A look at citrus yellow vein clearing virus as an emerging citrus disease in California

Authors: Yokomi, Raymond

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 9/3/2024
Publication Date: 10/23/2024
Citation: Yokomi, R.K. 2024. A look at citrus yellow vein clearing virus as an emerging citrus disease in California. Meeting Abstract. Presented at the California Citrus Conference, Visalia, CA, October 23, 2024.

Interpretive Summary:

Technical Abstract: The first detection of citrus yellow vein clearing virus (CYVCV) in the U.S. occurred in 2022 among citrus cultivars in dooryards in Tulare, California (CA). In 2024, CYVCV was detected in a lemon tree in Hacienda Heights in Los Angeles County. CYVCV was reported to be vectored by citrus whiteflies, aphids and mechanically transmissible in other countries. Our research aimed to confirm insect and mechanical transmission with California strain(s) of CYVCV in the greenhouse. Although we have not been able to confirm vector or mechanical transmission, our research continues using both plant and purified virus as acquisition sources for aphid and whitefly transmissions as well as mechanical transmission tests to non-citrus hosts. We observed a significant number of urban CYVCV-infected trees that were coinfected with citrus viroids, which are not insect-borne. This prompted a survey in 2024 for CYVCV in Tulare and Hacienda Heights quarantine zones where samples were tested for CYVCV and citrus viroids (Citrus exocortis (CEVd) and Hop stunt (HSVd)). Survey results showed that out of 843 samples, 35% were infected with viroids alone and 12.6% were infected with CYVCV of which 31.6% were coinfected with viroids. This suggests that some homeowner citrus propagation is occurring and since a third of the CYVCV-infected trees were coinfected with viroids, some CYVCV spread may be attributed to propagation by infected budwood or other mechanical means. Preliminary results from an ongoing greenhouse a citrus host range test indicate that a broad range of commercial cultivars including sweet orange, grapefruit, mandarin, tangelo, lemons, pummelo, sour orange and a few specialty citrus spp. support CYVCV replication. Strong symptoms (leaf deformities, yellow vein clearing on the adaxial side, watersoaked veins on the abaxial side, venial necrosis, and intermittent ringspots) were observed only in lemons and sour orange. Other cultivars exhibited intermittent mild vein clearing but symptoms were erratic, expressed under cooler conditions, and disappeared with hot (>35°C) greenhouse temperatures. Field observations and yellow sticky trap data from an organic Tango orchard near Reedley, CA showed that citrus aphid populations developed only when young flush was available. However, the citrus whitefly was found to overwinter as nymphs or pupae and emerge as adults in March and feed and oviposit on young flush. Citrus whitefly life history from egg stage to adult required approximately 55 days to complete in a lab colony and at least two generations per year were observed in the field. While CYVCV remains a threat to commercial citrus, these data suggest that keeping orchards under good hemipteran control in central California and maintaining quarantines, disease caused by this invasive virus may be contained to garden citrus.