Identification and pathogenicity of Alternaria species associated with leaf blotch disease and premature defoliation in French apple orchards

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.

Improvement of Alternaria Leaf Blotch and Fruit Spot of Apple Control through the Management of Primary Inoculum

“…These studies indicate that the distribution of Alternaria species causing leaf blotch and fruit spot in apple is different among regions, though A. mali is still the most commonly cited pathogen worldwide [ 9 ]. In Spain, three species have been found to be associated with these diseases, including A. arborescens , A. tenuissima and A. alternata (this work, data not shown), as it has been described in Australia [ 10 ], in Italy [ 8 ], in Israel [ 3 ] and more recently in France [ 11 ].…”

“…The occurrence of Alternaria leaf blotch and fruit spot has been reported in all apple-producing regions of the world [ 11 ], being one of the most important diseases of apple in Southeast Asia, including Japan, South Korea and China [ 9 ], southeastern USA [ 9 ] and Australia [ 10 ]. In Europe, Alternaria leaf blotch and fruit spot were first reported in 1996 in Yugoslavia [ 12 ] and later described in some important apple growing areas such as France, where they started to be a relevant problem in 2016 [ 11 ], Netherlands where they were detected in a survey in 2014 [ 6 ], and Italy, where the problem dates back to 1999 [ 13 ]. In Spain, the first symptoms, affecting mainly Gala and Golden Delicious varieties, were observed in the northeastern part in 2006, although damages of concern appeared in 2014–2015 [ 14 ].…”

Fungal Allergen and Mold Allergy Diagnosis: Role and Relevance of Alternaria alternata Alt a 1 Protein Family

“…Current phylogenetic studies have made significant changes to the systematic taxonomy of Alternaria by elevating 26 clades to the subgeneric taxonomic status of section [ 10 ]. Alternaria section consists of only 11 phylogenetic species and 1 species complex [ 71 , 72 , 73 ].…”

“…Molecular approaches based on barcoding the gene region or gene fragments, such as the internal transcribed spacer (ITS) [ 85 , 86 , 87 , 88 ], mitochondrial small subunit (mtSSU), large subunit ribosomal DNA (LSU) [ 87 ], A. alternata major allergen (Alt a 1) [ 87 , 89 , 90 ], glyceraldehyde-3-phosphate dehydrogenase (GAPDH) [ 86 , 87 , 88 , 89 ], anonymous genomics regions (OPA 1–3 and OPA 2–1) [ 85 , 88 , 89 ], translation elongation factor 1 (TEF1) [ 87 , 88 , 89 ], RNA polymerase, the second largest subunit (RPB2) [ 87 , 88 ], plasma membrane, ATPase, calmodulin [ 87 , 89 ] and actin [ 89 ], have been used to define the monophyly of Alternaria-Nimbya-Embellisia-Ulocladium in the Ascomycete family Pleosporaceae relationships [ 74 ]. Current advances, especially in multi-gene phylogeny and comparative genomics, have made it possible to redefine and delineate the different Alternaria sections, with accurate molecular differentiation and identification of isolates showing that the Alternaria section consists of 11 phylogenetic species and 1 species complex [ 71 , 72 , 73 ].…”

Coinfection of Two Mycoviruses Confers Hypovirulence and Reduces the Production of Mycotoxin Alternariol in Alternaria alternata f. sp. mali

“…A. alternata f. sp. mali is the forma specialis of A. alternata that infects apple ( Malus pumila Mill ), causing severe leaf blotch disease all over the world ( Fontaine et al, 2021 ). Considering that Alternaria blotch disease has become a serious threat to apple production in China, it is necessary to explore hypovirulence-associated mycoviruses that may be used as virocontrol agents against A. alternata f. sp.…”