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Eighteen producers are currently producing potatoes for the country’s fresh produce markets on 2,214 ha in the Northern Cape potato production region.
Approximately 60.74% of this region’s potato production comprises seed potatoes. The main cultivars earmarked for commercial production in this region include Lanorma, Sifra, and Mondial.
Douglas is located in South Africa’s dry continental area (Figure 1) with an annual average rainfall of approximately 200 mm for the last six years. Winters are cold with regular frost, while summers can be extremely hot.
The cultivar trial was carried out in sandy loam soil and laid out in a randomised block design with three replications per cultivar. Table 1 contains additional technical information relating to the trial.
The cultivar trial included cultivars with short and long growing periods.
Hence, growing periods can influence the eventual crop yield of certain cultivars. Growing periods are defined as the number of days from emergence to leaf senescence, depending on the season. A potato plant’s lifetime can be divided into five growth phases which include sprout development, vegetative growth, tuber initiation, tuber filling, and maturity.
Table 2 illustrates how the growth period differs from one cultivar to another. Environmental factors and management practices influence the different growth phases and their time of commencement.
Population density and haulm count influence tuber size and yield.
The number of eyes per tuber is dependent on the cultivar and can determine the number of sprouts produced per tuber. In this instance, the plant readiness of seed potatoes is very important seeing as better plant readiness leads to better sprouting in seed potatoes. Plant readiness of seed potatoes during the planting of this trial as well as population density and haulm count are indicated in Table 2.
The evaluation of new cultivars such as those in this trial provides, among others, results regarding yield and marketing index. The marketing index of the specific cultivars is calculated by classing and sorting each cultivar according to quality and size group distribution (for example, Class 1 Large or Class 2 Large medium). Prices are then compared to market prices as obtained during harvesting time. The performance of new cultivars cannot be based on one specific season only, as climate can vary from one year to the next. Therefore, cultivars are preferably tested across several seasons.
As with any crop, aspects such as temperatures, the availability of water (whether through good irrigation scheduling or rainfall) as well as heat units, are all important factors that have a fundamental influence during the potato plant’s growing period. These factors are thus taken into consideration when the performance of cultivars is evaluated.
Rainfall and irrigation
The season’s rainfall was substantially lower than the long-term average rainfall trend for the growing season (Figure 2). In this dry continental region, there is heavy reliance on good irrigation scheduling for potatoes.
Temperatures and heat units
Figure 3 indicates minimum and maximum temperatures. Temperatures below freezing point were recorded regularly since the end of May. These continuous below-freezing temperatures led to natural foliage die-off.
The collection of heat units during a growing period is an important factor in a potato plant’s development. The trend of heat units available for the season’s cultivar trial seemed to be somewhat higher than the trend of average long-term heat units. (Figure 4).
The yield index
Yield data collected during harvest was subjected to statistical processing using the GenStat® program.
The Tukey test of least significant differences (LSDs) was used to separate the mean. The cultivar effect during this specific trial (Figure 5) was statistically significant (p<0.05) and the coefficient of variation was acceptably low (12%).
These factors indicate that the trial was well executed and the results reliable. The yield of each of the cultivars is divided by the trial average (the trial average of all the cultivars is taken as 100%). A yield index is created and each cultivar’s performance is then read as a percentage of the trial average.
The average yield for the 2023 season trial was 44.88 t/ha.
The cultivars Sababa, Lilly, 11Z49A1, Mondial and Lanorma statistically produced the highest yields. Lilly produced the highest marketing index which can be ascribed to a good yield of Medium tubers as well as minimal Class 3 tubers.
Market prices for Class 1 Medium-sized potatoes were R5 more than Class 1 Large potatoes during the week of harvesting; this will influence the marketing index as indicated on the graph where Lilly produced the highest marketing index. The main reasons for the downgrading of each cultivar (Table 3) were investigated to determine which challenges the specific cultivars were faced with in terms of quality index. Moth damage, malformation and common scab were the main reasons for downgrading.
Lastly, to adhere to processing requirements, cultivars must meet the chip colour standard of >50 and specific gravity (SG) of ≥1.075. Only King Russet met the SG and chip colour requirements. (Table 4). – André Prins, GWK, and Enrike Verster, Potatoes SA
Special thanks to the producer and farm, the Northern Cape Potato Work Group as well as Anjé Erasmus, Damien da Cal and Dikgetho Mokoena of Potatoes SA. For more information, contact André Prins at andrep@gwk.co.za or Enrike Verster at enrike@potatoes.co.za.