Ceres cultivar trial under irrigation at Donkerbos in 2025/26

Producers in the Ceres production area plant roughly 1.27% of South Africa’s total potato hectares. Processing and table potatoes are planted under irrigation. 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, in the winter rainfall region. Summers in this region are warm, whereas winters tend to be cold and wet due to regular snow on the surrounding mountains. Planting is done mainly between October and November, while early or late plantings are at greater risk of 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 relating to the trial. Soil samples were taken prior to planting to determine the soil nutrient profile of the trial site (Table 2).

The cultivar trial includes cultivars with short and long growing periods, which can influence the final yield of certain cultivars. The length of growing periods depends on the nature of the seasons but is generally regarded as the time from emergence to natural leaf senescence. Table 3 sets out how these growing periods differ among cultivars. The plant readiness of tubers at the time of planting, as well as the density and haulm count observed later in the growing season, are also presented in Table 3.

Evaluating new cultivars, as in the Ceres cultivar trial, provides results regarding yield and marketing index. The marketing index of the relevant cultivars is calculated by classing and sorting each cultivar according to 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, cultivar performance cannot be based solely on a single season’s results, as climate varies from year to year. Therefore, cultivars are preferably tested across several seasons.

Impact of climate conditions

As with any crop, aspects such as temperature, water availability (whether through good irrigation scheduling or rainfall), and heat units influence the potato plant during its growing period. These factors are considered when evaluating cultivar performance. Applicable weather data is obtained from a weather station located on the farm where the trial was conducted.

Above-average rainfall was recorded during most of the active growing period (October to March) in the 2025/26 season (Figure 2). This summer rainfall in a winter rainfall region contributed to an above-average yield.

Minimum and maximum temperatures are illustrated in Figure 3. This season was at times characterised by large fluctuations in maximum temperatures. A total of 59 days with temperatures above 30°C and five days above 35°C were recorded – more than in the previous season.

Collecting heat units during a growing period is a key factor in the development of a potato plant. The trend of heat units available for this season’s Donkerbos cultivar trial appears to be slightly higher than the long-term data trend of heat units throughout the active growing period of November and December (Figure 4).

Yield and cultivar effect

Yield data recorded during harvesting were processed statistically 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 was low (10.6%). These factors indicate that the trial was well executed and that the results are therefore reliable.

Each cultivar’s yield is divided by the trial average, with the average across all cultivars set to 100%. This is used to determine a yield index, with each cultivar’s yield performance read as a percentage of the trial average.

The average yield of the trial in the 2025/26 season was 120.14 t/ha, which is higher than the previous five seasons’ average of 104.9 t/ha. This also represents another record yield for this cultivar trial. The increased yield can be attributed to a favourable combination of circumstances and sound management. This includes optimal planting and harvesting times, good soil, and efficient irrigation scheduling.

Most of the cultivars in the Donkerbos trial performed exceptionally well, showing no statistical yield differences. Lanorma, 91A3, and Foxy achieved the best marketing index. A good marketing index can be attributed to higher yields of large potatoes and/or a high percentage of good-quality potatoes. Size-group distribution and grading evaluations are essential for determining a cultivar’s marketability. The aforementioned cultivars with the highest marketing index performed well in these evaluations (Figures 6 and 7).

Cultivar performance

Given the nature of the seasons, cultivar performance will vary from one season to the next. This is simply because the climate is never the same from one season to the next. Thus, it is important to consider cultivar performance across multiple seasons. Figures 8 and 9 illustrate cultivar stability over the past three and five seasons.

The main reasons for downgrading are presented in Table 4. Common scab, malformation, insect damage, and stem-end rot were the primary reasons for downgrading potatoes. Stem-end rot typically occurs at the end of a growing season. This occurs when stolons become infected by the pathogen Fusarium spp., which spreads through the vascular tissue to the tubers.

Lastly, processing characteristics can also be evaluated when considering the internal quality of potatoes. To meet processing requirements, cultivars must, among others, record a specific gravity (SG) of ≥1.075 (Table 5). Various cultivars met the SG requirement. Internal quality was also assessed to determine whether the cultivars showed signs of hollow heart or brown spot. No internal defects were observed in any cultivar. – Enrike Verster, Potatoes SA

Special thanks to the collaborators at Donkerbos, the Ceres potato working group, participants FPD, GWK, RSA, and WesGrow, and relevant Potatoes SA staff. For more information, send an email to enrike@potatoes.co.za