Search

A sudden, unexplained decline and collapse of young apple trees on dwarfing and semi-dwarfing rootstocks has been reported across North America over the past decade. Although viruses have been detected in declining trees, no information is available on their potential causal role in the decline phenomenon. To this end, virus-inoculated apple trees were established in a high-density experimental orchard and monitored over five years. Tree decline was observed in year 4 (2022), resulting in 17% mortality, with declining trees exhibiting marked vascular tissue necrosis. However, none of the eight viruses and one viroid detected in the experimental orchard was significantly more prevalent in declining trees. Extreme temperature fluctuations in January 2022, followed by a severe water deficit in summer 2022, were recorded at the experimental orchard. Similar but distinct observations were made in a nearby commercial orchard with foliar nutrient imbalances documented in trees exhibiting symptoms of rapid decline. Together, our findings suggest that viruses are not primarily responsible for the rapid decline phenomenon and highlight the need for future work to investigate the roles of tree physiology and water stress in tree decline, as well as the potential efficacy of horticultural mitigation practices.

Ceresini treehoppers are present in northern California vineyard ecosystems, including the closely related Spissistilus and Tortistilus (Hemiptera: Membracidae). These membracids are not direct pests of wine grapes, but S. festinus is a vector of grapevine red blotch virus (GRBV). No information is available on the ability of Tortistilus spp. to transmit GRBV. In this study, Tortistilus were collected on yellow panel cards across 102 vineyard sites and surrounding areas in Napa Valley, California, USA in 2021–2023. Specimens were morphotyped, sexed and tested for GRBV ingestion and acquisition by multiplex PCR or qPCR. Phylogenetic analysis of the partial sequence of mt-COI and ITS gene fragments of a subset of 40 Tortistilus specimens revealed clustering in a monophyletic clade with T. wickhami with the former barcode sequence. Only 6% (48/758) of the T. wickhami tested positive for GRBV, but none of the heads with salivary glands (0%, 0/50) of the dissected specimens tested positive for GRBV, indicating no virus acquisition. In contrast, half of the dissected heads with salivary glands of S. festinus (52%, 12/23), from the same collection vineyard sites, tested positive for GRBV. Together, our findings confirmed the presence of T. wickhami in northern California vineyards and suggested a dubious role of this treehopper as a vector of GRBV.