Histopathology of postbloom fruit drop caused by Colletotrichum acutatum in citrus flowers
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Histopathology of postbloom fruit drop caused by Colletotrichum acutatum in citrus flowers João Paulo Rodrigues Marques & Lilian Amorim & Marcel Bellato Spósito & Beatriz Appezzato-da-Glória
Accepted: 17 October 2012 / Published online: 14 November 2012 # KNPV 2012
Abstract Postbloom fruit drop (PFD) is caused by both Colletotrichum acutatum and C. gloeosporioides and is a potentially serious disease in citrus that occurs when flowering coincides with rainfall. The fungus incites necrotic lesions in petals and stigmas leading to premature fruit drop and reduced yield. The mechanisms of infection and survival of the causal agents remain to be fully elucidated. In the present study, we investigated the histopathology of PFD caused by C. acutatum in the petals and stigmas of sweet oranges using electron and light microscopy. In the petals, pathogen penetration occurred intra and intercellularly and also through the stomata, with intercellular penetration occurring most frequently. The distinct tissues of the petals were colonised, including the vascular system, particularly the xylem. Acervuli were observed on both sides of the petals. Although the fungus did not penetrate through the epidermal cells of the stigma, C. acutatum caused necrosis and an increase of phenolics in this tissue. A protective layer rich in lipophilic and phenolic compounds was formed under the necrotic area and crystals of oxalate were associated with the sites where the pathogen was present.
J. P. R. Marques : L. Amorim : M. B. Spósito : B. Appezzato-da-Glória (*) Escola Superior de Agricultura “Luiz de Queiroz”, Universidade de São Paulo, Cx. Postal 9, 13418-900 Piracicaba, São Paulo, Brazil e-mail: [email protected]
Keywords Acervulus . Appressorium . Citrus sinensis . Fungi penetration . Protective layer . Ultrastructure
Introduction Postbloom fruit drop (PFD), caused by both Colletotrichum acutatum and C. gloeosporioides (Timmer 1993; Lima et al. 2011; McGovern et al. 2012), is one of the most important diseases affecting sweet orange trees [Citrus sinensis (L) Osbeck] (Feichtenberger et al. 2005). PFD is characterised by reddish brown lesions on the petals and necrotic lesions on stigmas that induce the abscission of fruitlets and retention of calices (Feichtenberger et al. 2005). Up to 80 % fruit loss is observed under favourable weather conditions, i.e., frequent rainy days during the bloom period (De Goes and Kupper 2002). Fungi of the genus Colletotrichum have developed different strategies for the infection and colonisation of hosts. Quiescent infection is commonly observed on immature fruits. The pathogen remains dormant until the host tissue becomes suitable for colonisation (Adaskaveg and Förster 2000). However, it is not known whether dormancy occurs in non-germinated melanised appressoria or after the formation of infection pegs (Wharton and Diéguez-Uribeondo 2004). Two distinct colonisation strategies have been described for Colletotrichum spp., intracellular hemibiotrophic and subcuticular intramural necrotrophic
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