By: Judit Monis, Ph.D., Plant Health Consultant
Grapevine trunk diseases occur worldwide and may be caused by bacterial or fungal pathogens, or both. This article will focus on trunk disease caused by fungal pathogens. It is important to note that the same fungal pathogens that affect grapevine are capable of infecting other fruit crops and landscape trees. It is my intention to provide information on the different species involved in the different diseases and learn how to prevent new infections in the vineyard.
If you read other articles I wrote or heard me speak, you already know that the only way to avoid diseases in the vineyard is to prevent the introduction of pathogens in the first place. Once established in the vineyard, there is no cure for graft-transmissible diseases and new infections are difficult to control. This means that care must be used when selecting planting material prior to developing a new vineyard block.
Below I describe the most common grapevine trunk diseases caused by fungi. As with viruses and bacteria, fungal pathogens can be found in mixed infections with other fungi, as well as with bacteria and viruses.
Petri Disease, Young Vine Decline, Esca
The disease is caused by Phaeoacremonium and Phaeomoniella species in young vines. In older vines (defined as older than 10 years), the disease is known as Esca. Esca disease can be chronic when vines are infected for a long period of time showing gradual decline symptoms or acute when the vines decline and die within a few days (apoplectic stage). It is not uncommon during the apoplectic stage to see dried up canes carrying grape bunches with raisins.
Bot Canker, Eutypa and Phomopsis Die Back, Other Cankers
Various pathogens can cause canker disease in the vineyard. Bot-canker is caused by different species in the Botryosphaeriaceae family. Eutypa dieback is caused by different species in the Diatrypaceae family. The best characterized and known species is Eutypa lata species. In my lab we characterized Seimatosporium as a canker species, but within the same group others have reported Pestalotipsis and Truncatella to cause cankers in grapevines. Other canker pathogens include Diaporthe (also known as Phomopsis). The canker symptoms observed when sectioning the trunk or cordon of a vine can look similar even when caused by unrelated fungal species, however, the life cycles and mode of infection may be different.
Black Foot Disease
Species of 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 are indistinguishable from Petri disease and the often the decline symptoms can be confused with Pierce’s disease.
Species of Armillaria mellea (Oak root fungus), Verticillium, Phytophthora, and Fusarium are soil born fungal species capable of causing decline and rots in the vineyard.
Disease Prevention and Control (Management)
As mentioned earlier, the best disease control measure is to prevent infection. Unfortunately, our certification programs do not test or exclude the infection of fungal pathogens in propagation material. The implementation of appropriate sanitation measures at the nursery is needed to produce high quality planting grapevine material. You probably heard me say that one infected vine can produce at least 100 -200 vines each year, potentially producing a significant number of infected grafted plants. Because fungal pathogens cannot be eliminated in the vineyard once introduced, it is important to inspect nursery material prior to planting. Since the effect of grapevine fungal pathogens generally will increase with the age of the vineyard growers must adopt management and control measurements as soon as the vines are planted in order to prevent the propagation and the dispersal of pathogens.
When planting a new vineyard, it is important to inspect the quality of the planting material (graft union integrity, lack of streaking or pitting) and plant in well prepared and drained soil, at the correct season. It is important to apply best practices in the vineyard (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.
Management at the vineyard should include trained personnel for pruning. 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, double pruning is recommended. In all cases, after pruning, the fresh wound produced should be protected using fungicides or SafeCoat VitiSeal. The recommendation of pruning as late in the season as possible is related to wound healing (the vine is more active in the spring and will heal faster) as well as most fungal trunk disease pathogens release spores during the rainy season and in the spring time the proportion of spores would have reduced to a minimum. However, wound protection is still 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.
A more drastic disease management practice includes vine re-training (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.
Economic studies performed by 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.
Other methods that have been reported for the management of fungal diseases include mustard greens cover crops and biological control agents such as Trichoderma and Mycorrhizal fungi.
New and more sensitive pathogen detection methods that apply next generation sequencing are now available 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 will 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.