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