Iris yellow spot virus (IYSV) is a viral disease of bulb and seed onion crops that is transmitted by onion thrips Thrips tabaci.

IYSV is characterized as genus tospovirus, virus family Bunyaviridae. The virus infects most Allium species and also is known to infect some ornamentals (iris, lisianthus) and some weeds (jimsonweed, tobacco, redroot pigweed) (UC-IPM).

California produces approximately 14,569 ha of onions with a total value of $144 million, making it the top onion producing state in the U.S.

Antelope Valley (Los Angeles County), and Imperial Valley (Imperial County) are both major onion producers, and in recent years have identified IYSV as an emerging threat to the onion industry. The virus was first identified in the Imperial Valley in May 2003, and since then has been positively identified in the Imperial Valley every year (Plant Health Progress, 2007). Onion plants with IYSV symptoms were diagnosed in 20 percent, 60 percent, and 40 percent of the Imperial County onion fields in 2004, 2005, and 2006, respectively (Plant Health Progress, 2007). Fortunately, the disease has not been severe enough to cause significant yield loss or damage. In 2005, IYSV was identified in the Antelope Valley in four onion fields. In two of the fields, IYSV was detected in more than 50 percent of the field; the other two fields had a 10 percent disease incidence of IYSV (Plant Health Progress, 2007).

In January 2008, IYSV was positively identified in Yuma County, Ariz., in two commercial onion fields located in the southwest corner of the county. The fields had a disease incidence of 25 percent, and 2 percent, respectively (Plant Health Progress, 2008).

The symptoms of IYSV are yellow/straw colored diamond shaped lesions on the onion scape or leaves of the onion plant. Chlorotic lesions may coalesce into large chlorotic streaks capable of causing lodging of the onion plant. Infection can reduce plant size and vigor of the onion plant. Infection by IYSV can potentially cause total field loss, but in the Imperial Valley there has not been any total field loss reported (Plant Health Progress, 2007). Stressed onion plants are more susceptible to IYSV than otherwise healthy plants.

IYSV is transmitted by onion thrips throughout the growing season, and weeds or volunteer onions may be a host reservoir for maintaining IYSV between cropping seasons. IYSV infection may be highest on the border of crops. The virus disease is common in the Imperial Valley, but still occurs erratically.

Management of the virus includes onion thrips control. Without the insect, the virus cannot infect nearby onion plants. Additionally, planting seed and bulb fields a safe distance from each other will decrease the spread of the virus between fields. Field practices that reduce volunteer onions and reduce weed populations may also help keep the virus in check.

Currently, research is underway to determine if common weeds of Imperial Valley are good hosts for IYSV. With this information, the epidemiology of the virus can be better determined to aid onion producers in management decisions.