Dieback of young vines and the Growth Arrestment Phenomenon along the Lower Orange River

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Dieback of young vines and the Growth Arrestment Phenomenon along the Lower Orange River

Neels Volschenk

Neels Volschenk & Kobus Hunter, ARC Infruitec-Nietvoorbij, Stellenbosch
INTRODUCTION
Dieback of young vines and the Growth Arrestment Phenomenon (GAP), also known as "Growth arrestment disease", are prevalent in the Lower Orange River area (Fig. 1). It has been a sporadic phenomenon ever since the first plantings of Sultanina on the fertile, low-lying, irrigated soils in the region. Much has been written since the 1950s about the phenomenon and speculated about its causes. In the 1960s a comprehensive investigation indicated that crop losses of up to 80% may occur. The region’s most serious incident ever of dieback, delayed budding and the GAP occurred in the 2000/01 season. Yields in several vineyards were so low that it was not cost-effective to harvest the grapes. Besides yields that suffer as a result of this phenomenon, it has been necessary to re-establish young vineyards twice and even three times before reaching profitable vine production.

FIGURE 1. A vineyard displaying typical delayed budding and symptoms of growth arrestment.

FIGURE 2. A one-year-old vine that died back during the winter with a number of shoots displaying strong growth above the graft union the following spring.

FIGURE 4. Necrotic sections on the flower bunch stem are early indications of Black bunch symptoms. Parts below the necrotic sections will dry out, while those above them could possibly develop further.

Initially the GAP was only observed on soils inside the irrigation scheme, but due to extensive vineyard plantings elsewhere, combined with cultivation practices, it has also been observed on the less fertile soils of the region. All cultivars and rootstocks used in the area are subject to dieback and growth arrest (GA). The phenomenon appears to be the result of climatic conditions, which is why GAP may occur in the region in approximately three out of ten years, with serious ramifications.
The phenomenon is not unique to the Lower Orange River, it also occurs in the USA (California), where it is known as Spring Fever, and in Australia, where it is known as Restricted Spring Growth.
SYMPTOMS
The phenomenon manifests in various symptoms. In young vines dieback usually occurs during late winter, two to five years after initial establishment. Dieback occurs in a downward direction from the long shoots or cordons up to the graft union. In spring, several strong shoots emerge below the dead trunk (Fig. 2). Vines may then be redeveloped from these shoots. In serious cases the entire vine may die, which means it will have to be removed and replaced with a healthy vine.
In older vines, spring budding on the long shoots may be uneven and as a result of dead or dormant buds, the bare sections of the shoot have a white appearance. The latter is also known as White shoot phenomenon (Fig. 3). Shoot growth in emerging buds is limited (±10 cm) and stops before flowering. The new shoots are dark green and generally have shortened internodes without active growth tips. The leaves are a dull green and misshapen. In many instances the growth tip dies at an early stage and the flower bunch dies and dries out at the same time (Grey bunch symptom). After flowering, shoot growth resumes from the lateral shoots of the affected shoots or from new buds on the long shoots. As a result of generally vigorous growth conditions, there is no difference between affected and normal shoots in the late season.

FIGURE 3. A two-year-old vine with GA and White shoot symptoms. Vigorous growth nevertheless occurs in the crown of the vine.

Shoots may also bud and grow evenly as usual, until shoot growth is arrested before or during flowering (Growth arrestment phenomenon). During this period, dark necrotic parts may develop on the flower bunch stems, causing partial damage to the flower bunches or causing them to dry out entirely and drop. The latter is also known as Black Bunch Symptom (Fig. 4). The GAP may also occur on shoots apparently growing normally in that bunch stems become necrotic after set, causing partial berry shed, whereafter bunches may recover to a certain extent or shed their berries further until nothing but the bunch stem remains.
Older leaves from affected vines usually have oily, shiny blemishes on the upper surface and in many instances leaves curl upwards (Fig. 5).
POSSIBLE CAUSES
Various factors, or combinations thereof, play a role in the incidence of the GAP.
Climate
The climate of the Lower Orange River area is probably one of the most important contributing factors to the GAP. Compared to Stellenbosch, big differences in summer and winter temperatures occur and the rate of decrease between summer and winter temperatures is noticeable (Fig. 6). High temperatures in the post-harvest period cause vines to grow actively until late autumn, leaving little time for the accumulation of carbohydrate reserves. The occurrence of the first frost may seriously harm this accumulation of reserves, especially if this takes place early in autumn. In 2000, heavy frost occurred as early as 4 and 5 May (Fig. 7), causing all leaf activity to cease at once. From 24 May to 4 June, daily temperatures dropped to below freezing point. During the remainder of June and July, temperatures repeatedly dropped to below freezing point, even as low as -7°C. It was considered a very early and cold winter. A similar temperature profile occurred in 2002. The 2000/01- and 2002/03 seasons were characterised by a serious incidence of GA and poor yields. On the other hand, the incidence of the GAP was very low in the 1996/97 and 2001/02 seasons and above-average crops were recorded (Fig. 8). The latter two seasons were characterised by moderate preceding winters when the temperature did not drop below freezing point until the middle of June and never below -2.5°C throughout the remainder of winter.

FIGURE 5. A curly leaf on the left and a normal leaf on the right.

FIGURE 6. Average monthly minimum temperatures in Stellenbosch and Upington.

FIGURE 7. Daily minimum temperatures for the Eiland experiment farm in Upington from 1 April to 31 October for 2000 and 2002 (years when GA was prevalent).

High day temperatures at the beginning of spring cause vines to bud and start growing. Low night temperatures cause soil temperatures to remain low, however, which limits root activity, such as the uptake of nutrients and water, as well as the production of growth regulators, with the result that sustained growth is not supported.
Soil
Although a potassium deficit has also been associated with GAP, it does not appear to be a direct cause; instead, GAP seems to be the result of excessive nitrogen uptake from the fertile, silty soils. During the post-harvest period, abundant nitrogen uptake promotes excessive vegetative growth, thereby delaying the accumulation and mobilisation of carbohydrate reserves, which may also heighten vine susceptibility to GA. At this stage, young vines, compared to older vines, are also inclined to grow actively for longer in autumn.
Diseases
The accumulation and mobilisation of carbohydrate reserves in the post-harvest period can be limited if diseases such as Downy mildew and Oïdium occur, both of which cause leaf losses or damage.
Irrigation
Experience has taught producers that winter irrigation is essential to limit the GAP. At the same time, excessive autumn and spring irrigation should be limited.
Rootstocks
All cultivars and rootstocks are subject to GAP. However, cultivars that have been grafted onto 143B Mgt appear to be not quite as susceptible to GAP.
Vine physiology
The above-mentioned factors impact on the physiological processes of the vine, which determine the occurrence, or not, of GAP. Sufficient nitrogen and carbohydrate reserves in the shoots, cane and roots of a vine are required for normal winter dormancy, budding and initial vegetative and reproductive growth as well as root growth.
During winter, vines are dormant, the repose enabling them to survive cold periods. Dormancy starts in summer already, with the ageing of leaves. Cane ripening occurs from December/January onwards when shoots start ripening and change colour from the base upwards. In the Lower Orange River area active shoot growth, especially in young vines, occurs until late autumn, and competes directy with the accumulation of reserves in the older parts of the vines such as the shoots, trunks and roots. At the inception of winter, insoluble starch that has accumulated in the vines is transformed into soluble sugars. The high sugar concentration protects the tissue against frost damage. Vines with properly ripened canes, i.e. with sufficient reserves and where the conversion of starch into sugar has already taken place, can tolerate temperatures of up to -15°C without any damage. At the end of winter the opposite conversion of carbohydrates takes place, i.e. sugars revert to starch. The starch that is synthesised in the post-harvest period is the first carbohydrates to be used at the start of the new growing season, for budding and the initial vegetative growth.

FIGURE 8. Daily minimum temperatures for the Eiland experiment farm in Upington from 1 April to 31 October for 1996 and 2001 (years when crops were above average).

During the first four weeks after budding the vine uses ±50% of its starch reserves. During the first four to six weeks after budding, growth takes place on reserves only, until sufficient leaves have been formed to supply the vine’s nutritional requirements and the root system has become functional and supporting. Under normal conditions, a vine should be able to grow on reserves for ten weeks. During the warm growing season nutrients mainly move upwards, towards the growth tips to support vegetative growth. Under normal, cooler post-harvest conditions, nutrients should move downwards into the trunks and roots for accumulation of reserves. The latter process appears to deviate from the norm in the Lower Orange River area, because the vines grow actively until late autumn.
If the normal growth and dormancy cycle of the vine is impaired by external factors, as discussed above, it may result in physiological abnormalities, such as GAP.
RECOMMENDATIONS
The occurrence of dieback in young vines and GAP in the Lower Orange River area is a given and producers will have to realise that the phenomenon will occur periodically and sporadically. The intensity of the phenomenon may differ from season to season. There is no comprehensive solution to the problem, but a few preventive measures may limit its incidence.

The establishment of vineyards on low-lying, cold and nitrogen-rich soils must be limited, especially on soils previously planted to lucern.

Cultivate grain without nitrogen fertiliser for at least one season if vines are to be established on former lucern fields.

No nitrogen fertiliser should be applied in the first year after establishment on nitrogen-rich, silty soils, especially not in the post-harvest period.

Use 143B Mgt as rootstock for plantings on low-lying soils to induce moderate growth and early cane ripening and leaf drop.

Overproduction should be prevented since this depletes reserves even further, especially in young vines.

Vines that have died back and where a number of shoots grow beneath the dead trunk, may be redeveloped in late spring. Only one shoot should be used to form new cordons or crowns, while the other shoots should be left in place and topped regularly to distribute the vigour.

Irrigation should be sensible so that the soils do not become too wet in autumn nor too dry in winter. Take great care with spring irrigation as the soil temperature decreases after each irrigation and root activity may thus be impaired.

Disease control should be effective, especially post-harvest, as leaf loss limits the accumulation of reserves and may also result in regrowth, thereby depleting reserves even further.

Vines established in localities that are potentially susceptible to early frost, should be positioned and the environment managed in such a way to create air currents that will remove cold air from the vineyards.

If possible the effect of early frost can be reduced by overhead irrigation, fans or smoke during the post-harvest period.

For further enquiries, contact Neels Volschenk at volschenkn@arc.agric.za.
ACKNOWLEDGEMENTS
Winetech and ARC for financial support.

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