Postbloom fruit drop of citrus: sensitivity to fungicides, molecular characterization and dispersal of secondary conidia of Colletotrichum spp

• Grant number: 15/19564-7
• Support Opportunities: Scholarships abroad - Research Internship - Master's degree
• Start date: December 01, 2015
• End date: March 31, 2016
• Field of knowledge: Agronomical Sciences - Agronomy - Plant Health
• Principal Investigator: Lilian Amorim
• Grantee: André Bueno Gama
• Supervisor: Natália Aparecida Peres Lauretti
• Host Institution: Escola Superior de Agricultura Luiz de Queiroz (ESALQ). Universidade de São Paulo (USP). Piracicaba , SP, Brazil
• Institution abroad: University of Florida, Gainesville (UF), United States
• Associated to the scholarship: 15/06007-2 - Postbloom fruit drop: definition of the action threshold for chemical control and identification of fungicide resistant strains, BP.MS

Abstract

Brazil is the largest producer of oranges in the world, being the citrus industry highly expressive in the country. São Paulo was responsible for approximately 62% of the production of this commodity in Brazil in the 2013 growing season. Postbloom Fruit Drop (PFD), caused by Colletotrichum acutatum and C. gloeosporioides, is an important disease in the southwestern region of Sao Paulo State. Growers spray the orchards based solely on the phenology of the plant, and do not consider the weather favorability for the disease development. This may favor the selection of resistant isolates of C. acutatum and C. gloeosporioides. This resistance happens naturally on the population, and this change of sensitivity to fungicides may begin after one or more mutations, for example. In São Paulo State, since 2010 growers are using a mixture of tebuconazole and trifloxystrobin to control postbloom fruit drop on the citrus orchards. Resistance of fungi to these fungicides has already been related in the literature. In order to better understand if there was a shift on the sensitivity of those fungi to trifloxystrobin and tebuconazole, it is essential to know how sensitive to the fungicides a population of the pathogen was before these chemicals were intensively used. Another aspect of this disease that is not fully understood is the potential of dispersal of secondary conidia produced on citrus leaves and, consequently, their role at this disease epidemiology. The sensitivity of the baseline population will be assessed through mycelial growth using the Spiral Gradient Dilution Method, so that the EC50, which is the effective concentration able to inhibit 50% of growth or germination of a pathogen, is obtained. The EC50 values will be compared through Tukey test at a significance level of 5%. Specific genome regions related to resistance to strobilurins already described in the literature will be searched, i.e., regions of the cytochrome b gene. For that purpose, PCR reactions will be performed using specific primers pairs (Cytb7/Cytb8 and Cytb11/Cytb12). The dispersal of secondary conidia of C. acutatum on leaves will also be determined. Leaves of citrus will be inoculated with spores of Colletotrichum acutatum. The inoculated leaves will then be washed with floral extracts to induce the production of secondary conidia from the appresoria of the fungi. Those leaves will be disposed inside a chamber under controlled conditions of light and temperature. A portable fan will be placed 50 cm behind the infected leaves. Petri dishes with media that is specific to the growth of C. acutatum will be disposed at known distances from the fan. Three treatments will be tested: source of inoculum submitted to wind; source of inoculum submitted to water and wind; source of inoculum submitted to floral extract and wind. After the treatments are applied, the Petri dishes will be then sealed and incubated on a growth chamber (B.O.D.), and after two days, the number of colony forming units will be evaluated and analyzed through non-linear regression. (AU)