Recipient Organization
UNIVERSITY OF CALIFORNIA, RIVERSIDE
(N/A)
RIVERSIDE,CA 92521
Performing Department
Entomology, Riverside
Non Technical Summary
Pierce's disease of grapevines has affected grape production in California for essentially as long as commercial vineyards have been present. Vineyards in the North Coast region have faced at least three major epidemics of this fatal disease over the last 40 years, including an ongoing epidemic in which grape growers are seeing not only an usually high number of diseased vines but also unusual patterns in where diseased vines are located. Despite substantial information about the biology and management of the insect vectors and pathogen causing Pierce's disease, a comprehensive understanding is lacking of what causes its periodic outbreaks. Other researchers and I are working to reassess our understanding of this important disease, identify any knowledge gaps that may exist, and modify management strategies accordingly. This includes documenting patterns in vector populations and diseased vines at field sites in Napa and Sonoma Counties, investigating whether additional insects may be more important vectors than previously believed, identifying which non-grape plants are contributing to vector populations and pathogen spread, evaluating whether recent environmental conditions contributed to the disease outbreak, and using mathematical models to simulate different management scenarios. The overall goal is to fill in information needed to develop more a strategic response to Pierce's disease during the early phase of the epidemic, which might include changing the timing or location of chemical control of vectors, targeting additional insects for control than is currently the case, and expanding the use of other disease control measures, such as removal of disease vines to limit their contributions to further pathogen spread.
Animal Health Component
50%
Research Effort Categories
Basic
50%
Applied
50%
Developmental
(N/A)
Goals / Objectives
For vector-borne pathosystems, disease epidemics may be attributable to a wide range of factors associated with the pathogen, vector, host, or environmental conditions. The generalist plant pathogen Xylella fastidiosa exemplifies the potential for multiple ecological or evolutionary triggers of disease epidemics, with outbreaks of X. fastidiosa diseases that have been ascribed to pathogen introduction, increased prevalence of reservoir hosts in the surrounding landscape, invasion by a new vector, and with climate likely playing an important role. Identifying the mechanism driving an epidemic is critical for mitigating impacts to agroecosystems, but is challenging for such pathosystems in which multiple triggers are relevant.X. fastidiosa infects a broad range of agricultural, ornamental, native, and weedy plants, though it does not cause significant disease in most of them. Among the most susceptible hosts are cultivars of Vitis vinifera, in which X. fastidiosa causes Pierce's disease (PD). Grapevine cultivars differ in infection level and disease severity, but typically express a suite of progressively worsening symptoms that include leaf scorch, raisining of clusters, irregular maturation of canes, delayed growth of shoots, defoliation, and vine mortality. Among the most severe PD epidemics occurred in the late 1990s and early 2000s in Southern California and the Southern San Jaoquin Valley following the invasion of the glassy-winged sharpshooter, Homalodisca vitripennis. Yet, PD epidemics predate substantially the introduction of this novel vector, and occur in areas where it is not established. Indeed, vineyards in the North Coast have long been impacted by PD, with at least three epidemics since the 1970s at approximately 25 year intervals. This includes an ongoing epidemic throughout the Coast Range, with reports of upward of 25% incidence of disease in some vineyards over the last few years. A comprehensive understanding of the episodic nature of PD in this region, or the cause of the current epidemic, is lacking.In North Coast vineyards, studies of PD dynamics have focused on the activity of the blue-green sharpshooter, Graphocephala atropunctata(BGSS) a native leafhopper that is distributed in coastal areas purportedly from Mexico to British Columbia, and which is the most efficient vector at transmitting X. fastidiosa to grapevines. Riparian corridors are a key source habitat for BGSS, with reproduction apparently limited to a handful of exotic and native forbs and woody plants. As a result PD pressure is greatest in vineyards nearby riparian habitat. However, BGSS does not disperse far, resulting in strong edge effects in PD distribution with few diseased vines in the interior of blocks. Moreover, BGSS is primarily active in vineyards during the late spring and to a lesser extent in mid summer. Vector activity concentrated relatively early in the season means that vineyard managers typically target that time window for vector control, especially on the edge of vineyards. Seasonality in vector activity may also be epidemiologically significant in that vines that become infected later in the year lose the infection over the winter at a high rate; greater than 95% of infections occurring after June recover. These seasonal features of the PD pathosystem are thought to limit the potential for vine-to-vine spread of X. fastidiosa within vineyards - instead disease dynamics are more consistent with primary spread into vineyards from surrounding habitat. More generally, they likely ameliorate disease incidence versus if they did not occur. The conventional view of PD epidemiology, described briefly above, is sufficient to explain why disease incidence might be moderate, or at least manageable, in most years. However, it is incongruent with episodes of high PD incidence, including the ongoing epidemic in the North Coast in which growers are reporting prevalence upwards of 25% of vines, higher than typical prevalence at sites distant from riparian habitat, and frequent disease hotspots in the interior of vineyard blocks where BGSS is not typically seen. Thus far, the available evidence is inconsistent with a novel (i.e. more virulent) strain of the pathogen being responsible, nor is there evidence of a new vector in the region. Collaborators and I will reassess the current understanding of PD epidemiology in coastal California vineyards to determine whether key assumptions are supported or if there have been substantive changes in the pathosystem that may be driving the PD epidemic. This will involve a combination of surveys and analyses to quantify vector and disease dynamics, impacts of non-grape plant community composition on vector and pathogen pressure, the relative importance of non-BGSS vectors, the role of recent climatic conditions as a potential driver, and mechanistic modeling to inform management strategies. This includes the following objectives:1. Field surveys to quantify the spatial and temporal dynamics of blue-green sharpshooter and Pierce's disease in North Coast vineyards2. Surveys and pathogen transmission studies to reevaluate the role of "minor", non-blue-green sharpshooter vectors in Pierce's disease dynamics in North Coast vineyards3. Surveys of plant community composition surrounding vineyards to identify those species contributing to vector and pathogen pressure in vineyards4. Analysis of recent weather station data, and degree-day modeling, relative to known impacts of climate on sharpshooters and Pierce's disease dynamics5. Development and analysis of a mechanistic model of PD dynamics to evaluate different management scenarios
Project Methods
Obj 1. Surveys will be conducted at 32 vineyards to capture contemporary patterns in BGSS and PD. At each site we will use sticky traps to monitor for BGSS adults throughout the year. Traps will be placed at the edge of vineyards near putative vector sources (e.g., riparian corridors, ornamental plantings) and at different distances throughout the vineyard block. Traps will be inspected at least monthly and the number of BGSS per trap will be recorded. BGSS will be removed, cleaned, and tested via qPCR to determine their X. fastidiosa infection status, to estimate the proportion that are infective. Finally, each fall we will map PD vines in all vineyards based on visual symptoms. We will compare observed patterns of BGSS abundance and natural infectivity to past studies' results over the season, at riparian versus non-riparian sites, and as a function of distance from vector sources. Spatial statistics will be used to analyze patterns of disease, locations of disease hotspots, and the scale of edge effects.Obj 2 - To better understand the relative importance of other potential vectors, we will estimate their abundance via a pair of surveys and test transmission efficiency of select species. In spring, we will randomly place quadrats throughout vineyard rows, visually inspect and record the number of spittlebug nymphal masses (Philaenus spumarius, Aphrophora sp.) within them, and estimate % cover of common weed and cover crop species to assess spittlebug host-plant preferences. Twice a week, during the spring and summer, we will sweep-net sample between vine rows, under vines within rows, on the edge of vineyard blocks, and in the vine canopy. Insects collected in sweeps will be identified and counted. We will quantify pathogen transmission by at least two poorly studied, but fairly common potential vectors - a spittlebug (Aphrophora sp.) and a leafhopper (Pagaronia sp.), using colonies established from field collected nymphs. Groups of adult insects will be confined on X. fastidiosa infected potted grapevines for up to 4 days, then confined in different group sizes (1, 5, or 10) on healthy grape seedlings for up to 4 days, which will be tested 8 weeks to determine the fraction that became infected. Spittlebug nymph counts per quadrat will be compared among sites, locations within site, and based on cover of common weed and cover crop species, sweep-net data will be analyzed with multivariate tests to understand differences in total abundance and composition of non-BGSS vectors. Finally, generalized linear models will be used to analyze X. fastidiosa transmission by Aphrophora and Pagaronia.Obj 3 - Transects will be established at all field sites, from the edge of the vineyard into the dominant native habitat or ornamental plantings adjacent to the vineyard. Plant composition will be quantified by identifying and counting all plants with stems > 1 cm diameter at their base that are within 2 m of the transect tape, and by inspecting and characterizing ground cover 10 x 2 m sections along the length of the transect. Ordination, including principal components analysis, will be used to capture the multivariate nature of plant diversity and relate it and the abundance of dominant taxa to cumulative BGSS number, the fraction BGSS testing positive for X. fastidiosa at each site, and PD prevalence of PD at each site.Obj 4 - As a first step toward understanding whether recent environmental conditions contributed to the ongoing PD epidemic, we will analyze weather station records relative to what is known about effects of temperature in this pathosystem. More than a dozen weather stations are located in the main grape-growing regions of Napa and Sonoma Counties, some with records dating back more than 70 years. We will use those records in analyses of broad differences in conditions during the years when the most recent epidemic began (approx. 2012/13) relative to historic averages. We will compare multiple metrics of "dormant season" (November through March) conditions, including 1) mean daily minimum temperature, 2) number of "cold" days (< 4.5 C), and 3) mean daily high temperatures. A similar set of calculations will be made for "growing season" (April through October) temperatures to determine whether recent years were warmer or had moderate temperatures for a longer period of the season: 4) mean daily high temperature, 5) number of "warm" days (>14.5 C). Second, we will use temperature records in degree-day modeling of BGSS oviposition and flight to understand whether in recent years BGSS was active earlier in the season than was historically case.Obj 5 - A discrete-time, vector-borne disease simulation model will be developed to describe vector and host dynamics in two patches - a vineyard and a nearby vector/pathogen source (e.g., riparian habitat or ornamental planting), coupled by vector dispersal between the patches. Parameter values will be selected based on literature reports or from data generated in earlier objectives, with sensitivity analyses used to explore the effect of potential management strategies on disease dynamics (e.g, vector control in source habitat, early season or late season vector control in vineyards, and roguing of diseased grapevines).