BY Dr. Kendra Baumgartner & Amy E. Warnock, USDA-ARS, Davis, CA

Armillaria root disease occurs in all major grapegrowing regions of California. Armillaria mellea, the causal fungus, infects woody grapevine roots and decomposes the underlying vascular tissue. Armillaria root disease affects vineyards planted on sites that were previously occupied by infected forest trees or orchard trees.

Despite its common name of “oak root fungus,” A. mellea has a broad host range, infecting over 500 species of woody plants.⁵ Native hosts of A. mellea include oaks and other common forest trees such as Douglas-fir, California bay laurel, and madrone.² Armillaria mellea also infects many planted hosts in California, including orchard trees (such as walnut) and woody landscape plants (such as rose).

Attempts at eradication
Some of the most toxic fungicides are only partially effective at delaying infection of grapevines by A. mellea. Pre-plant soil fumigation with methyl bromide either kills A. mellea mycelium in buried tree roots or weakens it, which predisposes the pathogen to attack by antagonistic soilborne fungi.⁴ However, soil fumigants typically do not reach all infected tree roots.

Efforts to eradicate A. mellea from infected grapevines through fungicide/biocide application to the surrounding soil are unlikely to affect the fungus, since it grows beneath root bark.

Cultural control
Results of previous studies demonstrated the potential of a cultural approach called “root collar excavation” for control of Armillaria root disease of grapevines.¹ The technique involves permanent removal of soil from the base of the vine’s trunk, to the depth at which main roots originate (the root collar), in an effort to protect this important part of the root system from vascular tissue decay. Root collar excavation improves yields and growth of infected grapevines, when applied before the fungus reaches the root collar and girdles the vine.

Research on root collar excavation demonstrated that grapevines can tolerate root disease and remain productive if the disease is slowed down early in the infection process. Focusing control efforts on infected grapevines that still produce normal clusters is a cost-effective strategy, given that replants are not immediately productive and are, unfortunately, likely to become infected by A. mellea.³

Biological control
The goal of the research reported here was to evaluate the efficacy of VESTA® (Biologically Integrated Organics, Inc., Sonoma, CA) to improve yield and growth of vines with Armillaria root disease.

It was previously shown that vines with Armillaria root disease have significantly fewer clusters, lower yields, and smaller clusters.¹ In terms of growth, symptomatic vines also have fewer shoots, lower pruning weights, and smaller shoots.¹ Therefore, amelioration of these impacts can be used as measures of the efficacy of VESTA.

Laboratory assays
VESTA is registered as an organic soil inoculant with the California Department of Food & Agriculture. Laboratory assays show that VESTA inhibits A. mellea growth in culture (BBC Laboratories, Tempe, AZ). Viable bacteria isolated from VESTA — Bacillus and Pseudomonas species which were found to inhibit A. mellea growth in culture — may function as antagonists of A. mellea in the field and/or as grapevine growth promoters.

VESTA results from a proprietary fermentation process of several composts that vary in age and raw materials. The product of this fermentation process is passed through a 100-micron filter in order to allow application of the soil inoculant to crops via drip irrigation systems. VESTA is extremely low in plant-available nutrients (0.001% N, 0.001% phosphoric acid, 0.001% soluble potash), but is thought to consist of complex C compounds that may help sustain the microorganisms in VESTA.

Field experiment
An experiment was designed that utilized VESTA as a therapeutic treatment (a product applied to a diseased plant in an attempt to prolong its life). The success of a therapeutic treatment depends on how much vascular tissue in the root system has already been destroyed, in addition to the ability of the treatment to decrease further colonization of healthy tissue. Based on past research on root collar excavation, we anticipated that timing applications early in the infection process might be critical to the success with the soil inoculant.

Observations were focused on vines that showed moderate symptoms of Armillaria root disease (stunted shoots approximately half the length of those of healthy vines, with some chlorotic leaves). Based on past research, the appearance of moderate symptoms signifies that the vine’s root collar is infected by a mycelial fan of A. mellea.³
In years following the appearance of a mycelial fan at the root collar, symptoms become more severe (shorter shoots with more chlorotic leaves, premature defoliation, and berry dessication) as the fungus eats through the underlying cambium and woody root tissue, thereby girdling the base of the trunk. Moderately symptomatic vines are more likely to benefit from a therapeutic treatment than severely symptomatic vines because they still have some capacity for root growth.

Vineyard sites
Research was conducted in two California North Coast vineyards, both with Armillaria root disease, but with different mortality rates. A Napa vineyard was planted in 1997 with dormant benchgrafts of Cabernet Sauvignon (Clone 337) on 110-R rootstock with 6 x 8 foot spacing. A Sonoma vineyard was planted in 1991 with dormant rootings of 3309C and field-grafted in 1992 with Pinot Noir, spaced meter x meter.

Armillaria root disease was diagnosed in the Napa vineyard in 2001. The Sonoma vineyard has a longer history of Armillaria root disease, with the first symptomatic vines having been identified in 1996. Before the start of this study, in 2001 and 2002, the disease severity was calculated by mapping the change in status of every vine in each vineyard. In the Sonoma vineyard, 10% of the vines died from Armillaria root disease. In the Napa vineyard, the mortality rate was only 2%.

Treatment strategy
Experimental treatments were replicated in each vineyard over two years. Each vineyard was divided into sections and, within each section, half of the rows were treated with VESTA (treated rows) and the other half with water (non-treated rows) in 2003 and 2004. There were three replicated sections in the Napa vineyard and two replicated sections in the Sonoma vineyard. Within treated and non-treated rows, 10 healthy and 10 symptomatic grapevines were randomly chosen for data collection, to give a total of four experimental treatments:

• healthy non-treated,
  
• healthy treated,
• symptomatic non-treated,
  
• symptomatic treated.

Symptomatic grapevines were characterized by having shoots approximately 50% the length of those of healthy grapevines. All symptomatic grapevines were diagnosed with Armillaria root disease by examination of their root collars for mycelial fans of A. mellea.

To avoid sampling symptomless grapevines that were infected on deeper parts of their root systems, grapevines immediately adjacent to a dead grapevine or a severely symptomatic grapevine were excluded. No mycelial fans were found at the root collars of healthy grapevines selected for data collection.

The soil inoculant was injected into the irrigation system of treated rows at the following phenological times and rates:

• budbreak (5 gal/acre),
• full bloom (5 gal/acre),
  
• 15% veraison (2 gal/acre),
  
• 85% veraison (2 gal/acre).

Petiole samples and soil samples were collected at full bloom in 2003 and 2004. Soil samples were collected using a hand auger to a depth of 15 cm. Samples were submitted to DANR Analytical Laboratories, Davis, CA, for nutrient composition analyses (for soil: total C, total N, NO3-N, P, K, Zn, pH, CEC; for petioles: total N, P, K, B, Zn).

In 2003 and 2004, on the day of harvest, clusters were counted, harvested, and weighed. During the dormant seasons, prunings from the same grapevines from which yields were collected were weighed. An analysis of variance (ANOVA) was used to determine the effects of vine status (healthy or symptomatic), VESTA (treated or nontreated), year (2003 or 2004), and their interactions on grapevine yield and growth parameters, and petiole and soil nutrients.

Results
Based on analyses of yield and growth parameters, VESTA had positive effects on yield and cluster weights of symptomatic grapevines in the Napa vineyard. Symptomatic treated grapevines had higher yields than symptomatic nontreated grapevines, but these differences were not statistically significant. (Figure I-A).

However, the yield increase observed with VESTA treatment of symptomatic grapevines from 1.56 kg/vine to 2.01 kg/vine, a 22% increase, would likely be considered economically significant, with material costs offset by increased yields.

Symptomatic treated grapevines had higher cluster weights than symptomatic non-treated grapevines, and these differences were statistically significant (Figure I-B). In fact, cluster weights of symptomatic treated grapevines were, statistically, as high as that of healthy grapevines.

In the Napa vineyard, pruning weight increases were not statistically significant (Figure II), but the same relative differences among treatment groups in both study years were observed.

In the Napa vineyard, treated grapevines had significantly higher soil C than non-treated vines (34.45 mg/g dry wt. versus 28.80 mg/g dry wt.). Effects of VESTA on the other soil nutrition parameters (total N, NO3–N, P, K, Zn, pH, CEC) were not significant, nor were effects on petiole nutrients (total N, P, K, B, Zn).

There were no significant effects of the soil inoculant on yield, growth, or nutrition parameters in the Sonoma vineyard. It is possible that symptomatic grapevines in this vineyard are more heavily infected with A. mellea than those of the Napa vineyard, given the longer history of Armillaria root disease. A higher application rate of VESTA may be required for vineyards that have suffered losses for more than five years.

It is also possible that the low yields on all vines in the Sonoma vineyard made it difficult to detect statistically significant changes. Given the extremely low yields among even the healthy grapevines (mean yield for both study years, 0.94 kg/vine), it is difficult to recognize significant yield increases, especially among the symptomatic vines (0.68 kg/vine).

The mechanism by which VESTA affects Armillaria root disease is not known. Bacteria present in VESTA may directly parasitize A. mellea mycelium and/or secrete fungistatic compounds. Increased soil C among treated grapevines suggests that higher microbial populations were encouraged by VESTA, but it is not known whether these microbes were the same species that have been identified from VESTA.

It is important to remember that the symptomatic grapevines in this study have undergone substantial root system destruction over the course of several years prior to VESTA applications. Their capacity has been steadily deteriorating since they were first infected by A. mellea.

Conclusions
In summary, VESTA had no effect on yields of healthy vines, but it did help symptomatic vines in the Napa vineyard with less Armillaria root disease. It increased yields (economically speaking) and cluster weights (statistically speaking) of symptomatic vines.

VESTA appears to be a promising approach for control of Armillaria root disease, as long as vineyards are treated soon after the disease is diagnosed. It is unrealistic to expect VESTA or any therapeutic treatment to fully restore yield and growth to infected vines in only two growing seasons.

Therapeutic treatments are more likely to be successful when used early in the infection process, once symptoms appear in the vineyard.

References

1. Baumgartner, K. 2004. “Root collar excavation for post-infection control of Armillaria root disease of grapevine.” Plant Disease 88:1235–1240.
   
2. Baumgartner, K., D.M. Rizzo. 2001. “Ecology of Armillaria species in mixed-hardwood forests of California.” Plant Disease 85:947–951.
3. Baumgartner, K., D.M. Rizzo. 2002. “Spread of Armillaria Root Disease in a California vineyard.” Amer. Jour. of Enology & Viticulture 53:197–203.
4. Munnecke, D.E., M.J. Kolbezen, W.D. Wilbur, H.D. Ohr. 1981. “Interactions involved in controlling Armillaria mellea.” Plant Disease 65:384–389.
5. Raabe, R.D. 1962. “Host list of the root rot fungus Armillaria mellea.” Hilgardia 33: 25–88.