What causes grapevine trunk diseases?
Grapevine trunk diseases can reduce the productive life of a vine and vineyard. We now know more about the organisms associated with trunk diseases in Oregon, as improved diagnostic tools have become available in recent years.
The vine is an ecosystem within itself, containing or supporting endophytes, epiphytes, insects and other invertebrates. Its roots and leaves secrete substances that influence the soil and leaf microbiome. In return, there may be organisms in the immediate environment of the vine that influence it in ways not yet understood.
Since the vine is an ecosystem, it cannot be separated from the greater ecosystem in which it exists. Factors such as soil type, soil pH, cultivar and rootstock type, water stress, heat stress, pruning method, and vineyard management all contribute to the overall health status of the vine. As a result, grapevine trunk disease is not due to a single, or even a couple, organisms. It is the shift away from health to a less sustainable vine (Figure 1), the outcome of an imbalance of both biotic and abiotic (nonliving) factors that allows pathogens to thrive.
Trunk disease should be considered a disease complex. There has been no systematic study of GTD in Oregon, so we have an incomplete picture of which pathogens are present. Presented here is what I learned from grapevine samples submitted to the Oregon State University Plant Clinic for disease diagnosis.
Fungi of the genera Botryosphaeria and Eutypa are big players in trunk diseases in Oregon. They may occur together, often in combination with other pathogens. Members of both genera get their start in a vine on pruning wounds, where spores that land on the cuts germinate and grow into the wood.
Worldwide, there are 26 Botryosphaeria (bot for short) and related fungi that cause GTD, and a similar number of fungi related to Eutypa that cause Eutypa dieback. E. lata is the most serious and common. In the OSU Plant Clinic, we have recovered and identified Botryosphaeria stevensii (=Diplodia mutila), B. dothidea, B. corticola (=D. corticola), B. quercuum, Lasiodiplodia and Neofusicoccum parvum from symptomatic vines. The names in parentheses refer to the imperfect stage of the fungus, where there is no genetic recombination during spore formation. The perfect stage is where genetic information is exchanged during spore generation.
Regarding Eutypa, E. lata is by far the most frequently recovered, although we have also identified from diseased wood E. leptoplaca. This fungus is known to infect other woody plants, such as bigleaf maple. In California, an isolate of E. leptoplaca from bigleaf maple was able to cause disease when inoculated to grapevine, although the extent to which this occurs in nature is probably limited.
How trunk diseases affect the grapevine
Once in the vine, these fungi grow in the xylem, killing it. Many illustrations have focused on wedge-shaped dieback as being typical of Eutypa diseases, but bot fungi may also cause this pattern, and other more irregular-shaped areas of discoloration (Figure 2). In general, whether wedge-shaped or not, dark brown tissue anywhere in the xylem is evidence of some type of trunk disease.
The pattern that shows up in a cross-section of a vine will depend on where the cut is made and how far away it is from the original infection point. Botryosphaeria fungi grow rather rapidly in a vine, and can move up to 8-10 inches in a year. The fungi associated with Eutypa dieback move more slowly. Both fungi, once established, may lead to abnormally short shoots in spring (Figure 1).
Managing the disease
Management of Botryosphaeria and Eutypa dieback is similar and focuses on preventing wound infection before it starts, typically in younger vines. Reduce the amount of spores in your vineyard by removing diseased plant material once identified. Disease-causing fungi reproduce and make spores in diseased wood.
Cutting out the dead wood is only the first step, and not the most important one. Removing the diseased material from the vineyard is essential. This can be done by burning, composting, mulching or burying — all of which will reduce the amount of fungal spores available to initiate new infections. Be sure not to pile diseased wood and prunings next to the vineyard for extended periods.
It is best to cut back infected vines 4-6 inches below the visible canker — the discolored wood. If internal discoloration is present at ground level, then the GTD is throughout the plant and you will need to replace the vine. For vines cut low but still above the graft union, a new, healthy shoot can be trained into position from a retained sucker. Fortunately, transmission of the fungi on pruning shears is not a concern.
Since spore release occurs with wet weather, pruning when the weather is dry is another successful strategy to minimize trunk disease. However, this is not possible in most areas of western Oregon.
Treating pruning cuts with fungicides can be effective when they are applied soon after the cut is made. Recent research in New Zealand has shown this works well when applied as a spray from a tractor, which reduces labor costs significantly.
There are a number of new products on the market based on strengthening plant defenses. These must be applied prior to disease development. The efficacy of these products has not been evaluated in the Pacific Northwest, but elsewhere their ability to control trunk diseases has been minimal.
Over a dozen natural active ingredients have been investigated worldwide as wound treatments, and some appear to have good potential, although their cost may be high. There have been two recent summaries (see Mondello et al. 2018; and Gramaje et al. 2018) of chemical and biocontrol tests. Results are difficult to interpret, as results vary depending on the pathogen involved. Efficacy is measured differently in different studies — one study will measure the ability to recover spores from pruning cuts, while another looks at the development of symptoms over time. However, these are the best and most recent summaries available.
Preventing the disease
Before considering which product is best to use, perhaps we should shift our focus to preventing disease in the first place. Plants under abiotic stress are less able to defend against infection.
Practices that worked in the past won’t work when the summers are drier and hotter than they used to be, and irrigation may be needed in our once dry farmed areas. Pay attention to vines and provide a good environment that will allow young vines to thrive.
I’ve seen young vines with roots that circle around the stem and do not extend farther than a couple of inches. This is due to an auger hole made in wet clay, resulting in shear walls difficult for young vine roots to penetrate; it can also be due to pot-bound nursery stock (J-rooting). Weak, poor quality planting stock just isn’t worth planting — especially when it comes pre-infected with crown gall or other problems!
As mentioned earlier, a grapevine is its own ecosystem. Scientists are investigating the microbial colonizers and inhabitants of a healthy vine to see what “healthy” looks like. Someday, microbial farming may be the way to optimize plant health. In the meantime, disease prevention through good viticulture is the best means to vine sustainability.
Gramaje, D., et al. 2018. Managing Grapevine Trunk Diseases With Respect to Etiology and Epidemiology: Current Strategies and Future Prospects. Plant Disease.
Mondello, V., et al. 2018. Grapevine Trunk Diseases: A Review of Fifteen Years of Trials for Their Control with Chemicals and Biocontrol Agents. Plant Disease.
Sosnowski, M.R. and D.C. Mundy. 2018 (preprint). Pruning Wound Protection Strategies for Simultaneous Control of Eutypa and Botryosphaeria dieback in New Zealand. Plant Disease.