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 five 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 25°C in the dark. After 5 days, colonies with dark gray aerial mycelia and brown conidia (n = 10; 17 to 40 μm long and 8 to 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 and KY548069) had 99% similarity with A. tenuissima and A. alternata. The GADPH sequences of BC1 and BC2 (MF374512 and MF374513) showed 100% similarity with A. tenuissima and A. alternata. Given A. tenuissima is synonymized under A. alternata (Woudenberg et al. 2015), the pathogen was confirmed as A. alternata (Fries) Keissler. For BC1 and BC2, 10 plants of cv. PI 274283 were spray-inoculated with conidial suspension (105 conidia/ml) 6 weeks after planting (Gaetán and Madia 2005). Ten control plants were sprayed with sterile distilled water. After inoculations, all plants were placed into sterile plastic bags. After 7 days, the plastic bags were removed and plants were maintained in the greenhouse at 27°C. The experiment was performed twice. For the two repetitions, at 12 days after inoculation, all inoculated plants had black spots on the leaves and stems (5% of the surface area using the disease assessment key of Tewari and Conn [1987]). The pathogen was reisolated from all inoculated plants and identified morphologically to complete Koch’s postulates. Control plants did not show any symptoms nor was the pathogen isolated from them. A. alternata was described as nonpathogenic on B. rapa in Canada (Duczek et al. 1999), but to our knowledge, this is the first report of A. alternata causing Alternaria black spot of B. carinata in South Dakota. This finding emphasizes that the need to manage B. carinata diseases in South Dakota will be greater with increasing production acres for biofuel.