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Distribution, Cultivar Susceptibility, and Epidemiology of Apium virus Y on Celery in Coastal California.

Apium virus Y (ApVY) is a potyvirus that was recently found to cause crop loss to celery (Apium graveolens) in California. Symptoms on leaves exhibit varying forms of chlorosis and necrosis. Depending on the cultivar, celery petioles could also exhibit extensive necrotic, sunken, elongated lesions. Severely affected plants were unmarketable. Disease incidence surveys found that a susceptible celery (cv. 414) showed 55% (2007) and 71% (2008) disease. Because it was noted that the Apiaceae weed poison hemlock (Conium maculatum) was present in almost all areas where ApVY affected celery, a 4-year survey collected overwintered hemlock from six coastal county regions and tested composite samples for ApVY using reverse transcription-polymerase chain reaction (RT-PCR) and ApVY-specific primers. These plants were consistently positive for ApVY. Seeds collected from these plants were also positive when tested with the same RTPCR method. However, when ApVY-positive hemlock seeds were germinated and the resulting seedlings tested, all results were negative. The failure of ApVY to be transmitted from hemlock seeds to seedlings was further documented by collecting newly germinated hemlock seedlings from the field and testing them with RT-PCR. All such seedlings were negative for ApVY even though large, adjacent, overwintered hemlock plants tested positive. Two crops of celery seed were produced from ApVY-positive mother plants; celery seed from these infected plants likewise tested positive for ApVY, but seedlings grown from the seed lots were negative for ApVY. Twenty-one celery and celeriac cultivars were inoculated with ApVY using viruliferous aphids, planted in a replicated field trial, and then grown to maturity. Seven cultivars remained symptomless, tested negative for ApVY, and showed signs of possible resistance. The epidemiology of disease caused by ApVY in California evidently involves poison hemlock as a common overwintering host with subsequent vectoring of the virus from hemlock to celery via aphids. ApVY was not seedborne in this weed host or in celery in our experiments. Our data suggest that growers can manage this disease by controlling poison hemlock weed populations and by planting celery cultivars that are not susceptible to ApVY.