Sources of Iris yellow spot virus in New York

Hsu, C. L., Hoepting, C. A., Fuchs, M., Smith, E. A., and Nault, B. A. 2011. Sources of Iris yellow spot virus in New York. Plant Dis. 95:735-743. Iris yellow spot virus (IYSV) has been found consistently in commercial dry bulb onion fields throughout New York State since 2006. Yearly recurrence of IYSV may result from annual reintroductions of the virus or persistence of the virus in overwintering host plants. To identify potential sources of IYSV, we surveyed onion transplants imported into New York as well as volunteer onion plants and weeds using a double-antibody sandwich enzyme-linked immunosorbent assay. IYSV was not found in any of 1,097 transplant samples tested in 2007 but 4 of 760 (0.53%) transplant samples tested positive in 2008. IYSV was found in volunteer onion plants in 3 of 10 (30%) onion fields sampled in 2007, in 4 of 27 (15%) onion fields sampled in 2008, and in 6 of 12 (50%) onion cull piles sampled in 2008. In all, 4 of 17 weed species (i.e., chicory [Cichorium intybus], common burdock [Arctium minus], curly dock [Rumex crispus], and dandelion [Taraxacum officinale]), were confirmed to be infected with IYSV using serological and molecular testing methods. IYSV may be reintroduced annually into New York through imported onion transplants but it also persists in volunteer onion plants and selected weed species. Iris yellow spot virus (IYSV), from the genus Tospovirus in the family Bunyaviridae, causes significant yield losses and grade reductions in onion (Allium cepa L.) bulb and seed crops in the western United States (9,11,12,15,28,38). Surveys of dry bulb onion fields in 2006 confirmed the presence of IYSV in New York State (20). Since then, IYSV has been found in onion fields across New York every year (21,32,33). Because of the widespread distribution of IYSV at the time the virus was first detected, it is likely that the virus was present in New York before 2006. Developing an effective, integrated management program for IYSV requires an understanding of basic epidemiological information about the virus pathosystem. This includes understanding primary infection sources, the mechanics of how the virus is transmitted from plant to plant, how the virus is introduced to new locations, how the virus survives between cropping seasons, and identifying factors that favor epidemics (24). Some of this information is known about Tospovirus spp. in general. Thrips are the only known vectors of Tospovirus spp., and acquire a Tospovirus sp. as larvae when feeding on infected plants (23,52). The ability of larvae to become infective (i.e., to both acquire and successfully transmit a Tospovirus sp.) decreases with age of the larvae (53,55). Tospovirus spp. replicate within the thrips host and are transmitted in a persistent, circulative manner; viruliferous adults are capable of transmitting a Tospovirus sp. to host plants for the duration of their lives (52). Aviruliferous adults feeding on infected plants can acquire Tospovirus spp. but cannot transmit the viruses (37,57). There is no evidence of transovarial transmission of a Tospovirus sp. in thrips (58), and there is no evidence that Tospovirus spp. are seed transmitted (37). The spread of some Tospovirus spp. to new locations closely mirrors the dispersal of invasive thrips as vectors of the virus (37). This general knowledge on Tospovirus spp. appears to hold true for the onion–IYSV–thrips pathosystem. More specific studies have confirmed that IYSV is not known to be transmitted through onion seed (25,40), and the only confirmed vector for plant-toplant spread of IYSV is the onion thrips, Thrips tabaci (Lindeman) (6,25,30). Where onion plants are grown continuously and in close proximity for bulb and seed production, thereby creating a “green bridge”, onion crops may be the primary sources of infection (37). In Colorado, IYSV incidence was very high in volunteer onion plants originating from the previous year’s crops, and virus symptoms were detected on volunteer onion plants before appearing on the crop plants (16), suggesting that volunteer onion plants may be a primary source for infection. In Georgia, volunteer onion plants originating from infected bulbs imported from Peru and then culled were suspected to have been the primary source of IYSV when the virus was first detected in that state (36). In addition to infecting Allium spp., IYSV has been found in at least 21 non-Allium plant species (44) and, depending on the geographic region, these alternate host plants may be a primary source and a reservoir for the virus to persist after onion bulb crops are harvested. So far, research suggests that primary spread of IYSV is more important than secondary spread within onion crops, though limited secondary spread can occur (16,37,45). In New York, T. tabaci is the dominant thrips species in onion fields (14) but, in some other locations in the United States, multiple thrips species are common in onion fields (10,31,46), or a species other than T. tabaci may dominate the onion system (36). For the onion–IYSV pathosystem in New York, there is relatively little information on the primary sources of infection, how the virus spreads to new locations, and whether the virus survives between onion cropping periods. The factors that favor epidemics of IYSV in New York are also unknown. Onion is typically grown as a seed crop or a dry bulb crop; onion plants produce bulbs in the first season but produce flowers and seed in the second season if exposed to adequate vernalization (2). In New York State, the majority of onion production is for dry bulbs (39). Planting begins in April and May, and bulbs are harvested in late summer and fall. Bulbs are sold out of storage throughout the winter (39). Fields planted to onion crops in New York are usually not rotated with other crops, and onion crops are not grown in the fields over the winter. In New York, approximately 85% of the onion crop is grown from seed and 15% is grown using onion transplants (39). Most transplants are bare-root transplant seedlings imported from Arizona, where IYSV has been reported since 1993 (15). In Colorado, between 0.4 and 5.0% of onion transplant lots imported from Arizona and California had symptoms of IYSV, and 18 to 91% of the transplant lots sampled were infested with T. tabaci (15). The persistence of IYSV in New Corresponding author: C. L. Hsu, E-mail: [email protected] Accepted for publication 16 February 2011. doi:10.1094 / PDIS-05-10-0353 © 2011 The American Phytopathological Society