Potato cultivar evaluation at the Cedara and Mkhathini research stations, 2024/25
The Cedara research station (29° 32’ 15 33 S, 30° 16’ 09 19 E) is situated in the moist mist-belt zone of the KwaZulu-Natal Midlands which lies 900 to 1 400 m above sea level. The area receives an average annual rainfall of between 838 and 1 140 mm, depending on locality (Figure 1).
The warm, wet summer conditions favour the development of early blight (Alternaria solani) (Sorauer) and late blight (Phytophthora infestans) (Mont. de Bary) in potatoes (Solanum tuberosum). The severity of these diseases, especially late blight, can result in yield losses of up to 100% if fungicides are not applied regularly as recommended.
Consequently, seed and table potato producers could incur substantial financial losses, whereas householders who lack resources for fungicides, could become food-insecure if their crop fails. A trial was therefore conducted to identify cultivars with good tolerance to late blight in potatoes.
Method and materials
Thirteen cultivars were evaluated under irrigation in a split-plot trial with three replications. The cultivars were planted on the main plot, which was divided into weekly fungicide applications from ridging until 90% senescence, and no-fungicide applications.
A combination of six fungicides and five insecticides was applied in a weekly rotation. The active ingredients included chlorothalonil + lambda-cyhalothrin, cymoxanil/famoxadone + mancozeb + abamectin, cymoxanil/mancozeb + cyromazine, difenoconazole + chlorantraniliprole, mancozeb + cypermethrin and metalaxyl-M + cypermethrin. In total, 12 sprays were administered. Insecticides were applied regularly across the entire trial.
The Cedara trial was planted on 19 September 2024 at a seeding rate of 37 037 tubers/ha-1. Each split-plot consisted of four rows, each 5 m in length and spaced 0.9 m apart. Data was collected from the two middle rows.
Fertiliser was applied according to FertRec recommendations for a 70 t/ha-1 yield, based on the results of a soil analysis. In total, 80 kg/ha-1 phosphorus, 180 kg/ha-1 potassium, and 240 kg/ha-1 nitrogen were applied. Twelve ratings of disease severity were done from 6 November 2024 to 22 January 2025. Harvest occurred two to three weeks after the senescence of each harvestready plot.
Climate conditions
During the 2024/25 planting season, total rainfall recorded between planting (14 September) and harvest (27 January) was 620 mm, indicating a slightly wetter season. This amount was almost 100 mm higher than the long-term mean (Table 1).
Irrigation is recommended to cease two to three weeks before harvest. However, heavy rainfall in January (167 mm) resulted in unfavourable conditions during the crop maturity stage, negatively impacting yields across all cultivars.
Plant population is important because it directly influences tuber yield. All cultivars were planted at 37 037 tubers/ha-1. Plant population did not differ significantly among cultivars, except for Lady Alicia, which had the lowest plant population at 32 498 plants/ha-1. Plant populations for the remaining cultivars ranged from 37 037 to 36 129 plants/ha-1. However, they did not differ significantly (Table 2).
Since the majority (92%) of the 13 cultivars did not differ significantly, plant population (plants/ha-1) can be treated as a confounding factor and is therefore assumed not to have influenced performance.
Number of haulms per plant
Cultivars differed significantly in the number of haulms per plant (Table 2). Foxy, Mondial, Valor, Moonlight and Lilly, had a higher mean number of haulms/plant, ranging from 5.5 to 9.9. Sound and Noya followed, with values ranging from 4.4 to 4.7 haulms/plant.
The lowest haulm numbers were recorded in 11Z55A5, Lady Alicia, Lady Luce, Lanorma, Palace, and Markies, with no significant differences among them. The cultivar’s genetic makeup and the environmental conditions in which it is grown largely determine the number of haulms/plant.
Late blight epidemiology
The interaction between leaf wetness, inoculum density, daytime temperatures (19 to 22°C), and relative humidity or RH (90 to 100%) creates favourable conditions for late blight spore germination (Rotem et al., 1971).
Late blight can build up rapidly, as one lesion can produce 9.8 x 105 sporangia from a single leaflet of a susceptible potato cultivar within 12 days under favourable conditions (Harrison, 1992).
Within days of infection, new sporangiospores emerge through the leaf stomata. Sporangia may form within four days of infection under favourable weather conditions (Tsedeley, 2014). The Cedara trial site is regarded as the most conducive environment for the build-up of late blight due to environmental conditions such as RH and temperatures.
Cultivar trial results
In the present study, at 83 days after planting (DAP) without fungicide application, Lilly (82%), Foxy (62%), 11Z55A5 (62%), Lady Alicia (38%), and Noya (35%) exhibited early signs of late blight infection between 35 and 82%.
In contrast, Lady Luce (23%), Sound (22%), Lanorma (18%), Mondial (15%), Markies (10%), Moonlight (10%), Valor (6%), and Palace (5%) had relatively low disease infection between 5 and 23% (Figure 2).
At 97 DAP, with no fungicide application, only Moonlight (28%), Palace (25%), Markies (23%), and Valor (22%) maintained infection levels below 30%, indicating strong disease tolerance. At 104 DAP, only Valor (39%) and Markies (45%) showed late blight infection below 50% (Figure 2).
With fungicide application, disease infection at 83 DAP was relatively lower (1 to 5%) across all cultivars, indicating the positive impact of fungicide application in suppressing late blight infection by more than a week (Figure 3).
With fungicide application, 11Z55A5 (83%) and Foxy (65%) were the only cultivars with late blight infection exceeding 60% (Figure 3).
The no-fungicide treatment took 83 DAP to reach a mean of 30% late blight infection, compared to 104 DAP with fungicide application. Similarly, the no-fungicide treatment took 97 DAP to reach 50% late blight infection, compared to the fungicide treatment.
Finally, the no-fungicide treatment took 111 DAP to reach 90% infection, compared to 118 DAP for the fungicide treatment (Table 3).
Results indicated that early infection ( 40 t/ha-1) and low-yielding cultivars (< 40 t/ha-1) showed no significant differences between fungicide and no-fungicide treatments (Table 4). High-yielding cultivars with no differences between treatments were Moonlight and Noya. Lilly, Lanorma, and Mondial were the only cultivars that produced high yields, specifically when sprayed with fungicides, with significant differences observed between fungicide and no-fungicide treatments (Table 4).
Major differences in tuber yield between fungicide and no-fungicide treatments were observed for Lilly (18 t/ha-1), Lanorma (15 t/ha-1), and Mondial (20 t/ha-1). For these cultivars, fungicide application is therefore crucial for high yields because they were more susceptible to late blight infection without fungicides in the 2024/25 planting season.
Moonlight and Noya were the only cultivars that were high-yielding (>40 t/ha-1) in both the fungicide and no-fungicide treatments. This strongly indicates tolerance to late blight, especially since there were no differences between the fungicide and no-fungicide treatments.
Tuber yield classes
Tuber yield percentages were evaluated for different tuber classes large, largemedium, medium, small, and baby for both fungicide and no-fungicide treatments. For tuber yield, only large and medium showed considerable differences between the fungicide (24.64%) and no-fungicide (19.79%) treatments (Figure 5).
A 9% tuber yield increase was recorded in the large class when fungicides were applied (p < 0.001). Similarly, in the large-medium class, a tuber yield increase of 3% was recorded with fungicide application (p = 0.05). No significant differences were measured in the medium class in terms of fungicide application. For small and baby tuber sizes, the no-fungicide treatment showed an 8% higher tuber yield than the fungicide treatment (Figure 5).
The study revealed a seesaw trend: Fungicide application increased the proportion of large and large-medium tubers, while no fungicide led to a higher proportion of small and babysized tubers.
Overall, some cultivars responded positively to fungicide application. Lanorma (22%), Noya (19%), 11Z55A5 (17%) Lady Luce (15%), and Palace (14%) showed a positive response above 10% to fungicide application for the large weight class (Figure 6). For the largemedium class, Lady Luce and Lady Alicia recorded yield increases of 17 and 12%, respectively, under fungicide treatment compared to no fungicide.
Specific gravity
Specific gravity (SG) differed significantly between spray treatments and cultivars (p < 0.001) (Table 5). Lilly (1.056) and Mondial (1.053) had the lowest SG values, with no significant difference between the two cultivars. Lady Alicia (1.083) was the only cultivar recorded with an SG close to the acceptable market processing benchmark of 1.085.
Recommendations
It is recommended that producers plant high-yielding cultivars to meet the yield and quality requirements of their intended markets. Based on the 2024/25 season study, high-yielding cultivars with more than 40 t/ha-1 when sprayed with fungicides are recommended.
These cultivars include Palace (54.54 t/ha-1), Lady Luce (51.57 t/ha-1), Moonlight (47.99 t/ha-1), Noya (47.11 t/ha-1),Lilly (44.3 t/ha-1), Markies (43.47 t/ha-1), Foxy (43.33 t/ha-1), Lanorma (42.24 t/ha-1), and Mondial (41.51 t/ha-1). For financially constrained producers, Moonlight (40.51 t/ha-1) and Noya (40.91 t/ha-1) were the only cultivars to yield above 40 t/ha-1 without a fungicide application, showing stronger tolerance to late blight. – Khethuxolo Mbotho, Sibonelo Gumede, and Taslos Magubane,
KwaZulu-Natal Department of Agriculture and Rural Development
For more information or references, send an email to Khethuxolo.Mbotho@kzndard.gov.za