Iris yellow spot virus in Zimbabwe: Incidence, severity and characterization of Allium-infecting isolates

Iris yellow spot virus (IYSV), an emerging pathogen of alliaceous crops, has been reported in many countries worldwide. The pathogen has recently been reported infecting garlic (Allium sativum), leek (A. ampeloprasum) and onion (A. cepa) in Zimbabwe. A study was carried to determine its incidence, severity and distribution in Zimbabwe. IYSV disease surveys were conducted in 18 districts across six provinces. Symptomatic alliaceous leaf samples were tested for IYSV by double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA). Furthermore, the pathogen was characterized by transmission studies, transmission electron microscopy and molecular assays. The pathogen was detected in garlic, leek, shallot (A. aggregatum) and onion crops in all districts, with disease incidences ranging from 26.7 to 78.8% and disease severities of up to 3.7 in some districts. Up to 70% of onion seedlings that were mechanically inoculated with sap from diseased onion plants produced typical IYSV symptoms characterized by green chlorotic tissues surrounded by tan-colored necrotic tissues. Electron microscopy studies revealed spherical enveloped particles that were 80-120 nm in diameter. Analysis of the partial nucleocapsid protein (N) gene of the Zimbabwean IYSV isolates revealed that they were at least 94.89% identical and similar to each other at nucleotide and amino acid levels. These isolates were closely related to isolates from Asia, Australia, Europe, South Africa and the USA, but were distinct from isolates from Brazil, The Netherlands and The UK. Further work to ascertain the pathogen's economic impact needs to be carried out. In addition, farmers need to be educated about the disease so that they can control it by growing resistant varieties and/or controlling the vector.