First Report of Alternaria Black Spot caused by Alternaria alternata on Brassica carinata in South Dakota

Abstract

During July 2016, Brassica carinata planted in experimental plots near Brookings, SD (44°18′37″ N, 96°40′25″ W) were observed with black spots (~2 mm diameter) on the leaves and stems (average disease incidence ~ 40%). Leaves and stems of 5 diseased plants were cut into 5 mm pieces, surface-sterilized, and plated onto potato dextrose agar (PDA). The PDA plates were incubated for 5 days at 25oC in the dark. After 5 days, colonies with dark grey aerial mycelia and brown conidia (n=10; 17-40 μm long and 8-15 μm wide) were consistently recovered from the diseased samples and tentatively identified as Alternaria sp. (Simmons 2007). Two representative colonies (BC1 and BC2) were hyphal-tipped and transferred to fresh PDA plates. DNA was extracted from the mycelium of both isolates and sequenced using the ITS (White et al. 1990) and GADPH (Berbee et al. 1999) primers. BLAST analyses showed the ITS sequences of BC1 and BC2 (KY548068- KY548069) had 99% similarity with A. tenuissima and A. alternata. The GADPH sequ...