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The Eastern Free State is a major potato producing region where 19% of the country’s commercial potatoes are produced on approximately 9 522 ha (2023 harvest year). The most prominent cultivars produced for table usage are Mondial, Panamera and Lanorma.
Bethlehem is located in South Africa’s moderate summer rainfall area (Figure 1) and received average rainfall of approximately 684 mm over the past 19 years. The region experiences warm summers and very cold winters with possible frost from mid-May to early September.
Trial design
The Bethlehem cultivar trial was planted in a randomised block design with three replications per cultivar. Table 1 contains additional technical information relevant to the trial.
Included in the cultivar trial were cultivars with short to long growth periods. Growth periods can, therefore, influence the eventual yield of certain cultivars. Growth period length is subject to the nature of the season, but is regarded as the amount of time that passes from emergence to natural leaf senescence. Soil analyses of the pivot on which the trial was executed are indicated in Table 2. Table 3 provides an outline of how these growth periods vary from one cultivar to the next.
Plant readiness of the seed potatoes at the time of the trial, as well as plant density and haulm count observed later on in the growing period, are also indicated in Table 3.
Classing and sorting
The evaluation of new cultivars in the Bethlehem trial delivered results regarding, among others, yield and marketing index. The marketing index of the relevant cultivars is calculated by classing and sorting each cultivar according to quality and size distribution, for example, Class 1 Large or Class 2 Large-medium. Price comparisons were then made to market prices at harvesting time.
Read more about Potatoes SA’s new cultivar trial dashboard here.
The performance of new cultivars cannot be based on the results of one particular season only, since climate and seed potato quality can vary from one year to the next. It is for this very reason that cultivars are preferably tested across several seasons.
Temperatures and water
As with any crop, temperature, availability of water (good irrigation schedules or rainfall), as well as heat units are important factors with a significant influence on the potato plant’s growth period. These factors are therefore taken into consideration when evaluating cultivar performance. Relevant daily and long-term weather data is obtained from a selected Agricultural Research Council (ARC) weather station that is as close as possible to the trial site.
Rainfall figures in the 2023/24 season (Figure 2) were generally similar to the long-term average recorded per month at the start. However, since January, less rainfall (close to the end of the growth period) was recorded. Overall, less rainfall was ultimately recorded for the duration of the trial.
Influence of heat units
Figure 3 illustrates minimum and maximum temperatures. This particular season was characterised by particularly significant fluctuations in maximum temperatures. Far-above average temperatures were recorded for a period of three months in the last months before harvesting.
Heat units are another important factor to consider, as the development of the plant is mainly based on the collection of heat units during a growth period. The trend of available heat units for this cultivar trial at Bethlehem was higher throughout in respect of the cumulative long-term data of heat units (Figure 4).
The yield index
Yield data collected during harvest day is statistically processed using the GenStat® program. The mean was separated using the Tukey test of least significant differences (LSD). The cultivar effect during this trial (Figure 5) was statistically significant (p<0.05) in terms of yield while the coefficient of variation (CV) was low (11.6%). 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 (the average of all the cultivars is accepted as 100%). This creates a yield index and each cultivar’s performance in terms of yield is read as a percentage of the trial average.
Trial yields
The average yield of the cultivar trial for the 2023/24 season was 78.91 t/ha. Lilly, Mondial, Palace, Sound and Foxy delivered the highest yield with no statistical differences in yield. Lanorma, Palace and Lilly achieved the best marketing indices of Large potatoes and/or the highest number of good-quality potatoes.
Size distribution and grading are indispensable evaluations when studying a cultivar’s marketability (Figures 6 and 7). Reasons for downgrading are indicated in Table 4. The main reasons for downgrading the quality of cultivars in this trial were moths, rot, and silver scab.
As seasons tend to fluctuate, the performance of cultivars changes from one season to the next. This is simply because the climate is never the same from one season to another. This was the first year that this trial was conducted, hence such a comparison was not possible.
Finally, when observing the internal quality of potatoes, processing characteristics can also be evaluated. To comply with processing requirements, cultivars have to comply with a chip colour norm of >50 and a specific gravity (SG) of ≥1.075 (Table 5). The cultivars 11Z55A5, Noya, Palace and Prince met the chip colour and SG requirements. – Enrike Verster and Anjé Venter, Potatoes SA
