Southwestern Free State cultivar trial under irrigation at Petrusburg in 2024/25

Estimated reading time: 8 minutes

The Southwestern Free State potato production region produces around 1.86% of the country’s commercial potatoes on 954 ha (2024 harvest year). 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 taken prior to planting in order to determine the soil nutritional 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 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. The plant readiness of tubers at the time of planting, as well as the 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 case of the Petrusburg cultivar trial, 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). 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 the results of a single season, as climate differs from one year to the next. Therefore, cultivars are preferably tested across several seasons.

Climate and weather conditions

As with any crop, factors such as temperature, water availability (whether through good irrigation scheduling or rainfall), and heat units have a significant influence on 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 is still acquired from an ARC station, as the Hortec weather station on the trial site has not been operational long enough to provide long-term 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 2024/25 season (Figure 2) shows considerably less cumulative rainfall than the long-term average. Post-harvest quality evaluations indicate that a persistently waterlogged soil profile was responsible for seasonal challenges. Considering the daily rainfall data, it is clear that fairly frequent and continuous showers had a major impact on quality and specific disease pressures.

Minimum and maximum temperatures are illustrated in Figure 3. The last (and only) frost was recorded on 1 October, roughly a month after planting. At the end of November, as well as throughout almost all of December and January up to harvest, numerous consecutive days with maximum temperatures above 30°C, and some even reaching up to 40°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 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 is subjected to statistical processing 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 (9.2%). 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 in the 2024/25 season was 102.6 t/ha, which is higher than the trial average of 94.5 t/ha from the previous five cultivar trials at Petrusburg (2021 to 2025). Good water quality can be regarded as a factor contributing to high yield.

More than half of the cultivars all statistically produced the highest yields (Figure 5). Commando, EXP B, and Lilly achieved the highest marketing index; this can be attributed to the higher yield of good-quality potatoes obtained 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 potatoes are classified (Table 4).

Moths, rhizoctonia, and enlarged lenticels were the main causes of downgrading. Rhizoctonia often appears as a result of cool, wet conditions and high soil moisture levels. The enlarged lenticels also indicate that the soil may have been too wet for extended periods due to weather conditions such as continuous light showers that kept the soil saturated. 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 climate varies from one season to the next. Thus, it is important to consider consistent cultivar performance across a number of seasons, rather than relying on the good performance of a single season. Sound and Sifra showed the least variation in the Petrusburg cultivar trials from 2021 to 2025 (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). Cayman and Palace met both the chip colour and SG requirements, while many of the cultivars only met the correct chip colour requirement. – Enrike Verster, Potatoes SA, and Johan Odendal, producer

Special thanks to Lushof Boerdery, relevant Southwest Free State working group members and participating seedhouses, FPD, GWK, Provision Agri, RSA and WesGrow.