Histopathology of black spot symptoms in sweet oranges

Abstract

In Brazil, citrus black spot (CBS) caused by Guignardia citricarpa is a major disease that has different symptoms on fruit. In this study, fruit of Citrus sinensis infected by G. citricarpa and showing the symptoms false melanosis, freckle spot and hard spot were cross-sectioned and analysed anatomically and histochemically by light microscopy. Immunohistological assays were performed. All symptoms were accompanied by a thickening of the cuticle. False melanosis lesions did not contain pycnidia and remained restricted to the epicarp or to the first layers of the mesocarp. The stomata in this type of lesion showed phenolic compounds in the guard cells and in the sub-stomatal chamber. In some samples, the guard cells and their surrounding cells lysed, and a wound meristem began to form underneath them. Freckle spot and hard spot lesions had very similar histological alterations to the epicarp and mesocarp, but in our samples only hard spot lesions contained pycnidia. Both of these symptoms were accompanied by protein inclusions. Epidermal and sub-epidermal cells located in the oil-gland region were obliterated, causing alterations in these structures. All symptoms had regions that stained strongly for lipids and phenols.

Phyllosticta citricarpa and sister species of global importance to Citrus

“…In addition, the movement of leaf litter from infected orchards through vehicle/ machine movement is also important (Dewdney et al, 2018;Silva-Junior et al, 2016a). Citrus fruit is not considered to be a realistic pathway for spread of P. citricarpa to new areas (USDA APHIS, 2010) for the following reasons: (i) the airborne ascospores cannot be produced on fruit, (ii) pycnidia are only produced in certain fruit lesion types (Brentu et al, 2012;FAO, 2014;Kotzé, 2000;Marques et al, 2012;OEPP/EPPO, 2009;Wager, 1952) and conidia are short-lived with low germination ability (Kiely, 1948b), (iii) conidium dispersal from fruit lesions is by means of short-distance (<1 m) wash-down dispersal (Kiely, 1948b;McOnie, 1965;Spósito et al, 2008Spósito et al, , 2011Whiteside, 1967), (iv) standard packhouse treatments and cold storage effectively control P. citricarpa infections (Korf et al, 2001;Lucon et al, 2010;Rappussi et al, 2009Rappussi et al, , 2011Schreuder et al, 2018;Seberry et al, 1967;Yan et al, 2016), and CBS lesions on fruit or discarded peel segments have a very low reproductive potential (Korf et al, 2001;Schreuder et al, 2018;Schutte et al, 2014), and (v) fallen leaves are not susceptible to infection (Truter et al, 2007). Inter-state movement of commercial packhouse-treated fruit from CBS present to CBS-absent areas is therefore permitted in the USA, in line with their Pest Risk Analysis conclusion that fruit is not a realistic pathway (USDA APHIS, 2011).…”

Scientific Opinion on the risk of Phyllosticta citricarpa (Guignardia citricarpa) for the EU territory with identification and evaluation of risk reduction options

“…During this latent stage, the pathogen develops between the cuticle and epidermis without significant host injury (McOnie, 1967;Marques et al, 2012). During this latent stage, the pathogen develops between the cuticle and epidermis without significant host injury (McOnie, 1967;Marques et al, 2012).…”

Pathogenicity ofPhyllosticta citricarpaAscospores onCitrusspp.

“…The long latent period of citrus black spot, generally 4 to 6 months before symptoms are expressed on fruit (Kiely 1948;Kotzé 1981), restricts the usefulness of detached fruit assays (Marques et al 2011), and studies using attached fruit also face many challenges. For example, Moran Lemir et al (2000) produced citrus black spot symptoms on lemon, the most susceptible citrus species, 6 months after inoculation.…”