During the fall of 2009, sunflower (Helianthus annuus L.) planted at the LSU AgCenter's Burden Center in Baton Rouge, LA exhibited severe stem and foliar blight symptoms. Symptoms on stems and petioles included elongated, slightly sunken lesions with dark brown margins. Leaf symptoms included irregular to circular, dark brown lesions with white centers and surrounded by a yellow halo. Several spots often coalesced to form large, blighted areas, and severely affected leaves turned yellow, followed by defoliation. The corolla and calyx exhibited similar lesions except for the yellow halo. Disease developed rapidly and the whole (100% disease incidence) field was blighted within a week following a rain (4 mm). Infected leaf and stem tissue was surface disinfested and plated on ¼-strength potato dextrose agar (PDA). Both leaf and stem tissue consistently produced dark olivaceous-to-black fungal colonies at room temperature under 12 h of fluorescent light per day. Conidia were 53 to 128 × 10 to 26 μm, borne singly on the conidiophores, hyaline to dark olivaceous, cylindrical, rounded at both ends, and with 6 to 10 transverse and 0 to 2 longitudinal septa. Conidiophores were single, unbranched, septate, hyaline to dark olivaceous, and measured 77 to 128 × 7 to 13 μm. Morphologically, the fungus was identified as Alternariaster helianthi (Hansf.) E.G. Simmons (= Alternaria helianthi [Hansf.] Tubaki & Nishih) (1). A single-spore isolate (PDC-4291) was obtained from the original culture and DNA from this isolate was extracted with a DNeasy Plant Mini Kit (Qiagen Inc., Valencia, CA). Primers ITS1 and ITS4 were used to amplify and sequence the internal transcribed spacer regions 1 and 2, and NCBI blast analysis of the 552-bp sequence (GenBank Accession No. JN208925) resulted in 100% homology with Alternaria helianthi isolated from sunflower infected with leaf spot and blight disease in India (GenBank Accession No. DQ156343). Pathogenicity was determined by inoculating 20 potted sunflower plants (Full Sun Improved TD, Fred C. Gloeckner and Company, Inc., Harrison, NY) with conidia from a 2-week-old culture of isolate PDC-4291. Each plant was sprayed with 25 ml of suspension containing 106 conidia/ml. Twenty control plants were sprayed with 25 ml of sterile distilled water. Inoculated and control plants were covered with plastic bags and maintained in a greenhouse at 28 ± 2°C. Plastic bags were removed 72 h after inoculation. Leaf spots similar to the original symptoms appeared on all 20 inoculated plants 5 days after inoculation. A few stem lesions were observed on 13 plants. Two weeks after inoculation, infected leaves turned yellow and blighted. Alternariaster helianthi (= Alternaria helianthi) was reisolated from the leaf spots and stem lesions. No symptoms developed on any of the 20 control plants. On the basis of morphology and sequence data, this pathogen was identified as A. helianthi, and to our knowledge, this is the first report of sunflower stem and foliar blight caused by A. helianthi in Louisiana. In Louisiana, sunflower is a popular ornamental that is grown in landscapes and gardens and by commercial flower growers who grow it for cut flower arrangements. Louisiana's hot, humid weather is ideal for disease development, which may discourage gardeners and commercial growers from planting sunflower.
Reference: (1) E. G. Simmons. Alternaria: An Identification Manual. CBS Fungal Biodiversity Center, Utrecht, the Netherlands, 2007.