Diversity of Alternaria alternata, causal agent of Alternaria leaf spot of sugar beet.
Alternaria leaf spot of sugar beet has long been a minor issue in the United States. Incidence of this disease has increased in Michigan, with up to 30% of the leaf spot damage in the 2016 through 2019 growing seasons attributed to Alternaria leaf spot. As part of the investigation of factors contributing to this increased disease prevalence, isolates were collected in the field and compared to isolates collected before 2012. Isolates from sugar beet also were compared to strains collected on rotation crops, such as potato and dry bean, and other plants with Alternaria symptoms in the region. All Alternaria isolates were morphologically similar to Alternaria tenuissima, producing small conidia in largely unbranched chains. However, as recently reported, isolates could not be genetically separated from A. alternata (small conidia in branched chains) using a multi-gene sequencing approach. In a three gene phylogeny, isolates formed at least three clades within A. alternata and did not cluster with isolates previously suggested as subspecies tenuissima. These results agree with previous reports that morphological differences do not reliably relate to genetic variability within A. alternata. In addition, isolates recovered prior to 2012 were found in the same genetic groups as those from 2016-2019. Thus, showing no evidence of a substantial change in the fungal population associated with the increase in disease incidence in the region. Isolates originally collected from sugar beet, dry bean, potato, and blueberry all caused lesions on sugar beet leaves with no significant differences in disease severity related to original host. The isolates from sugar beet also grouped with isolates from dry bean, potato, blueberry, and previously published strains from apple and pear, indicating there is no relationship between host and pathogen genotype. Our results support the potential for A. alternata from a diverse range of crops to infect sugar beet.