‘Non-pathogenic strains of Guignardia citricarpa’ belong to another species: Guignardia mangiferae
For several years a distinction between non-pathogenic and pathogenic strains of Guignardia citricarpa sensu lato (EU Annexes) has been made. In particular, specific requirements are stated in the EU Directive to avoid the introduction of pathogenic strains of G. citricarpa which cause citrus black spot. Pathogenic and non-pathogenic strains cannot be differentiated morphologically, but it had been observed that in culture non-pathogenic strains grew faster, and easily produced perithecia, whereas pathogenic strains remained sterile and produced many pycnidia. Non-pathogenic strains could be isolated from citrus, but also from many other hosts. The absence of morphological differences renders the identification of pathogenic strains on citrus consignments moving in trade quite difficult. Citrus black spot is traditionally diagnosed on the basis of fruit symptoms: black spot lesions containing pycnidia of G. citricarpa. But problems arise when no pycnidia are present, as black spot lesions can be caused by several other fungi (e.g. Diaporthe citri, Mycosphaerella citri, Colletotrichum spp.). Fungal culture is then required determine whether G. citricarpa is associated with lesions and whether it is a pathogenic strain. In practice, several factors make this rather difficult: competition in culture with other fungi, 14 days are needed to get mature pycnidia, isolates referred to in authors’ collections represent a continuum between the two groups. For these reasons, morphological and molecular studies have now been undertaken on a large number of strains of G. citricarpa sensu lato to investigate whether the distinction can be justified and whether reliable methods can be developed to differentiate between pathogenic and non-pathogenic strains.
Analysis of internal transcribed spacer (ITS) sequences showed that G. citricarpa sensu lato corresponds in fact to two distinct species. This was further confirmed by other molecular assays, and morphological and cultural studies were consistent with this proposal. All isolates from citrus fruits affected by black spot correspond to G. citricarpa sensu stricto. All other isolates from citrus (without black spot symptoms) and many other host plants belonged to another distinct species: Guignardia mangiferae. G. mangiferae (anamorph Phyllosticta capitalensis) is a cosmopolitan endophyte of many woody plants (including citrus). The following species previously described are probably synonyms of G. mangiferae: G. endophyllicola, G. psidii, P. anacardiacearum and P. theacearum. As G. mangiferae occurs in Europe on many host plants and does not cause citrus black spot, it would not be justified to include it in the EU Directive, in contrast to G. citricarpa which does not occur in Europe and represents a threat to citrus production. G. citricarpa and G. mangiferae cannot be reliably differentiated on the basis of morphological characters, but the ITS sequence studies led to the development of a PCR method which will later be described in another scientific paper.