International alert for the grapevine trunk diseases
Grapevine trunk diseases are considered the most destructive diseases of grapevine of the past three decades and are of rapidly growing concern in all wine producing countries. It is well accepted that almost the 20% of the vineyards are affected by these diseases around the world
Grapevine trunk diseases (GTD) have become a real problem during the last years, causing in the shorter or longer term the death of the vines and also, meaning a substantial economic cost for their replacement.
Grapevine trunk diseases are mainly caused by the Esca and Black Dead Arm (BDA), two typologies of fungi. There are many different types of fungi which develop several diseases like esca, excoriose, eutipiosis, phomopsis, black decay, petri, black foot, fusarium, verticilosis, etc. Each of these microorganisms have different ways to attack the plant (at the trunk level or also at the leaves and shoots), but a common pattern.
The life cycle and epidemiology are very similar for all the fungi known that cause trunk diseases. Fungal spores cause a sectorial or central necrosis joined with vascular discoloration at a wood level that causing a depressive status to the plants: weakened vegetative development or defoliation but sometimes chloroses weaknesses or also, some deformations of their leaves. Finally, the disease produces the slowly death of the vines. Other diseases, like Pierce disease, rugose wood complex or Grapevine leaf roll viruses (GlRaV) caused by bacteria and viruses are also associated to GTD, being responsible of a high rate of mortality in the vineyard (70%).
Generally, these trunk diseases damage above all, old plants but recently, it has been seen that they have become more rapid in their extension and also they start to affect young plants (2-3 years old) or vineyards over 7-years-old.
There are several factors which could explain the recently trunk diseases development, but mainly they could be summarized in the ban on the sodium arsenate (only method known to control the ESCA) from 2001 to 2003 in Europe; the high rate of asymptomatic stocks contaminated in the vineyard and non-appropriated cultural practices.
For instance, there is a real annual increase of the mortality rate from 0.5 to 1 % each year in plots of land where the treatment by the sodium arsenate has been stopped.
Methods of control and mitigation
Since the ban of arsenite sodium, control methods are mainly focused on preventive measures. Preventive or sanitation measures like removing dead/dying vines and pruning dead arms are used in order to reduce the source of spore inoculum originating from within a vineyard. Late pruning in the dormant period is also a recommended cultural practice, because wounds heal faster with high degree-day temperatures.
In order to limit the spread of inoculum, OIV recommended in 2006 (VITI 02/2006 resolution) some preventive measures: ensure good vineyard drainage; check solidity at the grafting area; remove vine stocks and old branches; pruning preferably during the dry period and reduce and protect pruning wounds, among others. However, for grapevine viruses’ control an early detection is the only preventive measure recommended.
Even if vineyard soils constitute the main source of inoculum for grapevine infections (management practices based on soil disinfestation and amendments, plant-based resistance to infection, and prophylactic cultural practices should be investigated) beside from some exceptions (e.g. Fomitiporia spp), fungal diseases and other diseases can spread by trading plant material and thus, they can be introduced in areas where they do not exist before. Some guidelines seem to be expected in this area (ie. Draft OIV Recommendations for Certification and Trading Material of Vine Plants/ VITI-PROTEC 14-565 A and B Et3).
Nowadays, there are not chemical products allowed for controlling trunk diseases because of their toxicity for the wine grower and their residues could be a risk for human health. Further researches in alternative solutions like the use of boron; organic products (acid molecules, biofumigants or plant fortifiers) or bio agents (microorganisms used in biological control; e.g. Trichoderma) are needed.
Future perspectives should go through research also in precision breeding: cultivars and clones are needed for preventing damages; for the genetic improvement of grapevine or their resistances. However, more wide spread and robust evaluations, must be done in the future in order to confirm the utility of cultivars (plants contain lot of genetic elements useful for grapevine) produced by precision breeding, which should be tested within next years.
Moreover, growing conditions and the climate change constitute a huge challenge: there is a direct effect on the fungal development in the vineyard and its externalizing symptoms. The indigenous microflora also, could be very important in order to limit and prevent the development of pathogens and thereby inhibit the onset of symptoms.