Technical Yearbook 2024

OCTOBER

Suitable rootstock for soil and wine goal By Heinrich Schloms

The choice of rootstock is an important decision which can determine the success of your newly planted vineyard. Various aspects (it usually follows a process of elimination) like soil, climate, irrigation, pests and diseases, scion cultivar characteristics and wine goal have to be considered. Soil surveys are an essential step to collect this information and thereby identify the most important limitations that will influence decision-making. The rootstock characteristics (Table 1, compiled by the late Prof Eben Archer) of nine rootstocks in the wine industry will be discussed in this article. SO4, 140 Ruggeri and 1103 Paulsen are not so popular in the industry anymore for various reasons, but will also be discussed. The following factors must be taken into account when choosing rootstocks: 1. Scion cultivar and wine goal The vigour and affinity of the scion must be considered in the rootstock choice. Scion vigour can be managed up- or downwards with the correct rootstock choice. If higher-quality grapes are required on fertile soils, a rootstock with more moderate vigour like 110 Richter or 101-14 Mgt will be recommended. 2. Soil potential Soil potential is determined during soil mapping by considering the physical properties of the soil influencing root growth. Higher soil potential results in a larger root system which in turn mostly creates a larger, denser canopy if all the other environmental factors are also optimal. Rootstocks with a lower vigour can be used on soils with a higher soil potential to balance canopy growth and yield. Rootstocks with a stronger vigour can be used on soils with a lower soil potential to ensure sufficient canopy growth and yield. In this way soil potential can be managed to achieve the optimal canopy density for the chosen wine goal. 3. Soil-chemical properties The presence of free lime and/or high levels of brackish salts like sodium chloride must be taken into account. Free lime in the subsoil must not be brought to the surface during soil preparation, as this action can aggravate iron chlorosis and other micro-element deficiencies. Brackish salts can be removed from the soil to some degree with gypsum applications, good

irrigation water and subsoil drainage, but a salinity tolerant rootstock like Ramsey will always be a priority on saline soils. 4. Signs of wetness Rootstocks’ tolerance of wetness in the subsoil varies drastically. Not all soils are suited to subsoil drainage (mostly due to a shallow, impenetrable clay layer). In such instances rootstocks that can handle the wetness better must be chosen. If possible, soils with good natural drainage and drainable soils should always be the first choice. 5. Availability and quality of irrigation water Water availability and quality is a growing problem in South Africa and a significant amount of research is currently being done on rootstocks that are highly drought and salinity resistant. Drought-resistant rootstocks are becoming increasingly popular as the availability of water decreases over time. Poor-quality irrigation water can lead to salinisation of the soil and then preference will be given to rootstocks that can handle the saline conditions. 6. Pests and diseases Fresh soil samples with living roots can be tested for the presence of nematodes, phylloxera, Phytophthora, crown gall etc. If any of the pests or diseases are present, it will influence the rootstock choice and certain control measures. Irrigation water must also be tested, as it can spread bacteria, fungi, insects and nematodes. Treatment of irrigation water with filtration, chemical treatments, UV light, ozone and biological control with help with this. 7. Logistical problems Ripening of large amounts of grapes at the same time can cause bottlenecks at cellars. To counteract this problem, the same cultivar can be grafted onto different rootstocks with varying vigour and phenology to distribute ripening more evenly. Rootstocks with stronger vigour, good fertility and late phenology can for example be used to delay ripening, while rootstocks with moderate vigour, low fertility and early phenology can accelerate ripening.

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TECHNICAL YEARBOOK 2024