Aerial photo showing a gradient in severity of iris yellow spot in an onion seed crop. Symptoms were most severe on the left side of the field, reflecting the direction of migration of the vector of iris yellow spot virus (IYSV), onion thrips, from the adjacent field that was planted to an onion seed crop the previous season. Because of the biennial nature of onion seed crops, the overlap in crops between sequential seasons results in a ‘green bridge’ effect, with movement of the vector and, therefore, IYSV from seed crops drying down for harvest to seedlings of the next season’s crops planted in close proximity.

Photo Source: Fred Crowe, Oregon State University Professor Emeritus