Investigation of Argentine ant biology and control methods in California vineyards

Argentine ants exacerbate grape and obscure mealybug populations in coastal California vineyards. Available ant controls are either toxic, broad-spectrum pesticide sprays, which disrupt natural enemies and are only moderately effective at controlling ants, or dry proteinaceous baits, which are unattractive to sugar-feeding ants. Over the last 5 years, we have worked to develop a sustainable ant control program, dispensing reduced-risk insecticides in sugar baits. These baits do not disrupt non-target species, nor do they contaminate groundwater (a concern with barrier insecticide sprays). Originally our system, although effective, was labor and material intensive, both of which were prohibitive to its adoption on a large scale by growers.

In 2004, we began testing a bait dispenser to replace previous models. We further updated the design of the dispenser in 2005, and report significant reductions in ant populations in treated blocks, with a corresponding decrease in economic damage to clusters. Subsequently, this bait station was accepted for use by CHEMSAC (US EPA), and can now be deployed in agricultural systems. In 2005, we also advanced registration of a product to fill these stations, one of which should be available no later than 2007. Registration of this product is based on 2 years of data demonstrating its efficacy in vineyard systems.

In 2005, we studied several aspects of the ant bait program in an attempt to decrease the cost of implementation and increase the efficiency of application. We delineated patterns of ant dispersion in two San Luis Obispo County vineyards, which show that during the winter months the ants were concentrated in riparian areas and moved progressively further into the vineyard from winter to harvest-time. This description provides better information to manage the spread of the pest, and suggests that bait stations may be strategically placed at the edges of fields to minimize ant movement into the field. We concluded studies of ant nest phenology, to ensure that baiting precedes and/or coincides with a period of intense brood production. We initiated a study of bait station densities and measured the relationship between the number of bait stations per acre and the level of ant activity. Initial data from temporal studies suggest that early-season baiting is preferable to late-season baiting, with significantly less economic damage to clusters in plots where baiting began in April as opposed to July. We studied Argentine ant foraging behavior and found that initial movement of sugar water baits is limited to the area within 50 feet of a bait station.

These studies provide the background information necessary to answer questions posed by growers regarding bait placement, initiation and duration of baiting, and density of bait stations per acre, allowing us to optimize the efficiency and effectiveness of the ant baiting program.

Control of trunk diseases by protecting new pruning wounds, and by trunk

Our long-term goals are to increase the options and cost-effective measures for control for grapevine cankers/dieback. In our current collaboration with Sutter Home vineyards in Arbuckle CA, we have determined that vines that historically would have been diagnosed with ?Eutypa dieback? are actually primarily infected by pathogenic strains of the fungus Botryosphaeria obtusa. Similar to Eutypa lataB. obtusa, at least primarily, infects through pruning wounds. Last year, we surveyed paints for use as protectants, and selected for those with high elasticity (i.e., are less likely to crack), ?breathability? (i.e., presumably less likely to build up mold under the painted surface), durability for exposure to uv light and rain, and fungal-resistance.

We had excellent results last year in both the laboratory and the field with Duration Exterior Paint (Sherwin-Williams), a ?self-priming? paint; we demonstrated it was non-phytotoxic, and even allows shoot out-growth if it?s accidentally painted over a bud. Moreover, it forms a film on freshly-cut surgical and pruning wounds on grapevines in the field that prevents fungal penetration for our current one-year study period, and we predict, for the remaining life of the vines. The California Department of Pesticide Regulation (CA DPR) indicated that Duration ?would be considered a physical barrier and would not require registration as a pesticide.? A Fusarium lateritium biocontrol formulation and an inferior paint were also included in our trials, but were deemed insufficiently impressive after year one, and have been discontinued.

In addition to selection of Duration for long-term testing, we have developed a pruner that dispenses the paint during pruning. In order to determine the extent to which the Duration paint will reduce spur death and improve vine productivity in comparison to untreated controls, the industry needs a long-term trial. To this end, in collaboration with Sutter Home vineyards, vines with symptoms of dieback (i.e., at least one dead spur) were surgically cut on the trunk in February of 2005. The surgical cuts were either treated with Duration or not, and new cordons were trained; our results show that new growth has been excellent. We have proposed to continue and expand this project with treatment (or not in the untreated controls) of all pruning cuts on these vines with Duration for the next several years until assessments can be made of spur death and vine productivity in aregenerated vineyard.

For part of our project last year, we examined grapevines for insights into the life cycle of B. obtusa. Results that will be useful in developing an efficacious control strategy include the following. We observed relatively few B. obtusa pycnidia (the spore-bearing structures) on the intact vines. However, shoots were asymptomatic but infected, and after prunings were dropped to the ground, pycnidia were produced. Consequently, we propose to further examine whether prunings on the ground are an important source of inoculum. In regenerated one year-old vines, we had shoot infection occur in the spring, and consequently the infection period may be longer than previously assumed. Finally, we developed a new spore trap device for rainstorm-splash disseminated spores.

Grapevine Canker Diseases in California

Grapevine cordon and trunk canker diseases constitute the primary causes of plant mortality and economic losses in the California grape industry. For many years it was thought that the decline and dieback observed in California grape growing regions commonly referred as Eutypa Dieback was due only to Eutypa lata. However, in previous studies, we isolated several other Diatrypaceae species as well as various Botryosphaeria spp from cankers found in the wood of declining grapevine. Fields surveys conducted in California since 2003 have showed the major significance of Botryosphaeria spp in canker disease of grapevine.

Surveys extended during 2005 have allowed our laboratory to obtain comprehensive identification and distribution of nine distinct Botryosphaeria spp associated with grapevine cankers in California. Also, Botryosphaeria species appeared as the main group of fungi isolated from wedge shape cankers in many California grapevine growing areas. Botryosphaeria spp were further distinguished based on specific morphological and genetic characteristics. Species concepts and identification were confirmed from phylogenetic analyses of the ITS, beta-tubulin and elongation-factor genes. Both morphological and molecular analyses have allowed our laboratory to build up tools for diagnosis and separation of the different Botryosphaeria species isolated from grapevine cankers in California. Study of the effect of temperature on colony growth revealed different optimal temperatures among Botryosphaeria species suggesting the specific adaptation and distribution of these different Botryosphaeria species to the various wine regions of California.

Study of cankers occurring in native plant species in the vicinity of vineyards were examined as to offer additional models of canker development to enlighten some of the major aspects of canker formation in grapevine. Following this type of investigation we were able to identify new alternate host plants and putative sources of inoculum for these major grapevine pathogens. Also, our observations have supported the idea that wood cankers in grapevine as well as in native trees are results of the activity of specific fungal association rather than a single fungal species as previously believed. AFLP (Amplified fragment-length polymorphism) is being applied on our assorted collection of E. lata isolates from California gathered in the past 5 years. Preliminary results did not detect occurrence of subpopulations of E. lata with regard to the geographical and ecological (host specificity) distribution of our isolates. Similarly, sequencing of the RPB2 gene for E. lata confirmed our previously proposed delineation of the species within the wide range of ecologically and geographically diverse isolate collection.

Results of pathogenicity tests showed that all Botryosphaeria species found in California were pathogenic to grapevines. However, pathogen virulence appeared to differ depending on the species. Boron-based chemicals have been shown to be effective to control E. lata when applied on pruning wounds. However, our results indicated that pruning wounds were susceptible to several fungal pathogens responsible for branch dieback of grapevine, and we also recovered several of these fungi in concert form wood cankers. Therefore, we are now testing the ability of boron-based material in comparison to other fungicides to control several wood decay fungi of grapes including E. lata, Botryosphaeria rhodina, B. dothidea, B. obtusa, B. sarmentorum, Phaeomoniella chlamydospora, Phaeoacremonium aleophilum and Pleurostomophora richardsiae

Epidemiology and control of trunk canker in grapevines in the Sacramento valley

An automated system was developed and tested for delivery of thick latex paint to a pruning wound at the time of pruning. The system works by coating the cutting blade of a pruning shear with fluid and then fluid transfer from the blade to the pruning cut. There are several major components to the system including a fluid pump, a solenoid valve actuated by a proximity sensor on the pruning shears, and pruning shears modified to allow fluid to be delivered to the cutting blade. The pruning shears can be manual shears or electronic shears. Variables tested in the proposal were blade type, fluid viscosity and fluid flow rate. While the system delivers fluid effectively to the blade, at fluid flow rates greater than 1 ml per pruning cut, fluid tends to leak from the shears. We believe the shears can be modified to prevent leaking.

The pathogenicity of Botryosphaeria obtusa and Eutypa lata isolates from symptomatic cankers was assessed using two-year old vines. Vines were inoculated with mycelia and the resulting lesion length measured after one month of incubation. The lesion lengths for B. obtusa isolates and E. lata isolates were 50.7 mm and 36 mm, respectively. Both were significantly different from the control, and indeed the data indicate that B. obtusacolonizes grapevine tissue more rapidly than E. lata. Koch?s postulates were completed by recovering the inoculated pathogen from the margin of the symptomatic tissue. The data indicate that B. obtusa is a pathogen of grapevine.

Trunk damage associated with mechanical harvesters is a potential infection site for vine pathogens. Of the 36 trunks examined, 33 had damage associated with mechanical harvesters. Twenty-eight of the vines with this type of damage had B. obtusa isolated from cankers associated with the damage. We examined the asymptomatic tissue at varying vertical distances from the canker margin to determine if B. obtusa was the pathogen at the advancing front of fungal colonization and if there were other pathogens associated with the canker. However, the prevalence of other wounds, particularly from shoot removal from the trunks, made it difficult to determine how far B. obtusa could be detected from the canker margin in asymptomatic tissue, and if sampling location biased pathogen diagnostics.

Effects of Atmospheric & Soil Moisture and Fungicide Physical Mode of Action on the Development of Botrytis Bunch Rot

Latent infections of berries were initiated at late bloom, then fungicides were applied at veraison before infections became active. Some clusters were treated a second time, 15 days later. A single application of Scala or Vangard provided almost complete control of latent infections established 60 days earlier at bloom. Elevate and Rovral were nearly as effective, but allowed a few latent infections to become active within clusters. Flint, Pristine, and Endura provided significantly less control. Following the second application, Scala, Vangard, and Elevate further reduced latent infection frequency to nil, nil, and 0.5%, respectively, but other materials were not similarly affected.

To assess protection of internal berry tissues exposed after injury or cracking, clusters were sprayed through veraison and injected with Botrytis spores2 or 3 weeks later. Scala, Vangard, Rovral, and Elevate provided excellent control, with 96 to 100%of the berries symptomless or showing only limited necrosis around the injection point. Pristine was comparably effective in preventing necrosis, but significantly less effective in limiting sporulation from infections that did occur. Flint and Endura provided the least control, with approximately two-thirds of inoculated berries necrotic and sporulating; sporulation was less intense with Flint.

Fungicides applied 1 day before flowers opened provided no control of infections initiated the next day. However, Elevate, Pristine, and Flint reduced the colonization of cluster debris by 73 to 93%; infected debris within clusters often initiates disease as berries ripen. When inoculated 8 days post-treatment, Elevate and Flint provided significant protective control of berry and peduncle infections; these materials and Vangard also significantly reduced colonization of cluster debris. In contrast, all materials provided excellent post-infection control when applied 3 days after inoculation at either early or late bloom; all materials also provided virtually complete control when berries were sprayed at veraison and Botrytis spores were applied to the surface 2 weeks later.

Individual Pinot noir berries were inoculated at veraison. When symptoms appeared about 1 week later, vines were assigned to one of the three soil moisture regimes: (i) High (soil maintained at ‘field capacity’, all evapotranspired water replenished thrice weekly); (b) Low (vines watered only to alleviate stress when volumetric water content fell below 12%); and (c) Medium (50% of evapotranspired water replaced thrice weekly). Relative to clusters in the ?Low? treatment, disease spread was approximately twice as great in the ?Medium? regime and three times as great in the High.

Bottom line:

  • All fungicides provided excellent protective control against late season berry infections. However, only Flint and Elevate provided consistent protective control of infections initiated at bloom.
  • Scala, Vangard, Elevate and Rovral provided excellent suppression/eradication of latent infections established at bloom when applied 60 days later at veraison; this activity was further improved by a second application 15 days later. These materials also provided excellent protection of internal berry tissues. Flint, Pristine, and Endura were significantly less effective in both manners, but all materials provided excellent post-infection activity at bloom.
  • Preharvest disease spread is more severe in well-watered vines than in those growing in dry soils.

Investigation of the Grape Mealybug Complex and its Natural Enemies to improve biological control

Mealybug (grape, obscure, longtailed and vine mealybugs) pest densities and economic
damage has increased in many Central and North Coast vineyards. Our previous research
showed that Argentine ant populations increased mealybug pest problems and that, in the
absence of foraging ants, natural enemies can often control mealybug populations.
Unfortunately, most of the insecticides that effectively control ants are more toxic than
those used to control mealybugs. We have therefore investigated the use of less toxic ant
controls suitable for IPM systems. In 2003, we demonstrated that dilute concentrations of
insecticides delivered as liquid sucrose baits are a suitable alternative to broad-spectrum
sprays for Argentine ant control. In 2004, we worked towards development of a
commercial program, which we hope to have available in 2007. First, two commercially
available ant baits (imidacloprid in a sugar-water bait and spinosad in a granular protein
bait) were evaluated in field trials. We found a significant reduction in ant density and
mealybug damage with the imidacloprid treatment while the granular spinosad bait did
not impact ant populations. Second, we designed and tested bait stations that hold 10
times more bait than previous models, and can remain in the field up to 6 months without
maintenance (whereas previous models required cleaning and refilling every 2 to 3
weeks). A larger bait station will be essential for lower maintenance throughout the
season. In this first year?s study with these large stations, we did not find a significant
difference in ant densities between treatments. We believe that the high ant densities
encountered required more than one year of bait treatment or an initially higher density of
bait stations. Therefore, ant-bait stations will probably be used to maintain low
populations rather than reduce already existing and large ant populations. We also
investigated Argentine ant foraging distance and ant biology to better determine bait
station dispersion in the vineyard and timing during the season. Results show that most
bait movement is limited to the vines in the immediate vicinity of a bait station, although
Argentine ants showed the ability to carry bait over 200 feet from a bait station and
moved both down the vine row as well as across rows. Therefore, we can reduce the
number of stations per acre, which is also essential in developing an economical program.
Results from the ant biology study highlight the importance of maintaining stations in the
field in early spring and post-harvest. Ant foraging activity is greatest at the stations
during these times of year, coinciding with an increase in egg-laying in spring and
absence of a more attractive food source after the grapes are harvested. In 2005, we will
conclude many of these studies in order to produce guidelines on bait station design,
placement in the vineyard and seasonal timing.

Can Grape Phylloxera Activity Damage Resistant Rootstocks?

This study had two primary objectives; the first was to gather evidence of damage to
resistant rootstocks and determine cause. In order to do so, we identified problematical
vineyard blocks and ascertained moderate phylloxera activity. Cane growth was
depressed and no other likely causes of vine damage were obvious. We selected four
vineyard blocks to investigate in detail, two on Teleki 5C rootstock and two on 101-14
Mgt. Nodosities counts on feeder roots were too variable for population estimates so we
used 2-liter soil emergence traps placed on the berm adjacent vine trunks to index
phylloxera timing and abundance. Populations were highly variable through the summer
but not extremely high. We found no phylloxera on mature storage roots, i.e.
tuberosities. Nematode counts were not well associated with damage. However, fungal
virulence in soil as determined by excised root bioassays was. We documented vine
damage by pruning weights in winter and found strong differences between the damaged
vines and undamaged.
The second object was to determine the mechanism of the field damage. We
hypothesized three possible mechanisms for phylloxera to damage resistant rootstocks.
Our first hypothesis was that a new phenotype might aggressively feed on rootstock
storage roots (i.e. tuberosities). This mechanism requires numerous vine-damaging
tuberosities. We searched each site but found none disproving this hypothesis.
However, our finding of nodosities suggested some change in the phylloxera. We used a
bioassay to test various rootstocks as well as V. vinifera collecting data on population
increase. Phylloxera collected from two 101-14 Mgt sites were virulent on young feeder
roots of a number of strongly resistant rootstocks showing greater virulence than the
common ?biotype A? and ?biotype B.? This suggests that these insects were changed
from previous forms in Napa and Sonoma Counties. Twelve microsatellite primers were
used to quantify genetic strains.
Our second hypothesis was that above ground damage may be due to very highly infested
feeder roots (i.e. nodosities). Populations of phylloxera were not high enough to support
this hypothesis.
Our third hypothesis was that Phylloxera normally growing on immature roots may
wander to storage roots in search of new feeding sites. Whether they successfully
establish or not, they may spread secondary pathogens by their probing (i.e. tasting)
activity. This mechanism suggests that probing of phylloxera spreads fungal infections
which severely damages the root system and is responsible for the above ground damage
seen. Presence of phylloxera nodosities as a source of the wandering insects and the high
fungal virulence observed at damaged sites supports this hypothesis. We did enclosure
experiments with excised roots in the laboratory and intact roots in vineyards to attempt
to disprove this possibility. Roots were treated with non-virulent eggs and virulent fungi
to determine whether fungal spread by probing occurred. The results suggested that
probing occurred, though at a low rate, and therefore we could not disprove this
Continued research is needed to determine that fungal necrosis is associated with the
observed damage and that probing is the likely cause. Additionally, we need to ascertain
whether other sites are involved and the spread of the problem.

Influence of Berry Shrivel on Mineral Nutrition on Cabernet Sauvignon

Berry shrivel is a disorder that becomes apparent between veraison and harvest. Berries on affected clusters become flaccid while the rachis appears to be healthy. This is unlike waterberry which is known in other grape growing regions as bunch stem necrosis, stalk necrosis, shanking and other terms related to necrosis of the rachis. Symptoms of berry shrivel do not include necrosis of the rachis. In affected bunches, all portions of the rachis, including the pedicles, usually appear normal throughout the season. Similar to waterberry, berries in clusters with berry shrivel do not ripen normally. They are not fully colored and the juice tastes sour and may have off flavors. The fruit is unacceptable to wineries and is often selectively removed prior to harvest. The cause of berry shrivel is unknown at this time.

In 2003, berry shrivel developed in two of the three vineyards monitored for this project. At one site, soluble solids were reduced at veraison, onset and harvest in both symptomatic and non-symptomatic clusters from affected vines when compared to clusters from non-symptomatic vines. Thus, even at veraison, prior to the onset of visual symptoms, each type of cluster from affected vines had reduced Brix when compared to clusters from unaffected vines. In addition, at harvest titratable acidity, malic and tartaric acids were greater in juice from symptomatic and non-symptomatic clusters on affected vines when compared to clusters on non-symptomatic vines.

At the second site, soluble solids were reduced in symptomatic clusters from affected vines only at symptom onset and later. Thus, Brix found on non-symptomatic clusters taken from affected vines was not significantly different than brix found on clusters from unaffected vines. At harvest, only juice from symptomatic clusters on affected vines had greater titratable acidity, malic and tartaric acids.

At each site, the ratio K/(Ca+Mg) tended to be lower in the rachis tissue of vines with berry shrivel at onset of symptoms and at harvest. This was due to potassium tending to be lower in affected vines at one site and calcium and magnesium tending to be higher in affected vines at a second site.

There was no consistent trend for differences in total percent nitrogen or ammonium-nitrogen in the rachis tissue of symptomatic and non-symptomatic clusters by sample date or site. Petioles collected at bloom, veraison, onset and harvest at both sites did not indicate any differences in total N, nitrate-N or ammonium-N between symptomatic and non-symptomatic vines in 2003.

At one site, leaf water potentials were measured at veraison and shortly after onset of symptoms. Vines that ultimately showed berry shrivel had higher leaf water potentials (less stressed) than non-symptomatic vines both at veraison and onset of symptoms. Leaf water potential measurements taken at the second site were not as clearly defined by vine symptom.

Studies in the Biology and Control of Armillaria Root Disease of Grapevines

Armillaria root disease (also known as oak root fungus) is a chronic problem on grapevines and has been known in California since at least the 1880’s (Gardner & Raabe, 1963). Despite the long history of this disease on grapevines, published research on disease progression in vineyards and control treatments for grapevines is scant. The purpose of our research is to gain an understanding of how oak root fungus infects grapevines and spreads within vineyards with the ultimate goal of developing a reliable management plan for control of Armillaria root disease. Armillaria mellea does not enter vineyards by wind-blown spores. Inoculum is present belowground in the form of infected roots. Removing all roots from the soil would likely ensure permanent control of root disease, but this is a difficult, and unrealistic, task. Pre-plant chemical treatments, such as soil-fumigation with methyl-bromide or sodium tetrathiocarbonate (formulated as Enzone, Entek), are temporarily effective; they often succeed in killing some of the inoculum. Unless fumigant reaches all underground inoculum, permanent control will not be achieved. Based on the lack of effective chemical controls and the frequency of A. mellea on the roots of native hosts, resistant rootstocks may provide long-term control of Armillaria root disease in vineyards. We have two screening experiments in progress that include 20 different rootstocks. Rootstocks that experience less mortality, such as 11 OR and Freedom, tend to exhibit defense responses, such as formation of callus tissue, or escape infection altogether more frequently than rootstocks that experience more mortality, such as Riparia Gloire or 039-16. Although final results for both experiments will not be gathered until Fall 2000, preliminary data show that some rootstocks are more tolerant of infection than others. Tolerance of infection may achieve control of root disease by decreasing the rate at which the fungus spreads on individual root systems which, in turn, allows infected vines to produce new, healthy roots in order to provide their shoots with an adequate supply of water. Decreasing growth of existing infections also provides less “food” for the fungus and, therefor, less energy to cause new infections. Our research on the epidemiology of Armillaria root disease in a Sonoma County vineyard over two growing seasons has provided valuable information on symptom development, replant success, and rate of spread of existing disease centers (clusters of dead/dying vines) in a meter-by-meter planting. In terms of symptoms, we found that most vines that succumbed to root disease developed symptoms in late August and died a few weeks later. However, 88%of the vines we observed with symptoms in 1998 are still alive, suggesting that appearance of symptoms is rarely followed by immediate death. The large replanting effort that was undertaken in this vineyard in 1995 is still promising; of the 191 replants, only 9 have died. Existing disease centers are spreading; of the 54 vines that died in 1999, 28 neighbored vines that died in 1998. Isolates of A. mellea collected from new disease centers were identified as the same fungal individual identified in existing centers.

Evaluation of Management Options For Organic Winegrape Growers

Spider mite presence in the Chardonnay vineyard (Fetzer Vineyards, Mendocino County) first became noticeable in early June 1998. At that time, the plots were delineated and the mite sampling protocol was initiated. Data for an array of viticultural parameters were also collected during 1998. Vine water status was assessed periodically from June through September using porometer readings for stomatal conductance and transpiration readings based upon three leaves per plot. In addition, petiole samples and berry samples were taken, with yield data being collected in early October. Regression analysis was used to determine significance of correlation between mite population and factors such as stomatal conductance, transpiration rate, sugar content, and yield. Comparison of mite infestation and stomatal conductance for 1998 revealed that in situations when vines had heavy mite pressure, stomatal conductance tended to be lower, and vice versa. This inverse relationship between mites and stomatal conductance was significant at the 0.01 level. Linear regression of mite infestation and transpiration rate again indicated a clear inverse relationship between mites and transpiration, with a significance levels of P = 0.021 for 1998. Comparison of mite infestation with sugar content of berries also indicated a significant inverse relationship during 1998, with P = 0.02. Unusual weather conditions (i.e., El Nino) in 1998 appeared to delay maturity throughout the crop. Linear regression of mite infestation and yield for 1998 indicated that the inverse relationship was highly significant (P < 0.01), although no significant relationship for these parameters was delineated (P > 0.05) in 1997. In conclusion, the study conducted at Fetzer Vineyards has clearly demonstrated that mite populations [i.e., Willamette mite (Eotetranychus willamettei)] may be negatively correlated with such viticulture parameters as yield, sugar content, stomatal conductance, and transpiration rate.