First report of Alternaria alternata affecting common foxglove ( Digitalis purpurea ) in Connecticut

Common foxglove (Digitalis purpurea, D. hybrida) is a popular annual or perennial crop grown as a potted flower for gardens and landscape plantings. Foxglove F1 hybrid ‘Dalmatian Purple’ was observed to have distinct target spot lesions at a retail greenhouse in Hartford County, Connecticut. The majority of plants (approx. 50) were affected by the leaf spot, with 5 to 35% blighted leaf area. Leaf samples were collected and brought to the laboratory in a cooler. A fungus was consistently isolated from surface sterilized leaf lesion pieces on water agar. Morphological traits of hyphal tip isolates (n=10) grown on 0.25% PDA at 20±1°C were observed and the pathogen tentatively identified based on conidial characters as Alternaria alternata. The fungus formed a brown to black colony and conidia with 1 to 4 transverse septa in chains 5-9 conidia long. Fifty conidia of isolates D-01, D-02, and D-03 were measured at 40× magnification using an Olympus microscope (Olympus, Japan). Conidia were 22 to 33 µm long and 5 to 12 µm wide, septate and dark brown, which is characteristic of the species (Simmons 2007). To confirm species identity, genomic DNA was extracted from each of the 10 isolates using the standard protocol of the Qiagen DNeasy Plant Pro kit (Qiagen, Valencia, CA). The Alternaria major allergen a1 (Alt a1), calmodulin, and glyceraldehyde 3-phosphate dehydrogenase (gpd) genes were amplified using Alt-for/Alt-rev (Hong et al. 2005), CALDF1/CALDR1 (Lawrence et al. 2013), and gpd1/gpd2 (Berbee et al. 1999) primer pairs, respectively, and sequenced with Eurofins Genomics (Louisville, KY). The PCR products showed 99-100% similarity to Alternaria alternata (Accession numbers OQ267742.1, OL862610.1, MK791193.1 for Alt a1, gpd, and CAL, respectively). The sequences were deposited in GenBank with accession numbers for Alt a1 (PZ391998 to PZ392007), gpd (PZ392008 to PZ392017), and CAL (PZ380466 to PZ380475). To confirm pathogenicity, leaves from 5 plants of potted foxglove ‘Dalmatian Purple’ were removed and inoculated on unwounded upper leaf surface with 5-mm plugs of agar containing mycelia from the margin of 7-d-old cultures of isolates D-01 and D-02. The leaves were enclosed in plastic boxes lined with moistened paper towels to maintain high relative humidity at 22°C. After 7 d, brown lesions formed that developed dry target spot centers. The pathogenicity test was repeated with similar results. Control leaves inoculated with uncolonized PDA plugs did not develop symptoms. The pathogen was reisolated from the lesions and was the same in appearance as the originally isolated pathogen. A. alternata is a common pathogen of bedding plants and potted flowers. Foxglove infected with A. alternata was found in Italy (Garibaldi et al. 2019). The foliar pathogen Septoria digitalis was found infecting D. lanata in Canada and suggested as being seed transmitted (Parmelee 1971). Buying treated and certified seed would be important to reduce the introduction of these pathogens, as Alternaria spp. can be seed transmitted on Digitalis spp. (Jacobsohn et al. 1985). To reduce losses, fungicide control measures may be necessary to protect common foxglove when conditions are favorable for the pathogen.