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Recent Introduction and Recombination in Colletotrichum acutatum Populations Associated with Citrus Postbloom Fruit Drop Epidemics in São Paulo, Brazil

First and fifth authors: Department of Plant Pathology and Nematology, University of São Paulo, CEP 13418-900, Piracicaba, SP, Brazil; first and third authors: Genetic Engineering and Molecular Biology Center, University of Campinas, CEP 13083-875, Campinas, SP, Brazil; second author: Department of Animal Sciences, Federal Rural University of Semiarid Region, CEP 59625-900, Mossoró, RN, Brazil; and fourth author: Fundecitrus, Fund for Citrus Protection, CEP 14807-040, Araraquara, SP, Brazil.

Citrus crops in São Paulo State, Brazil, have been severely affected by postbloom fruit drop disease (PFD), which is caused by Colletotrichum acutatum. This disease leads to the drop of up to 100% of young fruits. Previous studies have assumed that this pathogen exhibits a clonal reproductive mode, although no population genetic studies have been conducted so far. Thus, the genetic structure of six C. acutatum populations from sweet orange orchards showing PFD symptoms was determined using nine microsatellite markers, enabling inference on predominant mode of reproduction. C. acutatum populations exhibit a nearly panmictic genetic structure and a high degree of admixture, indicating either ongoing contemporary gene flow at a regional scale or a recent introduction from a common source, since this pathogen was introduced in Brazil only very recently. Sharing haplotypes among orchards separated by 400 km suggests the natural dispersal of fungal propagules, with the possible involvement of pollinators. A significant population expansion was detected, which was consistent with an increase in host density associated with crop expansion toward new areas across the state. Findings of moderate to high levels of haplotypic diversity and gametic equilibrium suggest that recombination might play an important role in these pathogen populations, possibly via parasexual reproduction or a cryptic sexual cycle. This study provides additional tools for epidemiological studies of C. acutatum to improve prevention and management strategies for this disease.

Additional keywords: fungal dispersal, microsatellites, reproductive strategies, sweet orange.