Monitoring and molecular quantification of inoculum, fruit susceptibility to Phyllosticta citricarpa infection and critical period for citrus black spot control with fungicide sprays in sweet orange orchards

Abstract

In citrus-growing areas of Brazil, the conidia and ascospores produced by Phyllosticta citricarpa, causal agent of Citrus Black Spot (CBS), is epidemiologically important. Monitoring and quantification of P. citricarpa inoculum have been performed with spore traps, which do not capture conidia. In South Africa, the susceptibility period of sweet orange fruit to P. citricarpa infection has been described to be 4 to 5 months after petal fall. However, in Brazil this period was reported as being up to 7 months in the field, and it may extend throughout the fruit maturation under controlled conditions. CBS management is mainly performed by applications of copper-based fungicides and strobilurins (QoIs). However, the critical period for fruit protection with these fungicides has not been clarified yet under SP conditions. Thus, the present project aim to: i) monitor and quantify the P. citricarpa inoculum in an commercial sweet orange orchard using qPCR; ii) assess the susceptibility period of sweet orange fruit to P. citricarpa infection in the field; iii) determine the critical period for CBS control with copper and strobilurin sprays in a late-maturing sweet orange orchard. Monitoring and quantification of inoculum will be performed using health sweet orange nursery trees as spore traps. The nursery trees will be kept for 14 days, from October to July, under and next to the canopy of adult sweet orange trees. The inoculum will be quantified by qPCR. In order to assess P. citricarpa susceptibility, fruit of late-maturing sweet orange from commercial orchard will be inoculated with different concentrations of conidia suspension, at intervals of 30 days, from October to July. This field trial will be conducted in Brazil and South Africa. The critical period for fungicide sprays will be determined in four experiments, two with copper and two with strobilurin sprays, at different times, from October to July. The project tenderer will request the Research Internships Abroad (BEPE) to conduct trials in South Africa aiming to compare the fruit susceptibility period in that country with the data obtained in Brazil. In addition, the recipients will also validate mathematical models of the interaction between weather variables and the inoculum (conidia and ascospores) or fruit susceptibility. It is expected with this project to optimize the CBS management and help citrus growers to spray fungicides only in the critical periods, in which the weather conditions are favorable to inoculum build-up and the fruit are more susceptible. (AU)