|
A Technical Guide for Wine Producers
|
|
RECENT ARTICLES | WYNBOER HOME
Responsibilities Of The Producer To Prevent Virus Distribution
It is generally known that viruses living systemically inside the plant, either entirely or partially, can not be directly chemically controlled. All known vine viruses or virus groups are distributed by infected plant material. Apart from infected plant material, leafroll viruses are also distributed by insect vectors above the soil, for example mealybug, as well as a few coccid species (scale insects). On the other hand, the fan leaf virus group (fan leaf, yellow mosaic and vein banding), not to be discussed here, is distributed from one vine to the next by the subterranean nematode, species Xiphinema index and X.italeae.
Plant material, both rootstock and scion, that is free or clean of harmful viruses, remains the preferred and obvious method to restrict virus distribution. Once mealybug is present, re-infection of the environment, including soil, is more or less a given. The distribution of mealybug should therefore be restricted to control leafroll distribution. From a practical point of view, therefore, control measures, either chemical or biological, can be directed at the mealybug vector to prevent the distribution which holds very serious implications for the S A wine industry.
Using the Winetech - ARC protocol as a guideline, the producer has the following responsibilities to prevent virus distribution with mealybug / ant control as the point of departure.
Vine plant material
Surely the most important factor in the distribution of viruses, it is of cardinal importance to use certified virus tested plant material, both rootstock and scion, when establishing a new vineyard. Infested rootstock and scion plant material is the source of infestation in a new vineyard. From there the infection can spread over the entire block, and also adjacent vineyards, if a vector such as mealybug is present. If the rootstock is infested, the infection will certainly be transferred by grafting, namely the graft joint, to the scion and vice versa. Unfortunately no plant improvement scheme can guarantee that so-called leafroll virus free plant material will reach the producer on farm level, since present conditions, despite the best efforts, preclude mother blocks from being declared 100 % leafroll "virus free". Nucleus vines are tested on a regular basis for the presence of leafroll and even foundation blocks are subject to very strict control on an ongoing basis.
Astronomic financial costs will have to be incurred by the industry if each vine in a mother plantation situated all over the viticultural areas has to be subjected individually to virus testing. First of all, producers must ensure that they obtain the very best plant material by making use of a recognised, reliable nursery. Control or select your own mother blocks (rootstock and scion) and inspect the nursery on a regular basis for the presence of virus and mealybug infection. Inspection of mother blocks should be done in late summer / autumn when symptoms are most visible. Nurseries usually have excellent administrative control systems and are able to trace each grafting combination. It is a prerequisite, however, for the producer to place the vine order with the nursery at least 18 - 24 months in advance.
If leafroll symptoms still occur in the young vines after establishment, the nursery should be notified immediately so as to be aware of the situation. In unsatisfactory negotiations with the nursery, the Wine Grape Improvement Association may be informed of the problem.
Hot water treatment of nursery vines (50o C for 30 minutes), with all its reservations and so-called disadvantages, is still a very useful treatment to get rid of a variety of fungi and plagues such as, inter alia, mealybug, but unfortunately not virus infections.
If any vines in a newly established block show leafroll symptoms during the first two or three years, it is preferable to replace those vines with healthy vines so as to prevent the virus from spreading further. Producers should ensure, however, that the symptoms are indeed those of leafroll and not another problem which damaged the phloem or bark close to the surface of the soil (e.g. strangulation, feeding damage, frost, wind etc.).
Mealybug distribution / virus distribution also occurs from older adjacent infested blocks through implements, human and wind movement. Take care not to plant young blocks between infested blocks.
The crux of the matter is: Keep your young vineyards free of mealybug and leafroll infestation for as long as possible, especially in low risk areas.
Top-grafting of adult vines to other cultivars
The need for alternative cultivars has created the trend to adapt existing vineyards to other cultivars in order to save time and expense. Since harmful viruses such as leafroll are transmitted by grafting, the existing vines should test clean of harmful viruses, otherwise the exercise, which entails considerable expense, will be futile! Unless the existing block tests free of leafroll virus on an individual vine basis, which is an uneconomic or impossible task, this practice cannot be recommended categorically. Some diseases such as Shiraz disease can also be latent and come to the fore after grafting. So far there is no quick detection technique for Shiraz disease. Even though the existing block might not show any virus or mealybug symptoms, such as with certain white cultivars, e.g. Chenin blanc, nobody can guarantee that the block is virus free. Remember mealybug occurs on roots up to 60 cm deep in the soil and is therefore not visible on the surface!
Soil preparation in re-establishment
Before establishing the vines, all old vineyard material should be removed from the soil and destroyed. Mealybug survives on old vine roots that remain alive under the soil for an extremely long period (up to 7 years). Another consideration is to spray the vines that still have active leaves directly after the harvest with a concentrated solution (2 %) of 2,4-D. The Americans claim that this treatment has killed vine roots to a depth of 3,0 m. Fumigation of soil and the effect as well as the economy thereof on mealybug in the soil warrant further investigation.
Since mealybug has many host plants, inter alia certain weeds, all living plants on the soil should preferably be sprayed with herbicides before the vines are planted.
Cultivation practices
Because of the presence of mealybug in the soil, producers are advised to start their cultivation practices in newly planted vineyards. From here they should move progressively to older and more infested blocks. They should also ensure that implements are clean by rinsing them. With regard to mealybug control above the soil, it should be borne in mind that topping and harvesting machines as well as the clothes and shoes of workers contribute to mealybug distribution. Human movement should always take place from the youngest to the oldest blocks. Even the wind can blow the mealybugs in their crawling stage from an infested vineyard block to a clean block. The prevailing wind direction can therefore influence the direction of mealybug and leafroll. Birds also transmit mealybug.
Natural enemies (predators)
Mealybug is attacked by 13 different kinds of natural enemies (ladybirds, wasps, etc.), while ants in turn protect the mealybug against its natural enemies. Dust harms natural enemies and consequently an attempt should be made to avoid dust formation, e.g. through permanent cover crops on vineyard paths, etc. The absence of natural enemies who are able to effectively control mealybug, may result in the formation of new mealybug colonies, thus benefiting leafroll distribution.
Ant control
The "unholy alliance" between ants and mealybug (even in the soil) complicates the distribution of mealybug and consequently leafroll even further and ant control is therefore of cardinal importance. Without ant control, biological control agents will be ineffective, which in turn results in a population explosion in mealybug populations. In addition to direct chemical ant control, it is recommended that sticky barrier strips be applied around the trunks, trellis poles and anchor wires. Through correct foliage control, an attempt should be made to prevent shoots from touching the ground, since these may serve as transit route for the ants to reach the vines. A cover crop reduces ants / ant nests, while also suppressing host plants and being beneficial to enemies of the mealy bug.
Weeds
Weeds are host plants for various mealybug species, one being a vector of the leafroll virus. High growing weeds also act as a bridge for ants into the leaf canopy of the vines, with dire implications for biological control. Effective weed control should take place in such a way that there are no weeds, on the berms in particular, at the beginning as well as throughout the growing season.
Monitoring for mealybug
Thorough monitoring is the key to success and economy of mealybug control. In November in particular, this deserves extra attention. An infestation of 2 - 3 % justifies chemical control. A survey of mealy bug infestation levels in problem blocks may be done as follows:
The key to effective chemical control of mealybug is early action. Prior monitoring is essential, however, since this information is used to decide whether control must be applied or not.
High mealybug infestation levels in the previous season (> 5%) mean that dormant or winter treatments must be executed in the problem areas. Thorough wetting with hand sprays is essential.
As a protective measure for the natural enemies of mealybug, the best pesticide must be easily degradable (within two weeks). Rapid degradation of the pesticide enables the natural enemies to re-invade the vineyard shortly after application. Chemical control in the course of the growing season is usually not as effective, with only a small section of the mealybug population being killed. Large mealybug populations are also protected against toxins by the wax- and honeydew secretions. Mealybug should be controlled as early as possible in the growing season!
Biological control
Biological control should be encouraged as much as possible since it is the most effective control measure. Under optimum conditions biological control will keep the mealybug population under 1 %. Optimum conditions include optimum short and long term cultivation practices, the absence of dust, ants and judicious chemical control.
The presence of a cover crop that flowers early in the season benefits the mealybug predators. The flowering period of the cover crop should be sustained for as long as possible in the season.
Long and short term practices
As partially discussed, long term practices involve the following:
|
|
Wynboer is incorporated in WineLand, magazine of the SA wine producers.
Visit our sister sites:
|
| UP |
COPYRIGHT (C) 2000 WineLand |