At the end of the 2007 growing season, Iris Yellow Spot Virus (IYSV) was detected on direct seeded and transplant onions in Ontario, leading to the first positive identifications of the disease in Canada. This tospovirus is picked up and persistently transmitted by onion thrips. Infected thrips are able to transmit the tospovirus for the remainder of their life to host allium crops and weeds. Research has shown that IYSV is not distributed uniformly throughout the host plant. In fact, the highest levels of IYSV are typically detected in the inner leaves where thrips tend to congregate and feed.

Figure 1. Iris Yellow Spot Virus - Onion

Why should Ontario growers be concerned? Once a plant is infected with IYSV, there are no curative measures.

IYSV symptoms include elongated or diamond-shaped lesions, ranging in colour from light green to straw-coloured necrotic regions. These lesions are generally more common on mid-aged leaves, spreading along the leaf to total necrosis starting at the leaf tip. If you see any bleached spots, take a closer look. However, researchers in Georgia have detected the tospovirus in plants with tip dieback and chlorosis in the absence of lesions. Although infected plants can produce good quality bulbs, severely affected plants will exhibit prematurely dried leaves and produce bulbs of reduced size.

What can Ontario growers do?

In Ontario, University of Guelph Laboratory Services Division is equipped to test for IYSV. A survey for IYSV in Southwestern Ontario is being conducted this growing season to determine the distribution of this virus. If you suspect IYSV in a field, contact OMAFRA or a local agri-business.