Examining Rootstocks and Vitis Species for Non-host Resistance to Pierce’s Disease

AVF Proj. ID: 9721 / /Cat.: ,
Primary Investigator(s): Andrew Walker, Bruce Kirkpatrickby

Major Objectives:

  1. To screen the following commonly used rootstocks for the ability to be symptomless carriers of, or non-hosts to, Pierce’s Disease Bacteria (PDB): St. George, 5C, 5BB, 161-49, 3309C, 101-14, Schwarzmann, 44-53 Mgt, 11 OR, 1103P, Ramsey, Freedom, Harmony, 039-16.
  2. To screen selections from rupestris X rotundifolia seedling populations for resistance/non-host status to PDB.
  3. To screen 4 accessions of each of a range southern Vitis species for susceptibility to PD; the ability to act as non-disease expressing carriers of PDB; and the ability to be non-hosts to PDB.
  4. To test the possibilities of grafting vinifera scions onto an infected stock/trunk combination, and grafting to non-hosts identified above under greenhouse and Davis field conditions, in preparation for future field studies.

Summary:

We have prepared plant materials for PD screening, developed the inoculation system, have optimized the PD – PCR detection system, and have seedlings from a cross of Chardonnay X (rupestris X rotundifolia). These plant materials will be inoculated with PD during summer 1998 and results should be ready near the end of 1998.

Determining the Factors that Influence Disease Onset from Overwintering Powdery

AVF Proj. ID: 9723 / /Cat.: ,
Primary Investigator(s): Doug Gublerby

Objectives:

  1. Determine how temperatures and chilling affect disease onset from grape buds harboring mycelium of Uncinula necator.
  2. Establish effective control strategies for bud perennation.
  3. Using the data from the first and second objectives, construct a model and subsequent risk assessment program for the control of the infected buds as primary inoculum sources.

Objective 1: From the first collection in December 1996, no bud infections were observed.

These buds were kept at 4°C for 8 months prior to planting in growth chambers due to lack of growth chamber space at the time. It is postulated that the extended length of constant cold temps may have had an affect on the buds. Surplus wood collected from the UC Davis Carignane vineyard is now being stored in a cold chamber maintained at 2-4° C. Buds will be randomly selected and pushed at 20°C to assess what the incidence of infection with different rates of chilling. The second collection was made in January 1998. Buds were observed daily for infection, sporulation, bud-swell, bud-break, flat leaf stage, and infection. Each shoot was measured and any symptoms of stunting were noted. Infected buds were observed in all temperatures except 30°C. The incidence of infected buds was highest at 15 and 20° C (Table 1). Preliminary data show a positive correlation between degree days and infection as well as between days to infection and temperature. Rates of sporulation of Uncinula necator do change with temperature but it is doubtful that is the only reason for the decrease in infected shoots at the higher temperatures. In fact, 25°C is close to optimum temperature for the growth of the pathogen. Further research will be needed to understand the dynamic association between accumulated chilling units and the response of the pathogen to the host growth rate. Material from the March collection is currently being assessed.

Objective 2: In year one of the study (1997), infected buds on each vine in each 1/3 acre replication were mapped (Fig. 1).

In April 1998 we will re-visit these blocks and assess disease incidence. We hypothesize that the vines in the treatment blocks will have a lower incidence of infected shoots in 1998 relative to those in the control blocks simply because early season infection in treated blocks did not occur on adjacent shoots. Therefore, bud infection on adjacent shoots should be reduced. After recording this information we will repeat the protocol as in 1997 and follow the treatment and control blocks into a third season (1999). Prior to treatment application in 1997, there were 734 infected buds in the blocks designated to be treated with Rally 4 oz/A and 762 infected buds in the non-treated control blocks.

Objective 3. Nothing as of yet has been constructed for a model.

Upon completion of mis years second growth chamber study and second field study, data will be analyzed. Model construction will be done in 1999.

Impact of Vein-Banding on Vine Performance: Interaction Between Grapevine

AVF Proj. ID: 9728 / /Cat.: ,
Primary Investigator(s): J.S. Semancik, J. A. Wolpert, M.A. Walker, M.V. McKenryby

Molecular characterization of the grapevine viroids associated with the yellow speckle (YS) and vein-banding (VB) disease syndrome as well as “mystery” disease tissues has identified specific grapevine yellow speckle viroid (GYSVd) variants associated with symptomatic tissues. Zinfandel-1 A from the UCD Foundation Vineyard and a yellow speckle Mission source from the Virus Collection at UCD were found to harbor homogeneous populations of’type 1′ (non-symptomatic) and ‘type2’ (symptomatic) GYSVd-1 variants. With the utilization of apple scar skin viroid (ASSVd) generic primers, capable of identifying members of the ASSVd group, RT-PCR amplification and nucleotide sequencing detected a previously unknown ‘type 3’ symptomatic GYSVd-1 variant in Italian sources. Heterogeneous mixtures of GYSVd-1 variants have been detected in vein-banding and ‘mystery’ diseased vines in Lodi and Napa Valley comprising of all of the above variants. All available viroid-free materials have been propagated for planting in an isolated site at UC Davis. Vines are available for grafting for the Viroid-free Performance Trial when a suitable location at Oakville field station is determined. Materials are being collected for the rootstock germplasm sensitivity study.

Management of Riparian Woodlands for Control of Pierce’s Disease in Coastal

AVF Proj. ID: 9733 / /Cat.: ,
Primary Investigator(s): Alexander H. Purcell, Joe R. McBrideby

The third year of this continuing project evaluated reducing Pierce’s disease (PD) of grapevines while restoring stream bank woodlands in north coastal California. By replacing wild grape, blackberry, and other plants used by BGSS for breeding with plants that are not favored by the BGSS for reproduction or feeding, the numbers of blue-green sharpshooters (BGSS) that we detected in yellow sticky traps were again reduced to significantly lower levels compared to undisturbed controls. Our tests of buffer strip plantings of redwood and Douglas fir between vineyards and riparian woodlands as a barrier to reduce the influx of BGSS into vineyards during spring have thus far been disappointing because of the long times required to generate buffer plants of a suitable size. Our second goal is to reduce the percentage of BGSS that are infective with the Pierce’s disease bacterium (Xylella fastidiosa) by replacing plants that support the multiplication, within-plant movements, and year-round survival of X. fastidiosa with plants that do not. Since June, 1997 we continued to record significantly decreased trap catches of BGSS in the removal and replanting treatments. Overall sticky trap catches of BGSS were reduced over 99%in the treatment in which vegetation was removed and replanted at our first site. At our second site (Napa River), where vegetation was first removed beginning in the growing season of 1996, we found that BGSS counts were reduced by over 70%, but most of the catches in the treated site were at one trap location, located near hillside vegetation that connected to the riparian zone. We established pretreatment (baseline) records of BGSS activity at a third site on Maacamas Creek in Sonoma County, where we began vegetation removal in late 1997, continued through the spring of 1998. Spring catches of BGSS in all sites were sharply lower than in previous years, probably because of cooler temperatures and rain. Effects of reduced numbers of BGSS on the spread of Pierce’s disease to adjacent vineyards will have to await the establishment of new vineyards being planted along some of the experimental areas.

Optimizing Tissue Culture Techniques for Virus Elimination in Grapevines

AVF Proj. ID: 9740 / /Cat.: ,
Primary Investigator(s): Deborah Golinoby

Microshoot tip tissue culture is the method of choice to eliminate virus infection from valuable grapevine clones. However, survival rates are often low or highly variable. The objectives of this research were 1) to optimize tissue culture protocols to increase shoot tip survival and growth rate, increasing efficiency of tissue culture treatment; and 2) to investigate the use of small single node cuttings for virus elimination. A new protocol was found which was a clear improvement over those used in the past, greatly increasing survival and efficiency of tissue culture treatment for virus elimination in grapevines. Four media were used in various sequence for a total of five different protocols. The effect of virus status on survival was studied utilizing the new protocol. We found no significant difference between healthy or virus-infected source materials in percent survival or percent root development in culture, although incidence of disease symptoms was significantly higher in plants cut from virus-infected source plants. The virus elimination rate in this study was 97%. For objective #2, we have found a significant percentage of “escapes” among relatively large explants cultured from virus infected plants. This suggests that as virus detection techniques become more sensitive it may be possible to regenerate clean stock far more efficiently using relatively large explants. We have found that plants continue to test negative for viruses by ELISA. This is further indication that the virus has been successfully eliminated. A woody index test is in progress to confirm this.

Studies in the Biology and Control of Armillaria Root Disease on Grapes

AVF Proj. ID: 9743 / /Cat.: ,
Primary Investigator(s): David M. Rizzoby

Armillaria root disease (also known as oak root fungus) is a long-term, chronic problem on grapevines and has been recorded in California since at least the 1880’s (Gardner and Raabe, 1963). The fungus attacks the roots of plants, killing the cambium and decaying the woody tissues. Symptoms of the disease include poor shoot growth, premature yellowing and dropping of leaves, dieback of vines, and eventually death of the vine. The disease occurs in distinct infection centers that expand over time. Spread of the pathogen is mostly by growth of fungal mycelium through susceptible host tissue and root to root contact, or through the formation of rhizomorphs (root-like aggregations of fungal tissue). The fungus can potentially survive for long periods in woody debris in the soil (up to 100 years) leading to long-term establishment on a site. Armillaria root disease is potentially a major constraint to hillside development of vineyards in north coastal areas and the Sierra foothills. Although most commonly associated with native oak stands, Armillaria species are natural components of most, if not all, forest ecosystems in California. In the coastal ranges, Armillaria is also associated with most native hardwoods and conifers (e.g., Douglas-fir, ponderosa pine). As an endemic species, Armillaria often does not cause major losses in native stands. The fungus is generally found in small lesions on the root system and root collar, and as epiphytic rhizomorphs. Lack of overstory mortality in these stands does not necessarily mean the absence of the fungus. As native trees are cut, the fungus colonizes the dying root systems, decays the wood, and slowly increases inoculum. Under the right conditions (susceptible host, favorable environment), Armillaria will then infect the planted crop. Little is known about the epidemiology of Armillaria in California vineyards. Large amounts of forest and orchard land are being cleared to establish vineyards. It has become clear that we need to identify the potential risk of Armillaria at sites that are being cleared of forest trees. If recommendations are to be made about site preparations and rootstocks, we must have a better picture of the identity and spatial distribution of the pathogen. We have identified two different species of Armillaria in forested sites of the coastal ranges and Sierra foothills: A. mellea, an aggressive pathogen, and A gallica, which acts primarily as a saprobe. By learning more of the ecology of these species in grape growing regions, we may be able to make risk assessment of new plantings. Control of the pathogen often relies on pre-plant fumigation, which is not feasible on hillsides. Due to the complete lack of control for already-infected vines, tolerance of infection relies on relative resistance of the rootstock and below-ground conditions affecting fungal growth. Given favorable growth conditions, changes in vineyard management to high-density plantings may compromise the relative resistance of the rootstock and exacerbate the problem. At the present time, there are no known Armillaria resistant grape rootstocks. Evaluation of rootstocks currently is done by planting vines in Armillaria infested sites. This method, however, often requires many years before adequate data is collected, and inconsistencies in the distribution of inoculum in the field make interpretation of these studies difficult. In addition, a screening program needs to take into account the variation in the pathogen. In 1997, we developed several techniques to inoculate grapevines with Armillaria. We will continue to refine these methods in 1998 and then expand the experiments in 1999 to encompass a wide range of rootstocks.

Xiphinema index and Grape Fanleaf Virus Disease Complex: Defining Nematode

AVF Proj. ID: 9750 / /Cat.: ,
Primary Investigator(s): Edward P. Caswell-Chenby

We have initiated experiments using large soil containers (4′ X 5′ X 5′) containing a coarse sand:clay mix. The containers were selected to allow us to simulate field conditions, with controlled soil conditions. The containers were planted in the spring of 1996 with the highly susceptible Rupestris St. George rootstock, to allow nematode (Xiphinema index) populations to increase. The plants were inoculated with Grape Fanleaf Virus (GFLV) with or without X. index during summer of 1996. Our objective is to determine how X. index and GFLV, separately and together, influence nematode population dynamics, virus prevalence, and plant growth and yield relative to rootstock. Upon sampling the units during late summer of 1997, we observed all nematode stages, (juveniles to adults), in all containers save one, indicating nematode reproduction. In addition, we observed nematode feeding damage on the roots, with large terminal galls at root termini. We counted nematode numbers in each experimental treatment, and interestingly, the nematode densities in the nematode and GFLV treatment were significantly lower than where the virus was absent. Although this is preliminary data, it suggests a previously undocumented interaction between the nematode and the virus, and that the presence of the virus reduces nematode reproduction. It is possible that this is due to a general decline in vine vigor due to the combined influence of the nematode and the virus. This research is continuing, and we should be able to ascertain details of the interaction between virus and nematode through more intensive sampling. In late spring of 1998 we move into the next phase of the experiments, and will introduce a resistant rootstock to half of the experimental units, so that we can document the influence of resistance on nematode dynamics, virus transmission and prevalence, and plant growth and yield.

A Study of Anagrus Egg Parasitoids Important for Biological Control of Erythroneura Leafhopper Pests of Wine Grapes in California

AVF Proj. ID: 9602 / /Cat.: ,
Primary Investigator(s): Dan Gonzalez, Serguei V. Triapitsynby

In many of California wine grape vineyards, especially in San Joaquin and Sacramento Valleys, the variegated leafhopper (VLH) has become the dominant pest since it was first reported there in 1980, replacing the western grape leafhopper (GLH) in importance. While the two species of mymarid wasps of the genus Anagrus, previously known as A. epos, provided more or less satisfactory control of GLH before 1980, parasitism of VLH is lower and does not currently provide sufficient control. Cornerstones of biological control are the proper identification of natural enemy species as well as evaluation of their effectiveness in the field following release. From data we have collected, we believe that several Anagrus species that were imported to California and released in selected vineyards in San Joaquin Valley during 1986-1991 either have not established or perhaps interbred with the local populations. One of the species native to California, A. erythroneurae, is the most abundant natural enemy of both leafhopper species throughout California, whereas lesser numbers (ca 10-20%of all samples) of a yet undescribed species (A. sp. 1) have been recognized from collections made in the Napa, Sonoma, and San Joaquin Valleys. Overall, more than 5,000 specimens of Anagrus were examined and identified during our study including those reared from potential overwintering refuges, i.e., almond, apple, blackberry, prune and wild grape plants. The conclusions resulting from this two-year study and our recommendations for further research and biological control programs are presented.

Aetiology, Epidemiology and Control of Measles

AVF Proj. ID: 9604 / /Cat.: ,
Primary Investigator(s): Douglas Gublerby

Isolations from symptomatic grapevines have consistently yielded several fungi including Cladosporium, Penicillium, Alternaria, Rhizopus, Paecilomyces, Aspergillus, Phomopsis, Eutypa, Botryodiplodia, Scytalidium, Phaeoacremonium as well as others. Several of these fungi are known pathogens including Phomopsis, Eutypa, Botryodiplodia, Scytalidium, Cylindrocarpon, and Phaeoacremonium. Symptom expression is well documented for Eutypa and Botryodiplodia. Little is known about the about Phaeoacremonium, Scytalidium, and Cylindrocarpon and the disease symptoms they cause. Pathogenicity tests using grape seedlings and cuttings has shown Phaeoacremonium spp. to be capable of growing in the xylem of grapevines and results in stunting (of both foliage and root system), leaf distortion and discoloration. As of yet no fruit symptoms have resulted from the inoculations but a method for inducing fruit production on young grape cuttings is being utilized in order to obtain fruit symptoms. Pathogenicity test using Scytalidium and Cylindrocarpon have not resulted in symptom expression and plants continue to be monitored. Biology and epidemiology studies of Phaeoacremonium spp. are underway to determine aspects of life cycle and effects of environmental parameters. Vineyards with a history of measles are being surveyed in Santa Barbara, San Luis Obispo, Monterey and El Dorado counties. Vineyards are being monitored several times throughout the year so a disease curve can be developed. The curve will be plotted against environmental conditions (temperature, RH, etc.) in order to correlate incidence of measles with the environment. Severity of the disease is also being determined by rating each vine for foliar and fruit infection and determine crop loss. All weed species and wood cuttings left in the vineyard are being investigated as potential host and inoculum sources and their possible role in the life cycle of Phaeoacremonium. Several types of media are being tested for selectivity to Phaeoacremonium spp. in order to detect inoculum sources in soil and bark. Media are being amended with chemicals such as Benlate, PCNB, and Rose Bengal which inhibit the growth of competing fungi. Disease control is the primary component of this work. Chemicals are being screened against Phaeoacremonium, Scytalidium, and Cylindrocarpon. Benlate and related compounds are effective against Phaeoacremonium.

Characterization of Grapevine Latent Viruses

AVF Proj. ID: 9610 / /Cat.: ,
Primary Investigator(s): Deborah Golinoby

Work on this project is aimed at three primary objectives: 1) to determine which virus or combination of viruses is responsible for these vineyard failures; 2) to test rootstock selections to determine their relative susceptibility to latent viruses; and 3) to apply new molecular tests to our characterized latent virus selections in hope of developing reliable, fast lab procedures to screen field selections. At the end of our first year of funding, we have made good progress in each area. Testing of latent virus isolates from field selected sites is progressing and useful information is accumulating. Evidence continues to suggest that the CB-100 ELISA test (which reacts with the leafroll isolate now known as French GLRaV-2) is useful in predicting latent virus problems. We have also established that the disease Kober stem grooving (KSG) occurs in the United States and that the ELISA test for GVA seems to detect the causal agent. We are working to optimize both these tests. Grafting experiments with various selected rootstocks and latent virus isolates are underway. Preliminary data was taken from a test grafted last summer, grafting was done for another test, wood was harvested this winter for setting up later tests, and cuttings were propagated for grafting next spring. Finally, good progress is being made in the goal of multiple technology testing of a large panel of latent virus isolates. Ultimately, this work will allow a grower to perform a set of selected tests on a proposed field scion selection, yielding an estimate of the risk of using it before it is grafted, and enabling better decision making when selecting a rootstock. Another valuable result of these studies will be to provide published information correlating new technology with the tests which are currently the regulatory standard. In turn, this will allow the new methodologies to be applied to federal quarantine regulations and grape certification programs. Without this information, the new methodologies will not be available for use in these important areas. Trials of specific latent virus effect on important rootstocks will establish the level of disease impact from specific latent viruses and providing nurserymen and growers with essential information about risk management.