Symptoms of Iris Yellow Spot Virus have been confirmed in onion fields in the Imperial Valley, with reports indicating increased incidence and severity this season, according to an April 27 announcement.
The detection of Iris Yellow Spot Virus (IYSV) is important for local agriculture because it poses a significant threat to bulb onion production. The virus can greatly reduce yields and is mainly spread by onion thrips. Current management relies on controlling these insect vectors along with specific cultural practices.
Growers are being urged to look for signs such as yellow or straw-colored lesions, dry elongated spots that resemble thrips feeding injury, diamond-shaped lesions more common on scapes than leaves, and concentric rings of green and yellow colors. Necrotic areas can also appear on leaves, which may be further colonized by secondary pathogens like Stemphylium vesicarium. Highest disease rates often occur near field edges and severe infections can lead to reduced bulb size or lodging during seed set.
The disease cycle involves IYSV overwintering in volunteer onions or infected host plants. Onion thrips acquire the virus as nymphs and transmit it as adults; tobacco thrips can also serve as vectors. The virus spreads when infected insects feed on healthy plants but cannot be transmitted through seed—though transplants may carry both the virus and thrips.
Recommended management actions include removing infected plants promptly, eliminating volunteer onions and wild Allium species nearby, controlling weeds around production areas, maintaining good soil fertility without excess nitrogen (which attracts thrips), planting densely where possible, using overhead irrigation to suppress thrip populations when feasible, separating seed from bulb production fields, and ensuring crop-free periods between plantings. No varieties are fully resistant to either IYSV or its vector.
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For suspected cases or questions about managing IYSV symptoms in onions locally, Ana M. Pastrana at UC Cooperative Extension – Imperial County is available for contact via email at ampastranaleon@ucanr.edu or phone at 442-977-5391.