We bring you an update on our rootstock series from the December issue — now including salinity tolerance and drought sensitivity. Information provided by Dr Piet Stassen with input from Petru du Plessis and other technical advisers.
The success of any rootstock is built upon healthy trees with an extensive network of efficient feeder roots. When nursery trees lack feeder roots, have inadequate reserves, or are not properly hardened off, they may struggle to establish in the orchard. This is especially true of clonal rootstocks, trees planted in sandy and stony soils, or with poor irrigation.
Tree production is a slow process — trees must be ordered two years in advance to ensure availability of the preferred rootstock. Failure to plan ahead could mean settling for an inferior second choice.
Flordaguard is an excellent rootstock for early cultivars grown on deep, well-drained, sandy soils in areas with low chill, but it is not always well hardened off.
SAPO 778 is not suitable for low-chill regions or early cultivars. Maridon does poorly in the Little Karoo but is outstanding in the Simondium region where bacterial canker may occur.
Rootstock choice impacts the entire lifetime performance of an orchard. Growers would do well to consult a specialist technical adviser when making their selection.
Table 1 contains information on plant attributes. The parentage of a rootstock will determine characteristics such as tolerance to calcareous soils and salinity, and susceptibility to fungal infections and nematodes.
The difference between early and late ripening was four to eight days. Rootstocks had little impact on the sugar content of fruit. In one trial SAPO 778 produced significantly lower sugars than other rootstocks when grown in sandy soils. Soil texture, climate and cultivar have an overriding effect on sugar content.
Soil texture plays an important role in the performance of rootstocks. Table 2 shows the soil texture preferences of stone fruit rootstocks.
Table 3 provides soil and climatic preferences. Most stone fruits are sensitive to salinity. Electrical conductivity of more than 300 mS/m and sodium levels of more than 9 mg/l in irrigation water constitute very high salinity for stone fruit. Refer to the footnote below for levels that will impact production.
Rootstocks with plum parentage can tolerate higher salinity than those with peach parentage. Peach-almond hybrids have intermediate tolerance. All available stone fruit rootstocks are sensitive to wet conditions.
Even short-term waterlogging will cause dieback in Flordaguard, Garnem and GF 677 from infections and root rots. Kakamas seedling will also be affected but is less prone to dieback. Marianna and Maridon are more tolerant during winter but sensitive in the initial growing period.
Table 4 summarises the resistance of the different rootstocks to nematodes and diseases. Bacterial and fungal infections are more common in wet conditions and in low-lying areas where fluctuating water tables may occur during heavy rains.
Nematode damage is often underestimated because the culprits are in the soil and aboveground symptoms are non-specific. Young root systems are more sensitive to high nematode numbers than established root systems.
Factors such as nematode resistance, drought tolerance and chill requirement shouldn’t be considered in isolation. Healthy, vigorous orchards result from the sum of all these characteristics. A holistic approach is the only way to achieve maximum production per hectare.