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Limpopo is a major potato production area where approximately 24.8% of the country’s total commercial potato harvest is produced by 121 producers on approximately 12 760 ha. The most prominent cultivars produced here for table and processing purposes are Mondial, Panamera, and Sifra.
Dendron is located in South Africa’s tropical summer rainfall area (Figure 1) and has recorded an average annual rainfall of 375 mm over the past 23 years, according to data from the nearest Agricultural Research Council (ARC) weather station. The winters in this region are cool and short, whereas the summers are warm and long.
During the winter months of May to July the region is characterised by average long-term temperatures of minimum 6°C and maximum 24°C, with very few days with temperatures below 0°C. The area lends itself to two production seasons: an early planting from January to March, and a main planting from April to July. Because of the two planting seasons, this region’s contribution to the industry is one of South Africa’s biggest.
Trial design and details
The Dendron cultivar trial is laid out in a random block design with three replications per cultivar. Table 1 contains additional 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.
For that reason, growing periods can influence the eventual 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 between cultivars. Table 3 also sets out the plant readiness of tubers at the time of planting, as well as the haulm count observed later in the growing season.
Marketing related results
The evaluation of new cultivars as in the case of the Dendron cultivar trial also 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).
Prices are 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 specific season, as climate differs from one year to the next. Therefore, cultivars are preferably tested across a number of seasons.
Effect of climate factors
As with any crop, factors such as temperature, water availability (whether good irrigation scheduling or rainfall), as well as heat units have a significant influence on the potato plant during its growth period. These factors are considered when evaluating the performance of cultivars.
Applicable daily weather data for the season is obtained from a weather station located close to the trial, whereas long-term weather data is obtained from the nearest ARC weather station.
Rainfall data reflects almost no rain for the period from plant to harvest, although the long-term average rainfall for this summer rainfall area is approximately 45 mm in total for the period from May to October. Sufficient irrigation scheduling is therefore very important, especially when it is warm during important growing periods.
Minimum and maximum temperatures are set out in Figure 2.
It reflects significant variation in maximum and minimum temperatures during the last three months of the season. While no frost was recorded by the nearby weather station, records did indicate frost at the trial site. Sound was the cultivar that stood out after these frost conditions with lush foliage that stood higher than that of the other cultivars. This alludes to Sound’s good frost tolerance in this area.
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 Dendron cultivar trial appears to be higher than the long-term data trend of heat units (Figure 3).
Cultivar effect
Yield data recorded during harvesting is statistically processed using the GenStat® program. The Tukey test of least significant differences (LSD) was used to separate the average. The cultivar effect during this trial (Figure 4) was statistically significant (p<0.05) and the coefficient of variation (CV) was good (8.4%). These factors indicate that the trial was well executed, and that 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.
Trial yield
The average yield of the trial for the 2024 season was 63.15 t/ha. This was higher than the previous four seasons’ average of 57.38 t/ha. Foxy, Sound, and Mondial delivered the best yield with no statistical difference in yield.
The same three cultivars also achieved the best marketing indices.
Unfortunately, no marketing index was recorded for Valor, seeing as the varieties were mixed during classing and sorting before Valor could be evaluated. A good marketing index is ascribed to a higher yield of large potatoes and/or a good percentage of good quality potatoes. Size group distribution and grading are significant evaluations when looking at the marketability of a potato cultivar (Figures 5 and 6).
The main reasons for downgrading are indicated in Table 4. Insect damage in the form of cutworm holes as well as soft rot had the greatest influence on the downgrading of potatoes in this trial.
Cultivar performance
Due to 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. Figure 7 illustrates the last three seasons’ variations in cultivar performance.
Lastly, processing characteristics can also be evaluated when looking at the internal quality of potatoes. To meet processing requirements, cultivars have to meet a chip colour norm of >50 and a specific gravity (SG) of ≥1.075 (Table 5).
ARC analyses reveal that all cultivars met the norms for correct chip colour, but only Lady Alicia and Prince met the SG requirement. However, during the internal quality evaluation Lady Alicia exhibited the beginning stages of hollow heart, which was detected on a larger and more progressive scale in Markies. – Enrike Verster, Potatoes SA
Special thanks to the producer Mossie Jongbloed, working group chair Schalk Grobbelaar, participants FPD and RSA, as well as Ané du Plessis and other Potatoes SA staff. For more information, send an email to enrike@potatoes.co.za