Estimated reading time: 8 minutes
Producers in the Ceres production area plant approximately 1.27% of the total area under potatoes in South Africa. Potatoes for processing and table purposes are planted under irrigation (651 ha in the 2024 harvest year). The most prominent cultivars produced in this region are FL2108, Sifra, FL2006, Avalanche and Lanorma.
The cultivar trial was conducted at Donkerbos in the Koue Bokkeveld, which falls within the winter rainfall region and has recorded an average annual rainfall of 647 mm over the past six years, according to data from the Donkerbos weather station located on the farm. Summers in this region are warm, whereas winters tend to be cold and wet due to regular snow on the surrounding mountains.
Planting mainly occurs between October and November, while early or late plantings have a greater risk of being affected by rain damage.
Trial design and details
The trial was conducted in sandy soil and laid out in a randomised block design with three replications per cultivar. Table 1 contains additional technical information regarding the trial. Soil samples were taken prior to planting to determine the soil’s nutritional profile at the trial site (Table 2).
The cultivar trial includes cultivars with short and long growing periods. Growing periods can therefore influence the eventual yield of certain cultivars. The length of growing periods is subject to the nature of the seasons but are generally regarded as the time that passed from emergence to natural leaf senescence. Table 3 sets out how these growing periods differ among cultivars. Table 3 also sets out the plant readiness of tubers at the time of planting, density, and the haulm count observed later in the growing season.
The evaluation of new cultivars as in the case of the Ceres cultivar trial also provides results relating to yield and marketing index. The marketing index of the cultivars involved are calculated by classing and sorting each cultivar based on quality and size groups (for example Class 1 large or Class 2 large medium).
Prices are then compared to market prices obtained at the time of harvest. However, the performance of cultivars cannot be based solely on the results of a single specific season, as climate differs from one year to the next. Therefore, cultivars are preferably tested across several seasons.
Effect of climate factors
As with any crop, factors such as temperature, water availability (whether good irrigation scheduling or rainfall), as well as heat units have a significant influence on the potato plant during its growth period.
These factors are considered when evaluating the performance of cultivars. Applicable daily weather data for the season is obtained from a weather station located on the farm where the trial was conducted.
The 2024/25 season (Figure 2) received below average rainfall during most of the active growing season. Because the trial was conducted under irrigation, optimal irrigation scheduling was applied and was likely one of the factors contributing to the above average yield.
Minimum and maximum temperatures are presented in Figure 3. During the season considerable fluctuations were recorded in maximum temperatures, with a total of 47 days of temperatures above 30°C and three days above 35°C. The collection of heat units during a growing period is an important factor in the development of a potato plant.
The trend of heat units available for this season’s cultivar trial at Donkerbos appears to be equal to the long-term data trend of heat units (Figure 4).
Yield and cultivar effect
Yield data recorded during harvesting is statistically processed using the GenStat® program. The Tukey test of least significant differences (LSD) was used to separate the mean. The cultivar effect during this trial (Figure 5) was statistically significant (p<0.05) and the coefficient of variation (CV) was low (7.1%). These factors indicate that the trial was well executed, and the results are therefore reliable.
The yield of each of the cultivars is divided by the trial average, with the average of all the cultivars taken as 100%. This is used to determine a yield index and each cultivar’s performance in terms of yield is read as a percentage of the trial average.
The average yield of the trial for the 2024/25 season was 113.66 t/ha – significantly higher than the previous six seasons’ average of 93.2 t/ha, and possibly a record yield in the history of this cultivar trial. The higher yield could be attributed to coincidence and good management practices, such as optimal timing of planting and harvesting, healthy soil, and good irrigation scheduling. Lilly, 91A3, Sifra, Foxy, Markies and Mondial delivered the highest yields, with no statistical difference in yield. Sifra, Lilly, and Palace achieved the highest marketing index.
A good marketing index can be ascribed to a higher yield of large potatoes and/or a good percentage of good quality potatoes. Size group distribution and grading are significant evaluations when looking at the marketability of a potato cultivar.
The cultivars Sifra, Lilly, and Palace reached a good marketing index and performed well in these evaluations (Figure 6 and 7).
Cultivar performance
Given the nature of the seasons, cultivar performance will vary from one season to the next, simply because climate varies from one season to the next. Thus, it is important to consider consistent cultivar performance across a number of seasons. The stability of cultivars over the past three and five seasons is presented in Figure 8 and 9.
The main reasons for downgrading are indicated in Table 4. Malformation and stem-end rot were the major causes for downgrading potatoes to Class 2 and 3. Stem-end rot typically occurs at the end of a growing season.
This is caused by infection of the stolons with the pathogen Fusarium spp., which spreads through the vascular tissue to the tubers.
Lastly, processing characteristics can also be evaluated by looking at the internal quality of potatoes. To adhere to processing requirements, cultivars must meet a specific gravity (SG) of ≥1.075 (Table 5). Many of the cultivars met the SG requirements. Internal quality was also assessed to determine whether the cultivars displayed signs of hollow heart or brown spot, but none were observed as having internal defects. – Enrike Verster, Potatoes SA
Special thanks to the collaborators at Donkerbos, the Ceres potato workgroup, participants from FPD, GWK, RSA, Wesgrow, Provision Agri, and other Potatoes SA staff involved. For more information, send an email to enrike@potatoes.co.za