Leafroll virus is not so innocent

A Technical Guide
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Leafroll virus is not so innocent (Part 1)

Glenneis Erasmus

The idea that leafroll virus has no impact on vineyards and that it might even help to ameliorate wine quality is starting to vaporise as mounting evidence is emerging that suggests the opposite. It is true that the virus does not destroy grapevines. It does however affect production and has been associated with crop losses of up to 60% under extreme conditions of infections.
Leafroll also has a definite impact on wine quality, as the virus affects metabolic processes. This in turn leads to late ripening and unbalanced fruit that are inferior to healthy plants in terms of sugar content; colour and wine flavour.

Fig 1 and 2: Leafroll virus has a negative impact on wine quality and can cause a reduction in yields of up to 60%. (Roleen Carstens, ARC Infruitec/Nietvoorbij)

A holistic approach is needed to combat the virus since it can be transmitted through vegetative propagation and grafting with infected material. The virus is not mechanically transmissible, for example by sap on wounds caused by pruning, but vectors - such as mealybugs, which have been identified as the most important vector of leafroll - can be dispersed by various means and cause the virus to spread from one vine or vineyard to another.

Fig 3: Hannes van Rensburg, Chair Winetech Plant Protection committee and Prof Gerhard Pietersen from Citrus Research International recently talked about leafroll virus at a South African Society for Enology and Viticulture open day held at Infruitec-Nietvoorbij in Stellenbosch. (Glenneis Erasmus, Famers Weekly)

Research by Prof Gerhard Pietersen from Citrus Research International has indicated that the disease spreads to low numbers of healthy plants initially, but has a typical infection curve where, in later years, the number of infected plants increases dramatically. The average infection rate until the vineyard is 100% infected is about 5-fold per year with a range of 0,7 to 13- -fold per year. Producers therefore initially only have a season or two to intervene if vines with the disease are detected. Failure to react can cause the virus to spiral out of control and might result in vineyards becoming 100% infected within 5 to 8 years,, Pietersen says.
For example, one vineyard block investigated by Pietersen containing 11 939 plants only had 487 infected vines (an infection representing around 4% of the total vines) in 2001. The vineyard was left untreated resulting in the virus spreading to infect almost half the vines (5115 vines) in only 4 years.
Using only certified virus free rootstock and scion material free from harmful viruses when establishing new vineyards is one of the most important measures to help prevent leafroll virus. Heat treatment and tissue culture of the nucleus plant is extremely effective in destroying the virus in clone material. The problem is that material can become exposed to the disease when planted outside, as is the case with most mother blocks.

Fig 4: It is sometimes difficult to identify leafroll virus on white cultivars. Producers should therefore treat all white vineyards as if they are infected when directing farm traffic. (Roleen Carstens, ARC Infruitec/Nietvoorbij)

Another problem is that plant material usually takes a long time before showing symptoms of a disease. Hence producers might be buying plant material that looks as if it is disease free, only to find that it is sick a few months later. Pietersen is conducting research to try and establish exactly how long this latent period can be as his current studies suggests that it can take longer than a year for infected plant material to show symptoms.
The risk of certified material having the virus is much lower than for uncertified material due to stringent propagation and standards set for this type of plant material, according to Pietersen. One of the ways in which nurseries are trying to maintain virus free status of plant material is by increasing the mother material in areas that are isolated or away from traditional wine producing region.
This practice even though it greatly reduce the risks of infection is however very expensive. "It costs almost R1,33m annually to ensure that nucleus material, around 7000 plants, complies with all the Vine Improvement Association’s specifications to be certified at KWV Vititec. Increasing plant material from nucleus material in isolated or new areas further increases prices as these areas are often far away which greatly increase transportation costs," says Nico Spreeth from KWV Vititec.
Johan Wiese, from Voorgroenberg Nursery, adds that the lower success rate experienced in increasing plant material in untraditional wine producing areas and the increased costs of production is difficult to justify to nurserymen as the market for these expensive plant material is quite limited even though there are more guarantees that the material is free from leafroll virus. In addition to this isolation renders it more difficult and expensive for nurseries or plant breeders to cover their costs by pressing grapes from these blocks. Management, pruning and picking costs are also greatly increased because of the new challenges posed by unfamiliar production regions.

Fig 5 (left): Vines that are affected by leafroll should be completely removed. Neighbouring vines should be treated with systemic insecticides to prevent the disease from spreading to other neighbouring vines in case they might have become infected but are not showing symptoms yet. (Roleen Carstens, ARC Infruitec/Nietvoorbij)
Fig 7 (right): Mealybugs has been identified as the most important vector responsible for spreading leafroll virus. (Roleen Carstens, ARC Infruitec/Nietvoorbij)

Plant breeders are in the process of producing new cultivars through the use of biotechnology to breed plants that are more resistant to leafroll. Producing such cultivars will greatly reduce production costs and allow producers to use more environmentally friendly production techniques, as producers will become less dependent on chemicals for the control of vectors that transfer the disease.
Jan Booysen, Executive Manager of Winetech, however points out that the market, even though demanding more environmentally friendly production is not willing to buy wine produced from GMO vines, currently shows the most promising approach in getting resistant cultivars. He adds that research is currently being done to ensure that South African does not fall behind in the use of this technology, but that it will take years to release such a cultivar due to all the stringent tests such a cultivar would be subjected to before it can be released.
While producers are awaiting the development of a leafroll resistant cultivar they will have to make use of a number of the strategies available to combat the spread of the virus. New vines should always be established in clean vineyards, treated with a systemic insecticide on planting, and monitored to ensure that they are leafroll-free even if it is certified plant material, Pietersen says.
Establishing new, certified high quality (3-star) vines on new land is obviously the safest measure to take, but very few farmers still have the luxury of this option, says Pietersen. Mealybugs can survive on live grapevine material up to 60 centimetres under the soil. If replacing old vineyards, the old vines should therefore be treated with a systemic insecticide before removal and measures should be taken to ensure that the whole vine, with root and all is removed. Research is currently in progress on which herbicides are the most effective in destroying vines.
Pietersen advises producers to use a fallow period of at least one year between the removal and planting of new vines. Research is also done to determine the minimum period to ensure there are no more virus- carrying mealybugs left when new vines are established. Theoretically, it is only necessary to have one generation of mealybugs not exposed to infected vines to create a vector population no longer carrying the virus.

Fig 6: One vineyard block investigated by Pietersen only had 487 infected vines in 2001. The vineyard was left untreated resulting in the virus infecting almost half the vines in the vineyard in only 4 years. (Prof Gerhard Pietersen, CRI)

The new vines should be treated with a systemic insecticide on planting, monitored over the next two seasons and any plant material exhibiting symptoms should be removed with root and all. In later seasons, neighbouring plants should be treated with systemic insecticides to prevent the disease from spreading to other vines in case they might have become infected but are not showing symptoms yet. These vines should also be monitored thereafter annually to establish whether they are also infected - These plants should be removed once it is certain that they have also contracted the virus.
Mealybugs are extremely small and can easily be dispersed by people moving from an infected to an uninfected vineyard. Strategies should therefore be in place to prevent this from happening: Workers should either only be assigned to specific vineyards or they should start working in healthy vineyards before moving to infected vineyards.
Alternatively workers would have to switch clothes before moving between infected and uninfected vineyards - the farm would need a washing facility if it wants to implement this last method of control. Pietersen advises that it is best to use more than one of these strategies. Farm implements, just as clothes, should also be spray cleaned or disinfected after being operated in infested vineyards before entering uninfected vineyards.
The assumption should also be made that white cultivars are all infected with the virus when directing farm traffic, as it is very difficult to distinguish between healthy and infected white cultivars.
Read more on effective vector control in next month's article.
Contact Information: Contact Information: The Pest Management Division of ARC Infruitec Nietvoorbij: (021) 809 3458 or prof Gerhard Pietersen gp@cri.co.za
Glenneis Erasmus (021) 919 8583

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