Abstract
Leaf blotch caused by Alternaria spp. is a common disease in apple-producing regions. The disease is usually associated with one phylogenetic species and one species complex, Alternaria alternata and the Alternaria arborescens species complex (A. arborescens SC), respectively. Both taxa may include the Alternaria apple pathotype, a quarantine or regulated pathogen in several countries. The apple pathotype is characterized by the production of a host-selective toxin (HST) which is involved in pathogenicity towards the apple. A cluster of genes located on conditionally dispensable chromosomes (CDCs) is involved in the production of this HST (namely AMT in the case of the apple pathotype). Since 2016, leaf blotch and premature tree defoliation attributed to Alternaria spp. have been observed in apple-producing regions of central and south-eastern France. Our study aimed to identify the Alternaria species involved in apple tree defoliation and assess the presence of the apple pathotype in French orchards. From 2016 to 2018, 166 isolates were collected and identified by multi-locus sequence typing (MLST). This analysis revealed that all these French isolates belonged to either the A. arborescens SC or A. alternata. Specific PCR detection targeting three genes located on the CDC did not indicate the presence of the apple pathotype in France. Pathogenicity was assessed under laboratory conditions on detached leaves of Golden Delicious and Gala apple cultivars for a representative subset of 28 Alternaria isolates. All the tested isolates were pathogenic on detached leaves of cultivars Golden Delicious and Gala, but no differences were observed between the pathogenicity levels of A. arborescens SC and A. alternata. However, the results of our pathogenicity test suggest that cultivar Golden Delicious is more susceptible than Gala to Alternaria leaf blotch. Implications in the detection of the Alternaria apple pathotype and the taxonomic assignment of Alternaria isolates involved in Alternaria leaf blotch are discussed.
Highlights
Alternaria spp. are ubiquitous fungi comprising approximately 300 different species (Simmons, 2007; Woudenberg et al, 2013)
The number of bases/residues that differed between isolates of A. alternata and isolates of other taxa of the Alternaria section Alternaria included in the analysis (A. arborescens SC, A. gaisen, A. longipes, A. gossypina and A. alstroermeriae) ranged from 6 to 37
The two phylogenetic analyses did, refine identification of two isolates: two Australian strains were assigned to A. alternata whereas they had previously been assigned to A. longipes (BRIP46356 and BRIP46455) by Harteveld, Akinsanmi & Drenth (2013) (Fig. 1, Supplemental Information 1)
Summary
Alternaria spp. are ubiquitous fungi comprising approximately 300 different species (Simmons, 2007; Woudenberg et al, 2013). Different taxa of Alternaria have been associated with Alternaria leaf blotch (ALB) and Alternaria fruit spot (AFS) diseases (Armitage et al, 2015; Gur, Reuveni & Cohen, 2017; Harteveld, Akinsanmi & Drenth, 2013). ALB is characterized by the development of round or irregular brown spots on leaves, bordered by dark brown to purple margins (Rotondo et al, 2012) These symptoms generally start in late spring or early summer, developing to yellowing leaves that can lead to early tree defoliation and a premature fruit drop associated with a reduction in tree vigour and fruit quality over the following years (Harteveld, Akinsanmi & Drenth, 2013; Rotondo et al, 2012). AFS may downgrade the fruit’s value, resulting in a significant financial burden to apple growers (Gur, Reuveni & Cohen, 2017; Harteveld, Akinsanmi & Drenth, 2014)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
