INTEGRATING VECTOR AND PATHOGEN PHENOLOGY TO OPTIMIZE X-DISEASE MANAGEMENT

X-disease is currently at epidemic levels in Pacific Northwest orchards growing cherries and other stone fruits. The difficulties in managing X-disease arise in large part from the multi-year lag between infection and symptoms and the wide host range of the X-disease phytoplasma and its leafhopper vectors, as well as high mobility of the leafhopper vectors. X-disease phytoplasma, the causal agent of X-disease, can commonly be found in groundcover weeds such as dandelions, which may provide sources of pathogen transmitted by leafhoppers that readily feed on these plants. In stone fruits (cherries, nectarines, and peaches) X-disease symptoms include small misshapen fruit, with more extreme symptoms in peaches and nectarines that include leaf yellowing and shot-holing (necrosis), branch dieback and tree death. Unfortunately, due to a paucity of research in the Pacific Northwest over the last sixty years since the previous outbreak in the region, little is known about these vectors. This has led to a rapid increase in research over the last 2-3 years to curb the outbreak, but more research is needed to overcome this devastating disease.In this proposed project we will develop degree-day phenology models for predicting transmission of X-disease phytoplasma by leafhopper vectors so precise timings can be prescribed for sprays and cultural strategies. This will expand upon a concurrent project that will develop predictive models for leafhopper abundance. Because leafhoppers are not economic pests if disease transmission does not occur, knowing the presence of both the disease and the leafhoppers will help growers spray precisely and only when necessary. Leafhoppers overwinter as eggs and transmission cannot be passed through generations, so leafhoppers must re-acquire phytoplasma each year before transmission can occur. Acquisition likelihood in vectors increases over the course of the season, so integrating the phenology of leafhopper abundance, disease titers, and leafhopper transmission is critical to adequately time management. Complementing a concurrent leafhopper phenology project, we will evaluate phytoplasma phenology in plant host from samples of cherry canopy leaves, cherry root sucker leaves, and dandelions in the orchard ground cover. Dandelions are one of the most common hosts of leafhoppers and X-disease phytoplasma in Washington and Oregon cherry orchards. All plants will be tested for phytoplasma and we will collect leafhoppers to test for phytoplasma in their salivary glands (measuring transmission) and guts (measuring acquisition). Phytoplasma phenology models will be similar to leafhopper abundance models but will differ slightly due to the nature of proportional data versus counts. Specifically, the objectives are to:Collect tissue samples from leaves of orchard trees, root suckers, and weedy hosts to develop phytoplasma phenology of acquisition potential.Collect leafhopper nymphs and adults to evaluate phytoplasma prevalence phenology.Develop phenology models and share them with stakeholders.