First Report of Iris yellow spot virus Infecting Onion in Zimbabwe

Onion (Allium cepa L.), an important food and cash crop in Zimbabwe, is prone to attack by several biotic agents causing up to 60% yield loss (Birithia et al. 2011). Iris yellow spot virus (IYSV) (genus Tospovirus, family Bunyaviridae) in Africa has been reported in Reunion and South Africa (Pappu et al. 2009), Kenya and Uganda (Birithia et al. 2011), Mauritius (Lobin et al. 2010), and Egypt (Hafez et al. 2012). Bulb onion plants, displaying symptoms typical of IYSV infection, were observed at two Harare farms during tospovirus disease surveys conducted in November 2014. These symptoms consisted of diamond-shaped, irregular chlorotic and necrotic lesions with green islands on the scapes. Disease incidence was estimated at 35% and 50% at the two farms. IYSV infection was confirmed when 18 symptomatic leaf samples collected from both farms were tested by double-antibody sandwich enzyme-linked immunosorbent assay using an IYSV kit supplied by LOEWE Biochemica GmbH (Germany) according to the manufacturer’s instructions. Twelve samples were positive for the virus. IYSV-infected leaf samples, together with healthy onion leaves, were subsequently collected and preserved in RNAlater (Life Technologies, USA). Total RNAs were extracted from both symptomatic and healthy onion leaves using Quick-RNA MiniPrep Kit (Zymo Research, USA) according to manufacturer’s instructions. First-strand cDNA was synthesized using RevertAid First Strand cDNA Synthesis Kit (Thermo Scientific, USA); followed by PCR using KAPA2G Fast HotStart ReadyMix Kit (KAPA Biosystems, USA), the forward primer IYSV-NCP2_F (5′-GGCGGTCCTCTCATCTTACTG-3′) and the reverse primer IYSV-NCP2_R (5′-GAAGTTCCAGGAGTGCATTTAGTC-3′) specific to the IYSV nucleocapsid (N) gene (Lee et al. 2011). The expected 236-bp amplicons, observed only from symptomatic samples, were excised and purified from agarose gel using Qiagen Gel Extraction Kit (Germany) prior to cloning into the pCR2.1 vector of TOPO-TA Cloning kit (Life Technologies, USA). Four positive clones were sequenced using M13 forward and reverse primers at Inqaba Biotechnical Industries (Pty) Ltd (Pretoria, RSA). The consensus sequence of the Zimbabwean IYSV (isolate 5c_Harare; GenBank Accession No. KT271469) shares nucleotide and protein sequence identities of 96 to 97% with corresponding region of IYSV isolates originating from Australia (KJ769187), Brazil (AF067070), Mexico (JX946658) and South Africa (EF579801). To our knowledge, this is the first report of IYSV infecting onion in Zimbabwe. The occurrence of IYSV in Zimbabwe underscores the need for systematic surveys to assess its incidence and its host range in order to develop effective disease management strategies. © 2016, American Phytopathological Society. All rights reserved.