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Iris Yellow Spot Virus on Onions – Do New York Growers Need to Be Concerned?

Christy Hoepting, CCE – Vegetable Program in Lake Plains and Finger Lakes Regions

IYSV is tricky to identify
IYSV produces straw-colored, dry, tan, spindle- or diamond-shaped lesions, without or with distinct green centers with yellow or tan borders on leaves and scapes. Symptoms vary greatly and are not always characteristic. Infected leaves or scapes will lodge during the latter part of the growing season. Several cases have been reported where symptomless plants test positive for IYSV by molecular testing. IYSV has been reported in both onion seed and bulb crops, volunteer onions within several rotational crops, other Allium species (i.e. garlic, leek), weeds (i.e. redroot pigweed), iris and an “experimental” host, Nicotiana benthamiana. More than one strain of the virus exists. For example, in Colorado, at least two distinct populations of IYSV have been detected. This suggests the possibility of multiple introductions and/or a continually evolving virus.

IYSV is spread by onion thrips
IYSV is a topovirus, similar to Tomato Spot Wilt Virus, which is currently thought to be vectored solely by onion thrips.  Onion thrips second star larvae and adults can transmit topoviruses only if the virus is acquired during the larval stage. Once a thrips has acquired a topovirus, it is viruliferous for life and can continue to spread the virus to new plants. Within an IYSV-infected onion plant, the disease is present at highest concentrations in the inner leaves near the bulb. Coincidently, this region is also most preferred by onion thrips as a feeding site. To date, IYSV has not been detected in onion roots or seed. It was detected in bulbs in Georgia in 2004.

Striving to understand and manage IYSV
Researchers at Colorado State University recently have undertaken research in depth to understand and manage IYSV. Research results to date have shown: i) disease incidence varies among varieties; ii) IYSV may be associated with plant stress (i.e. moisture, temperature extremes, salinity, soil compaction, pink root, etc. ); iii) in more susceptible varieties, IYSV incidence is initially higher along field edges and less in the center of the field (similar to the pattern of onion thrips). Note that this pattern of thrips distribution occurs when thrips immigrate from surrounding vegetation but their distribution may become more uniform in the field as the season progresses.  Cornell entomologists are currently studying the ecology of onion thrips and their movement patterns within and between fields; and iv) IYSV incidence decreases as plant population increases, possibly because onion thrips are challenged to locate a single plant when plant populations are high. In a IYSV pesticide trial, it was found that Actigard + imidacloprid resulted in a 34-38% yield increase of the jumbo class.  A complete integrated approach will be necessary for successful IYSV and thrips management.     

Should New York Growers be Concerned?

1.     In 2004, Colorado State University detected IYSV in 0.0 to 4.4% of commercial onion transplant seedlings from locations in Arizona and California. The contamination level varied among cultivars with red varieties most frequently being infected. New York State produces 12 000 to 13 000 acres of onions annually, a small portion of which are grown from transplants produced in Arizona and other states where IYSV occurs. The majority of onions grown from transplants in NYS are red and sweet varieties, both of which are more susceptible to onion thrips and IYSV than yellow cooking onions.  It is possible that IYSV may be introduced to NYS via infected transplant material from Arizona or other locations where the virus occurs.

2.     In 2004, IYSV was detected in onion bulbs in Georgia. Whether IYSV could be introduced into areas previously not known to be infected with IYSV via infected bulbs is unknown. NYS packers obtain bulbs produced in areas of western states where IYSV occurs for re-packing. Onion thrips might acquire IYSV from imported bulbs infected by the virus that end up in cull piles in NYS.  

3.     Onion thrips, the vector required to spread IYSV, can be prolific in NYS. The efficiency at which onion thrips could spread IYSV under NYS onion growing practices is unknown.  

4.     IYSV may reduce yield of the larger bulb classes significantly. In the absence of IYSV, it can sometimes be a challenge to achieve the large bulb size classes under NYS growing conditions.

5.     In 2004 in Georgia, IYSV was detected on several weed species around onion fields in the same year that IYSV was detected in onions. This suggests that the virus had been in the state before it was detected. Maybe IYSV is already in NYS?

Many questions regarding IYSV epidemiology and onion thrips ecology remain unanswered. Why did IYSV spread so rapidly in Colorado?  How is IYSV being spread among continents, states and growing regions within a state?  At this point, one can only speculate the risk IYSV poses to onion production in NYS. However, NYS onion growers need to be aware of this disease and be prepared to manage it should it occur in NYS. Knowledge is our best defense.

What NY growers can do

§      Onion growers and their scouts need to familiarize themselves with the symptoms of IYSV. Attend the onion session of the Empire Expo on February 16 where photos of IYS will be displayed. On the internet, visit www.narc2004.org/, click on “…powerpoint presentations”, scroll down to the oral presentations pest management section and view four IYSV presentations with many excellent pictures. Scout fields where transplants were imported from states where IYSV occurs. Seed crops and volunteer onions can be scouted too. IYSV symptoms tend to be more characteristic on scapes. Within fields, focus your scouting on field edges, stressed areas and where the stands are thin. If you suspect IYSV, contact your local CCE Extension Educator.

§      Bury all cull piles that may contain bulbs from states where IYSV occurs ASAP to avoid the possibility of thrips acquiring the virus and spreading it to production fields.   

§      Remove volunteer onions ASAP.

A special thanks and acknowledgement to Dr. Lorbeer, Dept. of Plant Pathology, Cornell, for his critical review of this article.