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The Southwestern Free State production region accounts for around 1.86% of the country’s commercial potato production on 954 ha. The most prominent cultivars produced as part of this region’s main harvest for table and processing purposes are Sifra (89%), followed by Mondial, Innovator, and Panamera. Petrusburg is located in South Africa’s dry continental region (Figure 1). The farm where the trial was conducted has experienced an average annual rainfall of 565 mm over the past 25 years, as recorded by the local Agricultural Research Council (ARC) weather station. This region is characterised by very hot summers and cold winters, with frost occurring from June to August.
Trial design and details
The cultivar trial at Petrusburg is laid out in a random block design with three replications per cultivar. Table 1 contains extra information relating to the trial. Soil samples were collected 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 is subject to the nature of the seasons but is generally regarded as the time that passes 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.
Marketing-related results
The evaluation of new cultivars, as in the Petrusburg cultivar trial, provides results on yield and marketing index. The marketing index of the cultivars involved is calculated by classing and sorting each cultivar based on quality and size groups (for example, Class 1 large or Class 2 large-medium). In this trial, all three replications were combined, washed, graded, and sorted by the storage facility.
Prices were then compared to market prices obtained at the time of harvest. However, the performance of cultivars 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.
Climate and weather conditions
As with any crop, factors such as temperature, water availability (whether through effective irrigation scheduling or rainfall), and heat units significantly influence the potato plant during its growth period.
These factors are considered when evaluating cultivar performance.
Applicable daily weather data for the relevant season is obtained from a Hortec weather station installed on the farm where the trial is conducted. However, long-term data are still obtained from an ARC station, as the Hortec weather station on the trial site has not been operational long enough to provide such data. The ARC weather station from which the long-term data was accessed is located 9 km from the trial site.
The rainfall trend for the 2025/26 season (Figure 2) shows cumulatively higher rainfall from mid- to late season than the long-term average. Post-harvest quality evaluations clearly indicate that wet and warm conditions presented challenges. Considering the daily rainfall data, it is evident that wet conditions mid- to late in the growing period had a major impact on quality and specific disease pressures.
The minimum and maximum temperatures are illustrated in Figure 3. The last frost was recorded on 18 September, roughly a month after planting. On the day of planting, temperatures below 0°C were recorded. At the end of November, and throughout almost all of December and January up to harvest, numerous consecutive days with maximum temperatures above 30°C, and even 35°C, were recorded.
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 cultivar trial appears to be somewhat higher than the long-term data trend of heat units (Figure 4). This can be attributed to the number of above-average hot days, particularly in December and January, which resulted in greater accumulation of heat units.
Yield and cultivar effect
Yield data recorded during harvesting are 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 (CV) was low (12.1%). These factors indicate that the trial was well executed, and the results are therefore reliable.
The yield of each cultivar 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 expressed as a percentage of the trial average. The average yield of the trial in the 2025/26 season was 96.8 t/ha, which is consistent with the trial average of 94.5 t/ha across the previous five cultivar trials at Petrusburg (2022 to 2026). Good irrigation scheduling and water quality can be regarded as among the factors contributing to high yield.
More than half of the cultivars statistically produced the highest yields (Figure 5). Buffalo, Sifra, and Sound achieved the highest marketing index, which can be attributed to the higher yield of high-quality potatoes from these cultivars. Size group distribution and grading are important evaluations when assessing a cultivar’s marketability (Figures 6 and 7).
The reasons for downgrading are taken into account when classifying potatoes (Table 4). Soft rot, moth, and other insects were the main causes of downgrading. Soft rot often appears during favourable (warm and wet) conditions. Brown spot and hollow heart were observed in a few cultivars.
Cultivar performance
Given the nature of the seasons, cultivar performance will vary from one season to the next simply because the climate varies. Thus, it is important to consider consistent cultivar performance across multiple seasons, rather than relying on a single season’s good performance. Sound and Sifra showed the least variation in the Petrusburg cultivar trials from 2022 to 2026 (Figure 8).
Lastly, when considering the internal quality of potatoes, processing characteristics can also be evaluated. To adhere to processing requirements, cultivars must meet a chip colour standard of >50 and a specific gravity (SG) of ≥1.075 (Table 5). Only Lanorma met both the chip colour and SG requirements, while many cultivars met only the chip colour requirement.
– Enrike Verster, Potatoes SA, and Johan Odendal, producer
Special thanks to Lushof Boerdery, relevant Southwestern Free State working group members and participating seed houses, FPD, GWK, RSA and WesGrow. For more information, send an email to enrike@potatoes.co.za.