In Part II Nico van Rensburg, secretary of the VIA, answers questions about possible reasons for reinfection of newly planted vines and steps producers can take to ensure that their plant material remains “clean” for as long as possible.
Why are there so many red vines (virus vines) in my young vineyard despite having planted certified material
Red leaves in a vineyard are not always due to Leafroll infection. Consequently experts should first ascertain whether there might not be other causes such as ringulation or potassium deficits. Leafroll symptoms might still occur due to reinfections by mealybug which could take place in the mother units, nurseries and the vineyard. Moreover the ELISA tests used in the case of 2 star and 3 star plant material have a threshold value, just like any other test, which means that a positive reading will only be obtained above a certain level of virus concentration and low concentrations cannot be detected. In the case of 1 star plant material the mother units are inspected visually only and the concentration of the virus particles may be so low that no symptoms are displayed.
Is it always the farmer’s fault if vines die or if virus symptoms appear
No. Vines may die and develop virus symptoms after planting due to several factors. Low concentration of viruses and latent viruses can start accumulating in the vines after planting and cause symptoms especially in vineyards that experience stress conditions. In the case of Shiraz disease, where the causative organism is not yet known, vines can die within two to three years.
Drying out is the main reason why young vines die. Consequently the handling of the nursery vines is very important after lifting and transportation to cold storage and subsequent care after planting of the vineyard. Other pathogens such as fungi, bacteria and nematodes could also make the establishment of young vines more difficult. Consequently warm water treatment is strongly recommended to limit these pathogens to the minimum.
What can I do to prevent the reinfection of new vines on old vineyard soils, e.g. how long should the soil be allowed to rest
Reinfection can be limited by removing as many roots as possible from the previous vines during soil preparation and leaving the soil to rest for at least one year by sowing and ploughing a cover crop into the soil. A practice that is increasingly common is to give the soil an application of a systemic insecticide a year before the vineyard is due to be uprooted, to try and eliminate any mealybug population that occurs on the roots. Research is currently being done to spray vines with a herbicide after the last harvest, so that the roots may also die. Once the vines have been planted, preventative mealybug and ant control must be strict. Movement of labourers and implements should always occur from the youngest to the older vineyards. Labourers should preferably wear clean overalls and tractors and implements must be sprayed clean with water when moving from one vineyard to the next. Vines that display leafroll symptoms in the first two years must be removed and replaced with certified vines of the same clone.
Is affinity still a problem
In the past affinity problems were caused mainly by virus complexes that result from the grafting of scions and rootstocks that are still heavily infected with viruses. Since the implementation of the Certification Scheme and the resulting improvement of plant material, the problem of affinity has largely disappeared. A new problem, however, known as Shiraz decline, induces the scion to callus very strongly and causes the rootstock to ringulate. One Shiraz clone that displays serious symptoms of this relatively unknown disease has already been withdrawn from the scheme.
Why don’t I always get what I ordered
In order to ensure that a producer receives the right scion and rootstock clone, orders must be placed with the nursery at least 18 months before the vineyard is due to be planted. It is also recommended that producers visit the nursery so that they may check on the development of the vines in the nursery and whether the right clones for their purposes have been grafted. In instances where varieties are required that are not common, the orders should be placed with the nursery at least 36 months before the vines are due to be planted so that the PIO can propagate the plant material. If the full complement of vines ordered cannot be supplied, due to a poor take percentage in the nursery, the nursery will purchase the vines from another nursery or inform the producer in good time to make other arrangements.
What is the difference between a nucleus, foundation; mother and commercial block
One of the core principles on which the scheme is based, is that plant material should move from a nucleus plant, to foundation units, to mother units and then to the nursery unit. Nursery vines that have followed this route and comply with the physical and phytosanitary requirements of the scheme are certified and then planted in production vineyards by producers. Although production vineyards are established using certified nursery vines, production vineyards cannot be used for further propagation under the scheme.
A nucleus plant derives from a local selection, breeding programme or an imported plant ex quarantine that is subject to strict virus tests and virus cleansing processes. Nucleus plants that test clean of the prescribed viruses and virus-like diseases are kept in a sterile soil medium in an insect-proof greenhouse with an impenetrable floor surface and tested regularly to ensure that the plants retain their status.
Plant material from the nucleus plants is gathered and propagated to establish foundation plants. Foundation plants may be kept in greenhouses or planted in open sites. Sites for foundation units must be 25 metres from other vineyards and the soil must be virgin soil and test clean of the vector X index that transmits fanleaf.
Plant material from the foundation plants are gathered and further propagated in a foundation nursery to establish mother units. Sites for mother units must be 3 metres from other vineyards and the soil should be virgin soil or test clean of the vector X index that transmits fanleaf.
Plant material from the mother units is collected and further propagated in nurseries to establish production vineyards. Sites for nurseries must be 3 metres from other vineyards and the soil should be virgin soil or test clean of the vector X index that transmits fanleaf.
Where are the “clean” nucleus plants kept
Nucleus plants that have been tested are kept in a nucleus unit, in sterile growth mediums, and that is insect-proof so that the nucleus plants cannot be reinfected.
What is being done by the VIA, PIOs and nurseries to ensure that our plant material tests ever cleaner of pathogens each year
Winetech has various research projects on viruses and plant improvement under way; the results are evaluated annually and incorporated in the protocols where possible.
Clones that have become obsolete as well as foundation and mother units that no longer comply with the requirements of the Scheme are scrapped annually and replaced with new, improved clones and planted in new foundation and mother units in areas with the lowest possible risk of reinfection.
Foundation and mother units are inspected annually and in some instances tested for leafroll. Units that have been tested on an individual vine basis in risk areas receive a 2 star grading and units in low risk areas receive a 3 star grading, whereas units that are only inspected visually receive a 1 star grading. As the status of the units is improved over time, the industry is continuously being provided with plant material of a higher standard.
In the case of nurseries the soil is left fallow for at least one year and sowed with a cover crop, while fungicides are used to limit fungal infections. Warm water treatment is strongly recommended, as it produced the best results in the research trials to eliminate a wide range of pathogens.
Are grapevine viruses harmful to humans
No. Grapevine viruses are very specific and only occur in live grapevine plant material. Consequently there are no health risks for humans. According to the latest research the leafroll virus does not occur in the grape berry.