Symptoms and Signs
Iris yellow spot is a disease of onions and other Allium species. Although garlic has not yet been found to be a host in the field, researchers have been able to infect it via inoculation.
Symptoms of iris yellow spot on onion include yellow to straw-colored lesions on leaves and scapes, and dry, elongated lesions or flecks that may resemble severe thrips feeding injury. Lesions may be diamond-shaped, but this occurs more commonly on scapes than on leaves. Severe disease on leaves may reduce bulb size.
Lesions on seed stalks may cause lodging (plant toppling) during seed set, when the weight of the umbel increases. Lodging at this stage reduces seed production.
Comments on the Disease
The pathogen is a tospovirus that is transmitted by the onion thrips, Thrips tabaci. Onion thrips must acquire the virus when it is a nymph to be capable of transmitting the virus as an adult. Excessive nitrogen fertilization tends to increase the severity of iris yellow spot by attracting onion thrips.
Some ornamentals (iris, lisianthus) and weeds (jimsonweed, tobacco, redroot pigweed) are also hosts of Iris yellow spot virus.
The highest disease incidence is typically near field edges. Degree of susceptibility varies among onion cultivars, and susceptibility increases with plant stress.
The following cultural practices can reduce problems with iris yellow spot:
- Maintain good fertility and adequate soil moisture to reduce plant stress. Avoid excessive application of nitrogen fertilizer.
- Practice good sanitation. Remove and destroy infected plants along with cull piles.
- Eliminate weeds in and around onion fields, especially volunteer onions and wild Allium species.
Disease severity is related to the presence of ONION THRIPS infected with the virus. Controlling onion thrips will reduce incidence of this disease; particularly spread within the field.