By: Judit Monis, Ph.D., Vineyard Health Consultant
Grapevine trunk diseases occur worldwide and can be caused by bacterial or fungal pathogens, and sometimes a combination of both. Pathogenic bacterial and fungal pathogens can be found colonizing the vineyard soil. It is important to note that important trunk disease fungal pathogens not only affect grapevines, but also cause disease in landscape and fruit trees. Grapevine stock can be infected with important pathogens which makes it important to screen nursery material for their presence prior to planting.
Below I describe the most common grapevine trunk diseases caused by bacteria and fungi. As with viruses, bacterial and fungal pathogens can be found in mixed infections (even viruses can be present), exacerbating the problem in a vineyard.
Crown Gall
The disease is caused by the tumor-producing bacterial species, Agrobacterium vitis. The bacteria penetrate the vines through mechanical injuries caused physical damage caused during vineyard operations or by freezing temperatures. The galls are generally visible at the crown area of the plant but can also be found in the upper portion of the vines and at the graft union of nursery produced vines. The bacterial-induced galls cause a reduction of the flow of water and nutrients that eventually cause vine decline and death. Although the disease occurs more frequently in the Eastern and Mid-Western United States vineyards, I have observed vineyards severely affected by A. vitis in Californian vineyards. The best practice to avoid the infection of this bacteria is to plant material from vineyards free of A. vitis. There are diagnostic tools for the detection of pathogenic (tumor-inducing) strains of A. vitis. However, often times the tests may yield false negative results.
Petri Disease, Young Vine Decline, Esca
The disease caused by Cadophora,
Phaeoacremonium and Phaeomoniella species in young vines is known as young vine decline. In older vines, the disease caused by the same fungal pathogens is known as Esca. The disease is chronic when vines express a gradual decline of symptoms over time, or acute when the vines decline and die within a few days. These acute symptoms are known as the apoplectic stage of the disease. It is not uncommon during the apoplectic stage of the disease to see dead vines carrying mummified grape bunches.
Bot Canker, Eutypa, Phomopsis Die Back, and Other Cankers
Various pathogens can cause canker diseases in the vineyard. Bot-canker or dead arm disease is caused by different species in the Botryosphaeriaceae family. The most severe Bot-canker species is Lasidiplodia theobromae, while weaker symptoms are caused by Diplodia spp. Eutypa dieback is caused by different species in the Diatrypaceae family. The best characterized and known species is Eutypa lata, but species of Criptovalsa, Diatrypella, and Eutypella can also cause canker disease in grapevines. In my lab we characterized Seimatosporium species as a fungal pathogen that causes decline and cankers in grapevines, but within the same fungal group others have reported Pestalotoipsis and Truncatella to cause disease in grapevines. Another canker pathogen includes Diaporthe (also known as Phomopsis). The canker symptoms observed in the sections of affected cordons or trunks in grapevines may appear to be similar but caused by unrelated fungal species, however, the life cycles and mode of infection may be different.
Black Foot
Species of Campylocarpon, Cylindrocladiella, Dactylonectria, and Ilyonectria (previously known as Cylindrocarpon spp.) are the causal agents of this complex disease. These fungi are soil-born and most active on compact soils with poor drainage. Symptoms above ground can be indistinguishable from young vine/ Esca disease described above. Additionally, the decline symptoms can be confused with Pierce’s disease, caused by Xylella fastidiosa, a bacterial pathogen.
Sudden Vine Collapse (previously known as Mystery collapse)
A couple years ago, Lodi growers in California reported a syndrome in which their vines collapse and die within a short period of time. In 2011, while working at STA, we tested vines with similar symptoms, not just from Lodi, but from California’s Central Valley, and Central Coast vineyards. We detected a combination of fungal pathogens (not always the same usual suspects) and viruses, namely Grapevine leafroll associated -3 (GLRaV-3) and the Vitivirus Grapevine virus F (GVF). Last year, researchers at the University of California at Davis with fungal expertise (Dr. Akif Eskalen) and viral expertise (Dr. Maher Al Rwahnih) analyzed symptomatic vines with this syndrome. The samples were subjected to high throughput sequencing for the discovery of novel viruses and to fungal culture diagnostics. The results were similar to those found in my laboratory: various fungal pathogens (not consistent in every sample), GLRaV-3, and Vitiviruses were detected in the collapsed vines.
Other Diseases
Species of Armillaria mellea (Oak root fungus), Phytophthora, and Verticillium are soil-born fungal species capable of causing decline and rots in the vineyard. Just as described above for black foot disease, these pathogens strive in compact soils with poor drainage.
Disease Management and Control
The best disease management and control measure I recommend is to prevent the introduction of pathogens in the vineyard. None of the US-certification programs exclude trunk disease pathogens. Therefore, propagation material is most likely infected with A. vitis and various fungal pathogens. It is encouraging to learn that work at Marc Fuchs laboratory at Cornell University has shown that it is possible to eliminate A. vitis from vines using the standard meristem tissue culture technique.
The availability of clean planting material (tested to be free of A. vitis) are most important in areas that are prone to freezing such as the North East and Mid-Western United States vineyards.
The implementation of appropriate sanitation measures at the nursery is most needed to produce high quality planting grapevine material. It is known that one infected vine can produce between 100 -200 vines each year, potentially producing a significant number of infected grafted plants. The use of hot water treatment (HWT) for 30 minutes at 50C (122F) at the nursery has shown a reduction of fungal pathogens in propagated vines. However, there are mix reports on the effect of the HWT on bud mortality. Reports in warmer winegrowing regions (e.g., Spain) have shown a lower effect on bud mortality compared to HWT in cool climate regions (e.g., Australia). Because fungal pathogens cannot be eliminated in the vineyard once introduced, it is important to learn and apply the best management practices available.
When planting a new vineyard, it is important to inspect the quality of the planting material (graft union integrity, lack of galling, streaking or pitting) and plant in well prepared and drained soil, at the correct season. The best practices in the vineyard must be applied (i.e., enough water, nutrients, etc.) as many of the fungal pathogens are endophytic (can live in the vine without causing damage) but can become pathogenic during stress situations.
It is known that the effect of grapevine fungal pathogens increases as the vineyard ages (the fungal pathogen population build up over time). Therefore, growers must adopt management and control measurements as soon as the vines are planted in order to prevent and minimize the propagation and dispersal of fungal pathogens.
Management at the vineyard should include expertly trained personnel for pruning activities. In California where the rainy season coincides with the pruning season it is recommended to prune as late as possible. If the vineyards are large, the double pruning method can be applied. This consists in the mechanical pre-pruning of vines, leaving canes of 1-2 feet long. In the spring or late winter, the pruning is completed by leaving the desired final number of buds per spur. In all cases, after pruning, the pruning waste must be removed from the vineyard as soon as possible. The freshly produced wounds should be protected using fungicides or SafeCoat VitiSeal.
The recommendation of pruning as late in the season as possible is related to the healing of the wounds. Since the vine is more active in the spring, it is expected that healing will occur faster. Another reason is that most fungal trunk disease pathogens release spores during the rainy season.
Therefore, by the end of the winter or early spring, the proportion of spores is expected to have been reduced to a minimum (in areas with predominantly winter precipitations).
However, wound protection will still be required because fresh wounds are more susceptible to infection and can remain susceptible for long periods of time. Things to avoid during pruning are: producing large wounds, cutting near the trunk, pruning after long periods of rain, and leaving vine residues in the vineyard floor. It is also important to respect the flow of sap, which is accomplished by cutting always on the same side of the vine.
Economic studies performed by Dr. Kendra Baumgartner and colleagues (USDA in UC Davis, California) has shown that preventative methods (late pruning, double pruning, and pruning wound protectants) are sustainable only if applied before symptoms appear in the vineyard. Adopting these methods in vines that are 10 years old or older will not recover the cost of investment.
A more drastic disease management practice includes vine re-training also known as remedial surgery. The procedure consists of training a new shoot from the base of the trunk to replace the old decayed vine trunk or cordons. The technique can help gain some years of production but will not cure the vines from the disease as likely the pathogens are systemically established in the vine. In areas with winter freezing temperatures, it is recommended to grow more than one trunk per vine.
If one of the trunks is compromised by disease, others are available to continue with the vine’s productive life. Keep in mind that the pathogenic fungi are systemic in the vine, and as mentioned earlier this method can buy some time before the vine declines and dies.
When replacing vines, the grower must understand that the A. vitis and fungal pathogens are able to survive in dead portions of the roots, therefore new vines that are planted (even if free of bacterial or fungal pathogens) can become infected over time if vine roots are not completely removed from the vineyard.
Other methods that have been reported for the management of fungal diseases include planting mustard (Sinapsis alba) plants as cover crops that act as a biofumigant and biological control agents such as Trichoderma species, and mycorrhizal fungi. In areas prone to crown gall infection, I have observed growers produce soil mounds to protect the trunk from freezing.
New and more sensitive pathogen detection methods that apply next generation sequencing (also known as high throughput sequencing) are now available commercially for the detection at the species level of microorganisms in plants and soil. It is my hope that in the near future, these methods will help reduce the infection levels of planting material and consequently translate into healthier vineyards.
Judit Monis, Ph.D. is a California-based plant health consultant, provides specialized services to help growers, vineyard managers, and nursery personnel avoid the propagation and transmission of disease caused by bacteria, fungi, and viruses in their vineyard blocks. Judit is fluent in Spanish and is available to consult in other important wine grape growing regions of the word. Due to COVID 19 Pandemic, Judit is available to perform virtual vineyard visits. Please visit juditmonis.com for information or contact juditmonis@yahoo.com to request a consulting session.
