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The Loskop Valley area is a potato-producing region where about 3.3% of the country’s commercial potatoes, mainly for processing purposes, are produced by 18 producers on approximately 1 715 ha (2024 harvest year). The most prominent cultivars grown in the area for processing purposes are FL2108, Hertha, Markies, and Innovator.
The trial was conducted between Marble Hall and Groblersdal. The area falls within South Africa’s temperate summer rainfall region and has recorded an average annual rainfall of 459 mm over the past 22 years. The region experiences very hot summers and winters, with the possibility of frost from July to August.
The cultivar trial was laid out in a randomised block design with three replications per cultivar. Table 1 contains relevant technical information regarding the trial. Soil analysis results are presented in Table 2.
Effect of growing periods
Included in the cultivar trial were cultivars with medium to 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 amount of time from emergence to natural leaf senescence. Table 3 outlines how these growth periods differ from cultivar to cultivar.
Plant density and the number of stems per tuber affect tuber size and yield. The number of eyes per tuber is cultivar-dependent and determines the number of sprouts produced per tuber. In this regard, seed maturity is very important, as ideal plant readiness generally results in better sprouting and more stems per sprout.
Table 3 sets out the plant readiness of tubers at the time of planting, as well as the haulm count observed later in the growing season.
Cultivar evaluation
The evaluation of new cultivars, such as in the Marble Hall cultivar trial, provides results relating to yield and marketing index. The potatoes in this trial were only sorted into size categories on a table intended for processing markets. Sizes were defined as large (70 mm in diameter), large medium (65 mm), medium (55 mm), small (50 mm), and extra small (45 mm).
The quality of the potatoes was very good, and no significant reasons for downgrading were observed or recorded. Price comparisons were made with average market prices 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 several seasons.
Water and temperature
As with any crop, aspects 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 Agricultural Research Council (ARC) weather station.
The rainfall trend for the 2024 season (Figure 2) shows considerably lower rainfall than the normal long-term average during the second and third months after planting (approximately from tuber initiation to tuber bulking). Irrigation scheduling was managed well during the growing period, which was mainly in the winter months. Higher rainfall followed in October during the late stages and at leaf senescence.
Minimum and maximum temperatures are shown in Figure 3.
A total of 40 days with maximum temperatures above 30°C and 12 days above 35°C were recorded during the season, including a week in mid-September when temperatures consistently exceeded 30°C and even 35°C. A dramatic fluctuation in maximum and minimum temperatures at critical growth stages can have a negative impact on tuber initiation and development. At the beginning of July, the co-worker recorded heavy frost on the farm where the trial was located, which played a role in the final yield.
The collection of heat units during a growth period is an important factor in understanding crop growth and development. The trend of heat units available for this season’s cultivar trial appears to be somewhat higher than the long-term heat unit data trend (Figure 4), with August and September showing high maximum temperatures.
Yield and 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 mean.
The cultivar effect during this trial (Figure 5) was statistically significant (p<0.05) and the coefficient of variation (CV) was within limits (16.6%). 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 season was 42.8 t/ha, a few tonnes per ha lower than the previous season. Sound, Moonlight, and P1 realised the highest yields, and the same three cultivars also reached the best marketing index.
Tuber size distribution is an important factor when evaluating a cultivar’s marketability in the processing industry (Figure 6).
Processors regard aspects such as size, quality, tuber shape, along with specific gravity (SG) as significant.
Quality characteristics for processing
Lastly, processing characteristics can also be evaluated by looking at the internal quality of potatoes. To adhere to processing requirements, cultivars must meet a chip colour norm of >50 and a specific gravity (SG) of ≥1.075 (Table 4).
All cultivars met the chip colour and SG requirements. Internal quality was consistently good, and no significant issues such as brown spot or hollow heart were observed. Table 5 contains photographs of the baking tests. – Enrike Verster, Aartappels SA
A special word of thanks to the producer Jaco van den Heever, Danie Marais and Frank Osler of PepsiCo, Jeanine van Jaarsveld of FPD, and Damien Da Cal and Dikgetho Mokoena of Potatoes SA. For more information, send an email to enrike@potatoes.co.za