Multiple large and small-spored Alternaria species causing leaf spot of potato in South Africa

Abstract

Early blight and brown spot of potato in South Africa are caused by Alternaria solani and A. alternata, respectively. However, elsewhere in the world, other Alternaria species have also been associated with leaf spot of potato. These species include A. grandis, A. protenta, A. arborescens, A. cantlous, A. infectoria and A. dumosa. Potato growing regions in South Africa were surveyed to determine which Alternaria species cause disease on potato. Samples were collected from September 2018 to April 2021 from 13 potato production regions in South Africa. Alternaria isolates obtained from collected samples were identified based on morphology and phylogenetic analyses of Alt a 1, tef 1-α, gapdh and rpb2 gene regions, depending on the morphological group the isolates were placed into. In addition to A. alternata and A. solani, two additional species were identified based on sequencing results, namely A. arborescens species complex (SC) and A. grandis. Virulent isolates were selected based on a detached leaf assay and inoculated onto potato plants in the greenhouse. Symptoms similar to those observed in the field were observed and the pathogens reisolated and identified. Alternaria grandis isolates caused symptoms similar to those of A. solani (early blight) and A. arborescens SC caused symptoms similar to those of A. alternata (brown spot). This is the first report of A. grandis and A. arborescens SC causing leaf spot of potato in South Africa.