, are susceptible to the pathogen. Consequently, disease control depends on frequent application of fungicides. In 2017, several mandarin producers reported failures in disease control, even after successive applications of fungicides from group of quinone outside inhibitors (QoI), popularly known as strobilurins. Recent work conducted by ESALQ, Fundecitrus and APTA, found resistance to azoxystrobin and piraclostrobin (fungicides QoI) in A. alternata isolates collected from orchards in nine municipalities of the State of São Paulo. The frequency of resistance was high: 73% of the isolates were resistant to piraclostrobin and 87% to azoxystrobin. As the fungicides of QoI group registered for the control ofA. alternata in mandarins are formulated in admixture with fungicides of the carbon 14 demethylation inhibitor (DMI) group, it is possible that the isolates resistant to QoI show low sensitivity to fungicides from the DMI group. Thus, the objective of this work is to evaluate the sensitivity of 90 A. alternata isolates, already tested for QoI fungicides, to DMI fungicides. The experiment will be performed with five isolates of the pathogen collected in 2003 and 2004, not exposed to IDM fungicides, and 90 isolates collected in 2017 and 2018, whose sensitivity to fungicides IQe has already been characterized. The isolates are being preserved at ESALQ and come from populations collected in mandarin orchards located at Socorro, Buri and Bariri municipalities. The dose at which the tebuconazole and difenoconazole fungicides inhibit 50% of the mycelial growth of each A. alternata isolate will be determined. In case of in vitro resistance, a sample of resistant isolates, as well as isolates collected in 2003 and 2004 will be inoculated in mandarin leaves, after application of fungicides at the field dose, to verify the 'practical resistance'. Finally, the efficiency of the fungicides formulated with QoI and DMI mixture will be verified in potted plants conducted in a greenhouse.