Determining the Causal Agents of Alternaria Leaf Blight and Head Rot Affecting Broccoli in the Eastern United States

<i>Alternaria brassicicola</i> is the causal agent typically associated with Alternaria leaf blight and head rot (ABHR) disease in broccoli and related crops in the Eastern United States. In 2020, a new species, <i>A. japonica</i>, was reported as causing disease in broccoli and other vegetables in this region. We conducted a multistate pathogen survey during the growing seasons of 2022 and 2023 to assess the distribution and occurrence of <i>A. japonica</i> in relation to <i>A. brassicicola</i> in five broccoli-producing states. Our approach specifically targeted collection of broccoli leaves with lesions typical of ABHR within commercially grown fields that were managed using either organic or conventional approaches in Connecticut, Massachusetts, New York, Virginia, and Georgia. Only typical ABHR leaf lesions were selected for pathogen isolation, and subsequently, sequencing of the <i>Alternaria</i> major allergen a1 gene was used to identify <i>Alternaria</i> species. The predominant species isolated was <i>A. brassicicola</i> (88% in 2022 and 94% in 2023), and the second most common was <i>A. alternata</i> (12% in 2022 and 6% in 2023), which was obtained from fields in Connecticut and Massachusetts in 2022 and in Virginia in both years. <i>Alternaria japonica</i> was not found in either year. Symptoms of <i>A. alternata</i> were indistinguishable from <i>A. brassicicola</i>, as were colony morphologies. Although <i>A. alternata</i> is considered a generalist and of little consequence for broccoli, it is considered a pathogen of significance on multiple crops (blueberry, citrus, pistachios), but there remains scant information on the disease etiology on broccoli. Therefore, we inoculated broccoli with <i>A. alternata</i> in controlled conditions to shed light on possible differences in infectivity of these species on broccoli. Results of our study showed that <i>A. alternata</i> is pathogenic on broccoli, capable of initiating infection and causing lesions typical of ABHR. This indicates that future disease surveys of ABHR should conclusively identify the species of <i>Alternaria</i> that are causing disease. Additional research is needed to determine the significance of this finding in relation to yield impacts, epidemiology, fungicide resistance, and management recommendations.