HPIPM:Onion Iris Yellow Spot

Taxonomy

Domain	Viruses
Realm	Riboviria
Kingdom	Orthornavirae
Phylum	Negarnaviricota
Subphylum	Polyploviricotina
Class	Bunyaviricetes
Order	Elliovirales
Family	Tospoviridae
Genus	Orthotospovirus

Scientific Name

Orthotospovirus iridimaculaflavi

Scientific Name Synonyms

Tospovirus Iris yellow spot virus

Tospovirus iridimaculaflavi

Common Name

iris yellow spot virus (IYSV)

Author: Howard F. Schwartz and Michael E. Bartolo

Identification and Life Cycle

Iris yellow spot is caused by the tospovirus Iris yellow spot virus (IYSV), and has recently become widespread in the western U.S. IYSV can also infect leek, Chinese chive, iris, lisianthus, and several other ornamental hosts. IYSV is reportedly vectored only by onion thrips (Thrips tabaci). Tospoviruses are acquired by first larval instars, but only can be transmitted (within 10 minutes of feeding) to plants after circulation and replication in the vector. The thrips becomes viruliferous for its entire life, but the virus is not known to be passed to thrips progeny through eggs. Weed hosts are largely unknown, but approximately 2% of redroot pigweed plants tested were positive for IYSV in Colorado. Onion seed, bulbs, and roots are not known to carry the virus, but volunteer onions are often symptomatic in early spring in Colorado. The virus likely overwinters in perennial and winter annual weeds, over-wintering onion, and adult thrips.

Plant Response and Damage

Iris yellow spot symptoms first appear as straw-colored, lenticular-shaped lesions on leaves. Lesions may have green centers or alternating rings of green and straw-colored tissue. Distinct spindle-shaped lesions are often found on scapes. Diseased plants may be scattered or widespread across a field, but the highest incidence of disease is often found on the field edges. Iris yellow spot is not known to have any direct affect on bulb quality, but bulbs from plants infected early in the season may not develop fully and store poorly. Total yield losses range from 1 to 10% or more in Colorado, but the disease can significantly reduce the percentage of colossal and jumbo grade bulbs in susceptible varieties.

Biological Control

No biological control strategies have been developed for Iris yellow spot.

Cultural Control

Plant high quality transplants free from thrips and Iris yellow spot. Practice a three-year or longer rotation between onion crops. Eliminate volunteers, culls, and weeds in and around onion fields. No varieties are completely resistant to Iris yellow spot, but varieties less susceptible to Iris yellow spot and thrips feeding injury have been identified and should be planted. Overhead irrigation can provide some thrips and disease suppression. Plant as densely as possible; avoid thin, patchy stands. Avoid crop stress, such as deficient or excessive irrigation, compaction, saline or alkaline soils, or herbicide injury. In Colorado, transplanted onions generally escape severe infection, but also may be a source of contamination. If highly susceptible varieties must be grown they should produced from healthy transplants.

Chemical Control

Thrips control may provide some reduction in Iris yellow spot, but thrips control alone is not sufficient to economically control the disease. Thrips resistance to commonly applied insecticides is widespread in Colorado and other onion production regions of the High Plains.

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.