2.4.2.e Grapevine fanleaf virus (GFLV)

Grapevine fanleaf is a very serious disease, caused by viruses of the Nepovirus genus, and is present in vineyards all over the world. In France, its name court-noué comes from one of its symptoms (shortening of the internodes). The name "fanleaf" refers to another symptom (leaf deformation in the shape of a palm leaf or fan).

It infects rootstocks and grape varieties, whether they are Vitis vinifera, other Vitis species or hybrid varieties. The derived symptoms are organ-specific, with a wide variability and range in severity from mild to massive mortality. As the disease is transmitted by soil nematodes, it can persist for a long time after a plot has been polled up, and reinfect the next plantation sooner or later. This virus, and its catastrophic spread which accompanied the post-phylloxera recovery, is at the origin of the implementation of French selection and the resulting certification process for wood and vines.

What damage does GFLV cause in the vineyard?

If the contamination does not originate from the use of infected plant material, GFLV first appears in patches within the vineyard, like other infectious diseases. The nematodes, by successively biting two neighbouring vines whose roots are in close proximity to one another, pass the virus from the diseased vine to the healthy vine.

GFLV is characterized by a large number of symptoms. Their type, distribution and severity are extremely variable and depend on many factors, including:

Varietal sensitivity, which is significant. Some grape varieties, even when strongly affected, display few symptoms and maintain a satisfactory production (such as Ugni blanc, Clairette and Alicante Bouschet). Others are much more likely to display symptoms (such as Muscat à Petits Grains, Chardonnay and Grenache).
The species and/or strain of nepovirus involved, and it's possible interaction with other viruses (synergy)
The age of the strains, probably associated with the infection period (vines contaminated in their youth visibly express more symptoms).
External conditions, e.g. stress, other diseases.
The time of year (symptoms are most visible at the beginning of the growth cycle).
Other factors relating to vigour, rootstock etc.

The main symptoms are:

At the base of the vine: gradual weakening which can be fatal. In the spring, the growth of vegetation can be slow, be delayed, stunted and with a bush-like appearance.
On branches: flattening and abnormal divisions at the nodes or merithalles (fasciation, "witches' brooms"), shortening of internodes, "zigzag" growth, chaotic tendrils, the emergence of numerous secondary buds, double-nodes, etc.
On leaves: sometimes spectacular deformations, vein anomalies (split, absent, anarchic), yellowing (lemon colour in the spring, turning white in the summer), reticulated or diffuse variegation of the foliage.
On inflorescences and bunches: fertility disorders (coulure, millerandage), uneven ripening, reduced yields and lower quality.

What is its vector and what is its biology?

Worldwide, 16 nepoviruses are known to produce the symptoms of this disease, whose vectors are soil nematodes of the Nemathelminthes order or roundworms, often living at substantial depths. 18 vector species have been identified, belonging to three genera: Longidorus, Paralongidorus and Xiphinema.

There are specific associations between nematode vectors and viruses. In France, there are two main types: GFLV (Grapevine Fanleaf Virus), which is found more often, and ArMV (Arabic Mosaic Virus). They produce the same symptoms and are transmitted in the soil by Xiphinema index for GFLV and Xiphinema diversicaudatum for ArMV. The X.index / GFLV pair is by far the most widespread in the world. To feed, the nematode punctures the growing roots with a stylet and transmits the virus if infected.

Nematodes are rarely found in the surface layers of the soil, which are disturbed by cultivation. They are located near the roots, often between 30 and 80 cm (where the root system is densest), but they can be found several meters deep if the root system permits it. They can survive for several years after vines have been pulled up by feeding on unremoved root fragments.
When conditions are unfavourable, nematodes can also go into quiescence and survive for more than five years without feeding, and then return to activity if conditions change. During this period, they may retain their ability to transmit the virus they carry. By themselves, nematodes do not travel more than a few centimetres per year, but the movement of soil contributes more widely to their dissemination (erosion, cultivation, earthworks, etc.).

Extract from Vigne Vin Occitanie.

Are there any preventive measures?

There is no specific curative treatment for GFLV. However, several prophylactic methods can be implemented to limit contamination and the survival of nematodes in the soil:

Uprooting management: when pulling up a plot with GVFL symptoms, great care must be used when removing the roots, as it is possible that some root fragments remain after uprooting. During the resting period, care should also be taken to remove any possible vine regrowth, which may enable the survival of the nematode vectors. Headlands and ditches should be maintained and any possible vine growth should be removed.
Soil rest management: when a plot is contaminated by nematodes, it is strongly recommended to respect a rest period of 7 to 10 years before replanting vines. This resting period can vary depending on the soil type and the degree of contamination. See the planting section.
Weed management: it is possible to introduce plant cover (spontaneous or sown) based on nematicide plants. As an intercrop (present at the same time as a sensitive crop), green manure or as a simple cover crop, certain plant species and varieties produce substances that are harmful to GFLV nematodes (Longidorus and Xiphinema). Examples of such nematicide plants include alfalfa, lupin, oats and marigolds.
Plant material selection: When replanting it is also strongly recommended to use certified healthy plant material. Alain Bouquet's Nemadex rootstock can significantly delay GFLV recontamination. However, it has certain agronomic constraints such as high vigour and poor drought tolerance.
Methods in the process of approval: Premunity, also called cross-protection, is a technique that consists of infecting the vine with a hypo-virulent strain of GFLV. Once infected, the vine is better able to withstand a hyper-virulent strain. This method is currently being approved and would delay new infections, as well as any symptoms.

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