Localized Distribution of Iris yellow spot virus Within Leeks and Its Reliable Large-Scale Detection

In a survey to determine the incidence of Iris yellow spot virus (IYSV) in crops of several host species, samples of one leaf tip/plant were collected at random. When tested by enzyme-linked immunosorbent assay (ELISA) using IYSV-specific antibodies and a blocking step that improved test reliability, the virus was detected only in leek and onion. It was found in 11 of 21 leek and 2 of 26 onion plantings with apparent incidences of 1 to 7 and 1%, respectively. However, the figures for leek crops greatly underestimate IYSV incidence due to localization of infection within plants. Thus, in tests on multiple subsections from individual plants, IYSV was detected in one or more leaves but never in all leaves. Within infected leaves, it was localized in patches of infection found mainly in the middle and top subsections of the unfurled leaves, but infrequently in their bases. It never was found in the furled leaves that make up the stems, or in the basal plates or roots. Therefore, to obtain reliable estimates of IYSV incidences in largescale surveys of leek crops, the randomly collected samples tested by ELISA should consist of combined tissue subsections from the tops and middles of several leaves from each plant sampled.