Small‐spored Alternaria spp. (section Alternaria) are common pathogens on wild tomato species

Abstract

The wild relatives of modern tomato crops are native to South America. These plants occur in habitats as different as the Andes and the Atacama Desert and are, to some degree, all susceptible to fungal pathogens of the genus Alternaria. Alternaria is a large genus. On tomatoes, several species cause early blight, leaf spots and other diseases. We collected Alternaria‐like infection lesions from the leaves of eight wild tomato species from Chile and Peru. Using molecular barcoding markers, we characterized the pathogens. The infection lesions were caused predominantly by small‐spored species of Alternaria of the section Alternaria, like A. alternata, but also by Stemphylium spp., Alternaria spp. from the section Ulocladioides and other related species. Morphological observations and an infection assay confirmed this. Comparative genetic diversity analyses show a larger diversity in this wild system than in studies of cultivated Solanum species. As A. alternata has been reported to be an increasing problem in cultivated tomatoes, investigating the evolutionary potential of this pathogen is not only interesting to scientists studying wild plant pathosystems. It could also inform crop protection and breeding programs to be aware of potential epidemics caused by species still confined to South America.

Untargeted metabolomics reveals PTI‐associated metabolites

“…A. alternata was isolated from the leaves of the wild tomato plant ( Solanum chilense ) showing signs of leaf spot (Schmey et al, 2022). A. solani (1117‐1) was isolated from tomato in Freising.…”

“…A. solani (1117‐1) was isolated from tomato in Freising. The identity of the fungal isolates was confirmed based on morphological (Simmons, 2007) and molecular features (Schmey et al, 2022).…”

Alternaria diseases on potato and tomato

“…Many studies proved that A. alternata alone can infect tomato and potato leaves (e.g., Belosokhov et al., 2017) and Schmey et al. (2023) showed that small‐spored Alternaria are true pathogens on wild tomato species (Schmey et al., 2023).…”

“…Panels (b) and (c) are modified from Schmey et al. ( 2023 ). All three pathogens were collected from wild tomato species: (a) from Solanum lycopersicoides , (b) from S. habrochaites and (c) from S. chilense .…”

“…However, most studies report genotype diversities far higher than expected for purely asexual reproduction. This holds true for studies looking at A. solani (Einspanier et al., 2022 ; Leiminger et al., 2013 ; Lourenço et al., 2009 , 2011 ; Upadhyay et al., 2019 ; van der Waals et al., 2004 ), A. alternata (Gherbawy et al., 2018 ; Meng et al., 2015 ) and studies that dealt with both large‐ and small‐spored species (Adhikari et al., 2020 , 2021 ; Ozkilinc et al., 2018 ), as well as diverse Alternaria species from wild tomato hosts (Schmey et al., 2023 ). Possible evolutionary mechanisms leading to the high genetic diversity are mutation, gene flow and selection (Lourenço et al., 2011 ).…”