A greater threat than Phylloxera?

At the outset, I want to make it very clear why I am writing this article. I cannot claim to be an authority on grapevine trunk diseases (GTDs), although I have attended scientific meetings and correspond regularly with some of the leading scientists in the field. The reason I write this article, and particularly in this publication, is to raise industry awareness of the issue, which myself and others believe is currently lacking. It is my experience that growers in many countries, and even professional viticultural scientists, do not have an adequate understanding of the threat posed by grapevine trunk diseases to the world’s vineyards and to wine supply.

The title is deliberately provocative. Every student of viticulture and wine around the world will know about the story of the American phylloxera insect, and the destruction which it caused in Europe in the 19th and 20th centuries, and its spread to many of the vineyards of the world. I will recall the history of phylloxera here, so it may be seen in some perspective beside that of grapevine trunk diseases. Trunk diseases are caused by a group of fungi; some of these have been known for a long time, others have come to the attention of vine pathologists more recently. Some of the fungi have the ability to spread in vineyards and many of the diseases are currently spread by nurseries. At the moment there is limited control available, which is part of the cause for concern.

Let me also state initially that phylloxera is quick acting in its damage to grapevines. When I studied phylloxera infestation of own-rooted vineyards of New Zealand in the 1980s, we found that the vines would typically show symptoms in the first or second year after infestation of the root system. Depending on the soil conditions and water supply, be it by rainfall or irrigation, the vine would soon show depressed vigour and die within a few years. However, since the reaction was not immediate, it was not always possible to identify grapevines in the early stage of infection.

A similar yet more protracted situation exists for most trunk diseases. Typically they are insidious compared to phylloxera. That is, the symptoms develop slowly over a period of years, although vine death may still occur. Because the reaction to the disease is slower, vine to vine spread is less obvious and growers are typically unaware that a disease is spreading in their vineyards. Trunk disease will kill vines, as does phylloxera, and if left unrecognised, it can spread.

A look back at Phylloxera

Phylloxera is a small, yellow insect which feeds on the roots of grapevines and allows them to be invaded by fungi and bacteria, leading to root death, and subsequently death of the vines. The effects of phylloxera were first noted in France in 1863. Subsequently, it was shown to be an accidental import from America, perhaps along with imported ornamental plant species, which were quite the rage in Europe at the time. Phylloxera was initially found in the vineyards of southern France and had spread to Bordeaux by the 1870s. Man was the principal means of spread, by moving either infected nursery plants or soil. Initially there was debate and confusion among leading scientists of the time, who could not agree on the cause. Initially there were hopes that soil injection of carbon bisulphide, and/or flooding would halt the progress of phylloxera. Eventually, by 1880, the solution of grafting on American species as the rootstock was developed, and is now the standard commercial control mechanism for phylloxera worldwide, and the basis of the present extensive grapevine nursery business. Phylloxera is now widespread around the world, including most of Europe and part of the New World and Asia. Chile and parts of Australia are at present free of the pest.

Interestingly, as vines were grafted to rootstocks which were typically free of virus disease symptoms, there was an increase in virus diseases in vineyards. These diseases have now been largely overcome by the use of certified virus-free mother vines for propagation.

Grapevine trunk diseases

There are three main groups of grapevine trunk diseases, which include three main diseases affecting grapevine wood: Eutypa dieback, Esca decline and Botryosphaeria dieback, which occur in the main vine-growing regions of the world. In Europe until the late 1990s, the most common trunk diseases were Eutypa and Esca, but by the turn of this century a third disease caused by Botryosphaeria was becoming recognised in France and other vine-growing regions. This disease was previously called Black Dead Arm (BDA) in Hungary, and that name will be used here, although it is also commonly called Botryosphaeria canker disease. There are other fungi causing trunk diseases such as Ilyonectra causing Black Foot, which is also a problem with young vines.

There are several fungal species involved in these major diseases: Phaeomoniella chlamydospora and Phaeoacremonium aleophilum are involved in Esca as is the fungus causing wood decay, Fomitiporia mediterranea. Recently there has been a reclassification of the different types of Esca disease. The two fungi Phaeomoniella chlamydospora and Phaeoacremonium aleophilum are also included in the grapevine trunk disease called Petri disease, or Black Goo, which is important in young vine decline. BDL is caused by Botryosphaeria species, 21 of which are associated with grapevine decline causing wood cankers and dieback. Eutypa trunk disease is caused by Eutypa lata. Both Eutypa and the white rot form of Esca are typically associated with older vines and larger pruning wounds, whereas Botryosphaeria can infect one-year-old pruning wounds.

Several important questions need to be asked about these diseases. Firstly, why do they appear to be so recent, and so widespread, especially the Botryosphaeria diseases? The fact that they are new explains why the diseases are neither well understood nor well recognised around the world. Several features point to them being of recent concern. For example, consider California in the 1990s. The development of the so-called ‘Biotype B’ phylloxera laid waste to extensive grapevine plantings on the most important, but phylloxera-susceptible rootstock, AXR1, although that susceptibility was widely known in Europe. There is conjecture as to whether this was a spontaneous development of a new biotype, or perhaps more likely a result of importing field-grown grafted vines from New York when there was a substantial local shortage. There is every likelihood that the phylloxera originally introduced to Europe was from the East coast of the USA, and a different biotype to that which was present in the West, which allowed the wide spread planting of an otherwise phylloxera-susceptible rootstock like AXR1. In any event, the new plantings on rootstocks which were resistant to phylloxera involved problems with a “young vine decline” to be termed Black Goo, which was later found to be Petri disease, as was previously described. In any event, the very conspicuous xylem staining of the rootstock was associated with poor growth and vine death.

This problem and others around the world led to the formation of the International Council on Grapevine Trunk Diseases (www.icgtd.org) which had its first meeting in California in 1998. Meetings of the ICGTD have been held every two years since, the last being in Valencia Spain in 2012.

A final point relates to a recent survey of grapevine trunk diseases in France, published in 2013. The review covered seven regions, 329 vineyards and 12 cultivars. There were big differences between regions, with 54% to 95% of vineyards showing the incidence of GTDs. The proportion of plants infected with trunk disease also varied by region from 3% to 32%. The review stated that GTD are of major concern for the sustainability of certain French vineyards and vine cultivars. Trunk diseases have been described as the “new phylloxera” in France.

Comparing trunk diseases and Phylloxera

How do trunk diseases in the first decade of the 21st century compare with the phylloxera epidemic beginning around 150 years previously? There are several points to consider:

Both phylloxera and GTD are widespread around the world, but trunk diseases probably more so. The leading researcher, Jose Urbez Torres, says that trunk diseases occur wherever vines are grown. Phylloxera does not.
Both phylloxera and GTD spread in vineyards. The size of Phylloxera-affected zones double in size each year, as judged by symptomatic vines. The spread of trunk disease is probably slower. Botryosphaeria spreads by spore infection of winter pruning wounds in wet and humid weather, typically one vine each side in the row is affected and also the downwind row. This is typically slower than for phylloxera.
Both phylloxera and GTD kill vines, though undoubtedly phylloxera acts more quickly. Both phylloxera and GTD have a bigger impact on vines which are already under stress.
The impact of phylloxera is very visual, being quite quick to induce vine death and with conspicuous spread to adjacent vines both within and across rows. This resulted in a panic response in France, beginning 150 years ago. There is not such an awareness of the widespread infestation of trunk disease because it is more insidious. This means it is happening, but escaping attention.
Phylloxera is being controlled by the modern grapevine nursery industry. Trunk disease appears to be spread by the modern grapevine nursery industry, as has been shown in recent studies in most grape-growing countries of the world.
Phylloxera was overcome by locating sources of resistance within the grapevine species. Not unexpectedly, these were American species, the home of phylloxera, classical Darwinism. There is no known source of genetic resistance to GTD.
There are various management procedures to control GTD, including vine surgery, protection of pruning wounds and removal of infected vines. There is an urgent need for more research.

Conclusion

I have no doubt that science will produce solutions to GTD as was done for phylloxera approximately 110 years ago. Perhaps it will happen more quickly, for science is now better developed. However, the task appears more complex, since there are no natural sources of resistance to utilise as was the convenient solution for phylloxera.

I despair for the role of nurseries around the world in spreading GTD. Typically they deny that they are part of the problem, yet that is not my observation in many countries nor from what I read. I invariably come across instances of infected nursery stock being delivered to unsuspecting growers, who in many instances are blamed for poor young vine performance.

There are methods to control GTD in nurseries. The principal problem seems to be contaminated rootstock mother vines, which need to be tested and replanted as happened for the detection of virus problems. There are other procedures in the nursery that also require modification. Hot water treatment is successfully used by some Australian grape vine nurseries without detriment, yet the majority of grape vine nurseries across the world shun this approach claiming it will damage the vines. There is an urgent need for more research and extension on this point.

The GTD problem will be solved, but firstly the problem must be recognised. Sadly, such recognition still seems a long way off.