First report of Alternariaster helianthi causing leaf blight disease of sunflower (Helianthus annus) in Sudan

Abstract

In Sudan, oilseed crops rank second in production area after cereals, and sunflower (Helianthus annuus) is considered a promising oilseed (Mohammed, 2010). Leaf blight caused by Alternariaster helianthi is a destructive disease of sunflower. During autumn 2018 and 2019 diseased sunflowers with dark brown to black leaves and stem spots were observed in the Blue Nile state, Sudan. The disease incidence in the major rain-fed areas, approximately 690 hectares, was 90 and 88% in 2018 and 2019, respectively. Symptoms on the leaf surface were observed most frequently after flowering. The disease appeared in the form of small scattered brown spots on the leaf lamina. Leaf spots were irregular, circular and 5–10 mm in diameter. Later these spots increased in size and coalesced covering a larger leaf area, with a dark brown margin and yellow halo. Necrotic lesions also appeared on the stem, petioles and sepals (Figs. 1, 2) with spots on younger plants exhibiting a yellow halo (Fig. 3). Infected leaf and stem tissues were surface-sterilised with sodium hypochlorite (1% v/v) for three minutes, then rinsed three times with distilled water and plated on potato dextrose agar. The plated tissue produced dark olivaceous to black colonies after five to seven days. The conidia were solitary, non-beaked, borne on simple unbranched conidiophores, cylindrical to elongate and elliptic (Fig. 4). The conidium colour was subhyaline to very dilute tan, becoming slightly darker in the most mature conidia, the conidia were variable in size from 18.1 to 23.9 μm wide and 86 to 112 μm long, and borne singly on the conidiophores. Based on the conidia and conidiophore morphology as described by Simmons (2007), the pathogen was identified as Alternariaster helianthi. For molecular identification, DNA was extracted from a representative isolate (designated AHE-2) using a DNeasy Plant Mini Kit (Qiagen, USA). PCR using a species-specific primer pair AhN1F/AhN1R (Udayashankar et al., 2012) amplified a 314 bp product which was sequenced for confirmation of A. helianthi. The resulting sequence was deposited in GenBank (Accession No. OM618612). BLASTn analysis showed that the sequence had 99% (289/291 bp) identity with an rDNA ITS sequence previously identified as A. helianthi (MK968068). Pathogenicity was determined by inoculating 30 sunflower seedlings (30 days old). Conidia from a one-week-old culture was applied to the seedlings using a 10 ml of suspension containing 106 conidia/ml. Twenty control plants were sprayed with sterile distilled water using a hand atomiser sprayer. Inoculated and control plants were covered with plastic bags and maintained in greenhouse for 24 hours after inoculation. Symptoms similar to those observed in the field appeared on all 30 inoculated plants after a few days, the percentage leaf coverage with symptoms was 38%. Alternariaster helianthi was re-isolated from the symptomatic plants, the re-isolated culture and spores were similar in morphology to the inoculated isolate. No symptoms were observed in the control plants. To our knowledge, this is the first report of A. helianthi in Sudan. Alternariaster helianthi has been reported in Uganda, Kenya, and South Africa (Kgatle et al., 2018; 2019). In this study, high disease incidences were observed, and significant yield losses may occur (Calvet et al., 2005), therefore, management strategies are needed to help prevent further spread.