Estimated reading time: 7 minutes
The Eastern Free State is a major potato production region where nearly 19% of the country’s commercial potatoes are produced on roughly 9 523 ha. The most prominent cultivars produced in this region for table consumption are Mondial, Panamera, and Lanorma.
Reitz falls within South Africa’s temperate summer rainfall region (Figure 1) and has received average annual rainfall of about 509 mm over the past 20 years. This region experiences warm summers and very cold winters, with frost from mid-May to early September.
Trial design and details
The cultivar trial at Reitz is laid out in a random block design with three replications per cultivar. Table 1 contains extra information relating to the trial.
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 2 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 2.
The evaluation of new cultivars, as in the case of the Reitz cultivar trial, provides results relating to 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).
Prices were then compared to market prices obtained at the time of harvest. Cultivar performance cannot be based solely on the results of a single season, as climate differs from one year to the next. Thus, cultivars are tested across several seasons.
Seasonal aspects
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 growing period. These factors are considered when evaluating cultivar performance.
Applicable daily and long-term weather data for the past season are obtained from a nearby Hortec weather station and a selected Agricultural Research Council (ARC) weather station located as close as possible to the trial site.
The rainfall trend for the 2024/25 season (Figure 2) shows considerably more rainfall than the cumulative long-term average. Excessive, above-average rainfall was recorded particularly during the final months before harvest, which resulted in a higher percentage of rejected potatoes due to their quality (Figure 7).
Figure 3 illustrates the minimum and maximum temperatures. The relevant season experienced particularly large fluctuations in maximum temperatures, especially during the first three months of the growing period.
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 Reitz appears to be close to, but slightly higher than, the long-term data trend for heat units (Figure 4).
Yield and cultivar effect
Yield data recorded during harvesting day are 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 (13.5%). 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 39.8 t/ha, which is slightly lower than the average yield of the previous season.
The trial average for the four seasons in which the trial has been conducted to date is 41.1 t/ha.
Lady Luce and Lanorma produced the highest yields, with no statistical difference in yield. These cultivars also achieved the best marketing index. A good marketing index can be ascribed to a higher yield of large potatoes and/or potatoes of good quality.
Size group distribution and grading evaluations in potatoes are essential for determining a cultivar’s marketability (Figures 6 and 7).
The main reasons for downgrading are indicated in Table 4. Moth and soft rot were the major reasons for the downgrading of cultivar quality in this trial. The latter can be partly attributed to the excessively wet conditions during the period before harvest.
Given the nature of the seasons, the performance of cultivars 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 various seasons. The stability of cultivars over the past three seasons is illustrated in Figure 8. Mondial and Sound showed the greatest consistency across the three seasons.
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).
Several cultivars showed the required chip colour, but only Lady Alicia met both requirements.
Special thanks to the participating farm and collaborator, Fick & Son Bdy, the Eastern Free State working group, as well as participants FPD, GWK, and RSA. For more information, send an email to enrike@potatoes.co.za.