How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS AND SIGNS

Symptoms of iris yellow spot on onion include yellow- to straw-colored lesions on leaves and scapes. Dry, elongated lesions or flecks may resemble thrips injury. Lesions may be diamond shaped (this occurs rarely on leaves, more commonly on scapes). Late in the season, infected seed stalks and leaves may lodge. Plant vigor and bulb size are reduced.

COMMENTS ON THE DISEASE

The pathogen is a tospovirus that is transmitted by onion thrips, Thrips tabaci. The virus infects most Allium species, although garlic is not thought to be a host. It also infects some ornamentals (iris, lisianthus) and some weeds (jimsonweed, tobacco, redroot pigweed). The highest disease incidence is typically near field edges. Otherwise healthy plants may show few symptoms and maintain decent growth, while stressed plants may be killed.

MANAGEMENT

Maintain good fertility and adequate soil moisture to reduce plant stress. Practice good sanitation, and remove and destroy infected plants along with cull piles. Eliminate weeds in and around onion fields, especially volunteer onions and wild alliums.

Chemical Control
Disease severity is related to thrips populations; controlling ONION THRIPS will help reduce incidence of this disease.

PUBLICATION

UC IPM Pest Management Guidelines: Onion and Garlic
UC ANR Publication 3453
Diseases
R. M. Davis, Plant Pathology, UC Davis
B. J. Aegerter, UC Cooperative Extension, San Joaquin County
Acknowledgment for contributions to Diseases:
F. F. Laemmlen, UC Cooperative Extension, Santa Barbara/San Luis Obispo counties
R. E. Voss, Vegetable Crops, UC Davis

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