INTRODUCTION

Potato (Solanum tuberosum L.) is known to be one of the top three food crops that are consumed most in the world. It contributes the highest percentage of carbohydrates to the diet of millions of individuals in South America (Aguilar et al., 2024). It is also reported that the production of potatoes in Latin America has been increasing steadily over decades (Devaux et al., 2024).

With a history that spans over 8,000 years, Bolivia has been the center of domestication and genetic diversity for the potato (Morante, 2019;Verastegui-Matínez et al., 2023). While Bolivia has significant potato production potential, it presents various challenges for potato production due to adverse climatic events, inadequate disease controls, insufficient training of producers, etc. (Aguilar et al., 2024).

In particular, potatoes are highly susceptible to various pathogenic organisms, and diseases like late blight can lead to significant losses in both quality and yield of potato production (Aguilar et al., 2024). Consequently, potato cultivation requires the extensive use of pesticides to mitigate the negative impacts of diseases (Vilvert et al., 2022). In fact, in the Cochabamba region of Bolivia, pesticides and fungicide should be applied about 6 to 8 times during a single cultivation cycle, which amounts to approximately 2 liters of pesticides and 5.3 kg of fungicides per hectare.

For the pesticide use in potato cultivation in Bolivia, there are mainly two ongoing issues. First, their overuse and improper application of pesticides persist. The extensive use of pesticides is inducing detrimental health effects on the population and to contribute to increased pollution of land and water in Bolivia (Barrón Cuenca et al., 2024). Second, the seed treatment chemicals used for seed potatoes in Bolivia can be highly toxic. This is because the regulations on pesticides in Bolivia are weaker than those of other countries, and moreover, considerable amount of pesticides are reported to be illegally imported bypassing the regulations. For example, CRUISER, which is one of the popular seed treatment chemicals used in Bolivia, has been banned since 2018 in the whole European countries because its active ingredient, thiamethoxam, is bee-harming material. Numerous studies have emphasized the link between pesticide exposure and genotoxic damage, which is often measured through DNA strand breaks and/or micronuclei formation (Barrón Cuenca et al., 2024).

To address these issues, we explored the use of plant ash, an eco-friendly farming technique, for disinfecting seed potatoes. The ash treatment to seed potato pieces is a traditional eco-friendly method that has advantageous effects on disinfection and fertilization. First, it raises alkalinity on the cut surface so that it provides an unfavorable environment for many bacteria and fungi to grow, including the emergence of potato common scab (Bang-Andreasen et al., 2017). Second, applying dry materials like ash directly on the cut surface of seed potato pieces is known to promote the formation of callus during the curing process (Jennings et al., 1964). Third, the components of the plant ash including potassium, calcium, and phosphorus can supply nutrients to the potatoes (Mahmood et al., 2003).

Alongside this, we also compared the yields of experimental groups that were planted as whole tubers with those treated by cutting the seed potatoes. Those methods are also expected to help Bolivian farmers who face financial issues purchasing pesticides to continue potato production.

MATERIALS AND METHODS

The materials and methods used in this experiment are described below to provide a detailed account of the experimental setup and procedures.

1. Plant material

The variety evaluated was “Jatun Puka”, a promising new cultivar developed by Peru and being investigated by the Bolivian National Institute of Innovation in Agriculture and Forestry (INIAF). The potato variety has been produced since 2020 for its high tuber yield (60 to 70 t/ha) and its resistance to Phytophthora infestans. Currently, this variety is grown in high-altitude areas such as the Altiplano, as well as in low-lying areas such as the mesothermal valleys. The “Jatun Puka” plant is tall (up to 1.5 m) with thick stems with dark hues. The mature tuber is elongated and oblong, with a reddish skin and yellowish flesh.

The “Jatun Puka” seed potatoes for the experiments were all supplied from Chuquisaca in September 2024. They were sowed in October 2023 and were harvested in March 2024. After that, the potatoes had been stored at warehouse by the providers until we purchased them.

2. Description of study area

The experiment was conducted in three locations: KOPIA Bolivia Center and two farmers’ fields. The KOPIA Bolivia center is in Sipe Sipe (Latitude: -17° 26’ S, Longitude: -66° 22’ W), which i s at 23 km to the west part of the c ity of Cochabamba. One of the farmers’ fields is at Buena Vista Community of Mairana (Latitude: -18° 06’ S Longitude: -63° 55’ W), which is 3 hours to the west part of the city of Santa Cruz. The other farmers’ field is at Sapanani (Latitude: -17° 30’ S, Longitude: -66° 10’ W), which is in the east part of the city of Cochabamba. The experiment was conducted from September 2024 to March 2025. A general description of the experimental sites is provided in Table 2, as presented below. In addition, the information of the average temperature and precipitation at the regions during the period between sowing and harvesting is also shown in Table 3.

3. Experimental design and treatments

The experiment was conducted using a Randomized Complete Block Design (RCBD) with four replications. The experimental treatments were categorized into three primary groups: ash-applied (T1, T2), non-treated (T3, T4), and those subjected to chemical seed treatments (T5, T6) (Table 4). Each primary treatment group was further subdivided into two experimental conditions: whole tuber planting and cut tuber piece planting. Each experimental plot covered an area of 6.3 m² (2.1 m ^* 3.0 m) and comprised three rows. The inter-row and intra-plant spacings were 70 cm and 30 cm, respectively, while the spacing between plots and blocks was 50 cm and 70 cm, respectively.

4. Experiment procedure

First, the seed potatoes for T1, T3, and T5 were halved before sowing. In detail, they were cut along the natural orientation of the potato, from the rose end (where most eyes are) towards the stem end. We also ensured that each piece had at least one strong eye. Also note that the knives used in the process were sanitized before by putting it in boiling water.

After that, for T1 and T2, rice huck ash was applied on the cut surfaces of the potato pieces or on their surfaces of the whole tubers. In case of T5 and T6, chemical treatments were applied instead. Then, for the cut seed potatoes, 7 days of curing process were performed, drying them at ambient temperature.

Fertilization that consisted of N:P:K 18-46-0 (500 kg/ha) was applied at the time of sowing, with a subsequent application of N:P:K 20-20-20 51 days later. The seed treatments employed in treatments T5 and T6, as well as the pesticides utilized throughout the experiment, represented the most commonly used agrochemicals in the respective regions. Detailed information on these treatments and pesticide applications is provided in Table 5.

5. Data collection and analysis

Data were collected from 33 plants in each 6.3 m² experimental unit based on the following response variables.

A. Germination rate

Three weeks after germination, the number of plants in each experimental unit was counted, then this data was estimated as a percentage of the total number of seed potatoes planted in each experimental unit (33 seed potatoes).

B . Number of stems p er p lant

During flowering time, in the central row of each 6.3 m² experimental unit, the number of main stems of five representative plants was counted.

C. Tuber yield (t/ha)

During harvest time, the fresh weight (kg) of the total tubers in each experimental unit of 6.3 m² was recorded.

The data were analyzed with the statistical program SAS Version 6.12 (SAS Institute Inc., Cary, USA), and the PROC GLM command was used for the analysis of variance in multi-locations of the following response variables: germination rate, number of stems, and tuber yield. The comparison of means within the treatments was carried out with the PDIFF command and the H statistic. The means were separated when the differences were significant at 95% probability (Pr>F=0.05) and at 99% probability (Pr>F=0.01). In addition, for the process of Pearson correlation analysis between germination rates, number of stems, and yield, R language (4.5.1) was used.

RESULTS

In this section, we present a set of consecutive graphs showing the results of potato cultivation experiments in six different treatments, repeated in three different areas in Bolivia. The variables evaluated are the germination rate, the average number of stems, and total yield in weight. Each treatment is denoted as T1, T2, …, T6, respectively.

1. Germination rate

T4 was a generally effective option for all three locations while the result patterns differed greatly among the locations. In Mairana, there was a significant tendency where the germination rates were more than two times higher when using whole-tuber seed potatoes (T2, T4, T6) compared to their corresponding methods using cut seed potatoes (T1, T3, T5). When using whole-tuber seed potatoes, the germination rate of T6 was slightly higher than that of the other no cutting methods, T2 and T4. Among cutting methods, T1 showed the highest germination rate. In Sapanani, however, it was hard to find a clearly differentiated output for all six methods. What can be observed was that using chemical pesticides (T5, T6) slightly harmed the germination rate compared to the others in that area. Lastly, in Sipe Sipe, the e ffect o f the application of a shes a nd chemicals was weak for methods without cutting treatment (T2, T4, T6). When using cut tubers (T1, T3, T5), the germination rate was lowest for T5 and highest for T3.

Overall, as shown in Table 6, the germination rate was high at Sipe Sipe. Regarding after-treatment, the effect of ash treatment had little effect to the germination rate compared to no treatment group, while chemical treatment worsened the germination rate moderately. It was also shown that cutting seed potatoes before sowing had negative effect to germination rate in this experiment.

2. Number of stems

Figure 2 shows the average number of stems for each treatment. Overall, using whole-tuber seed potatoes (T2, T4, T6) was generally better than using cut seed potatoes (T1, T3, T5). Such a tendency was particularly clear in Mairana and Sipe Sipe, while it was vague in Sapanani. Among whole tuber methods (T2, T4, T6), applying chemicals (T6) showed the best result. It was also noticeable that the result of applying ashes to whole-tuber seed potatoes (T2) was still comparable with that of chemicals (T6).

Overall, as in Table 7, the number of s tems was high at Sapanani. Similar to the case of germination rate, the effect of ash treatment was not significant compared to no treatment group. However, in contrast, the effect of chemical treatment was positive. In addition, the difference of the numbers of stems was considerable between the cut seed potatoes and whole-tuber seed potatoes.

3. Yield in weight

We also examined the total yield of potatoes in weight for each experimental setting. In Mairana and Sipe Sipe, using whole tubers (T2, T4, T6) was always a better choice than u sing cut tubers ( T1, T3, T5). Moreover, especially when using whole seed potatoes, applying chemical pesticides resulted in higher yields than applying ashes in the two locations. However, the situation was different in Sapanani. In Sapanani, applying ashes to whole-tuber seed potatoes (T2) showed the highest amount of production among all six methods.

Overall, the yield per area was highest at Sapanani, and was lowest at Mairana. In terms of after-treatment, the yield of non-treated group was the highest, and the yields of ash treatment group and chemical treatment group were similar. Lastly, the whole-tuber seed potatoes showed better yield than cut seed potatoes, as in the case of germination rate and number of stems.

4. Correlation analysis

As in Figure 4, we calculated the correlation between germination rate∼number of stems, number of stems∼yield, and germination rate∼yield for Mairana, Sapanani, Sipe Sipe, and all regions. For each case, a single r value was calculated via Pearson correlation analysis where a value close to 1 or -1 indicates a strong correlation between the two variables. In particular, it is notable that the r value is nearly zero for all kind of correlations at the Sapanani region, while the r values of the correlations at the other regions are all at least over 0.75. It could be interpreted that there were some kinds of significant environmental factors or managemental factors during the experiment at the region.

The multi-location analysis of variance of germination percentage, number of stems, and tuber yield (Table 9), shows highly significant differences (Pr>F=0.01) for locations, treatments, and interaction location ^* treatment, except in the variable tuber yield, where the interaction was not significant. These results indicate that the effect of the treatments on the germination percentage and number of stems was different in at least one of the locations, while for tuber yield, the effect of the treatments was not different in the three locations. In other words, the pattern or tendency of the effect of the treatments was consistent in all three locations for tuber yield, without a combination of treatment and location showing exceptional result.

DISCUSSION

In this study, we explored the effect of seed treatment using plant ash, known for containing various essential nutrients, as well as the practice of planting seed potatoes as a whole versus cutting them. Although both of which are believed to increase yield, the strong interaction with the environment of the localities masked the effect.

Mairana was the region where the negative effect of using cut seed potatoes was the most severe among the three regions, also recording the smallest amount of yield. Considering the humid conditions and high temperature of Mairana (Table 2 and Table 3), the cut tubers would suffer from excess moisture and elevated temperatures, which promote the development of phytopathogenic microorganisms under such conditions. High humidity and temperature are known to create a favorable environment for the growth and spread of plant pathogens, particularly fungi and bacteria that cause soft rot, damping-off, and other tuber-borne diseases (Agrios, 2005;Garrett et al., 2006).

Moreover, in the Mairana case, the cut tubers were planted late after they had been cut, cured, and sent to the field because farmers were mobilized to extinguish forest fires nearby. It would be one of the causes that cutting seed potatoes showed significantly low germination rates in Mairana.

In Sapanani, the germination rates were low in general, with all methods less than 80%. This could be because the plot suffered a drought period due to the absence of an irrigation system for rainwater. In addition, Sapanani was the only location where the application of chemical treatment worsened the germination percentage.

In the other hand, the application of ash even worsened the productivity in Mairana and Sipe Sipe, but only excluding Sapanani. The diminished effectiveness of the ash treatment may be attributed to the soil’s high alkalinity in both Mairana and Sipe Sipe regions, where the pH levels were measured as 8.2 and 8.17, respectively (Table 10 and 12). Since the ash itself is also alkaline, the use of ash would raise the soil pH above the optimal range for potato growth (pH 5.5–7.0), which may have hindered the effectiveness of the treatment (Rengel, 2002;Waterer, 2002). In fact, high soil pH levels are associated with the increased incidence of common scab (Streptomyces scabies) in potatoes, a disease that thrives in alkaline conditions and can significantly affect tuber quality and yield (Lacey et al., 2001;Wanner, 2006). In Sapanani, the pH level was measured to be 7.1, which may justify the higher yield in that region (Table 11). In contrast, in South Korea, where ash treatment has been used effectively, the general pH level of agricultural soils is reported to be around 6.0 (Kang et al., 2012;Kong et al., 2015). Therefore, in the future, substances that lower soil pH, such as elemental sulfur, can be examined as eco-friendly seed treatment candidates for high-alkalinity soils in Bolivia (Chen et al., 2011).

In Sipe Sipe, the result of germination rates indicates that both the ash and the chemicals would negatively affect the formation of sprouts in the tuber when the seed potato is cut. It is also notable that the tendency continued for the number of stems and the yield, reassuring the negative effect of both ash and chemicals to cut tubers. In addition, the experimental field in Sipe Sipe suffered from exceptional rainfalls at the later stage of cultivation (Table 3), while the region has a dry climate normally (Table 2). It can be thought that the effect of the rainfall would have affected the final yield.

There is also a possibility that certain seed treatment chemicals were not proper for cutting seed potatoes (Nolte et al., 1987). A notable finding emerged when chemical seed treatment was used: while planting whole potatoes led to better germination and yield compared to other experimental groups, cutting the tubers resulted in lower yields than those of the other treatments. It means that it is needed to explore different seed treatment chemicals or their alternatives that are suitable for cut tubers of potatoes.

Lastly, while the effect of ash treatment was positive in certain condition, cutting the seed potatoes worsened germination rates, number of stems, and yield for almost every cases. However, since only the half of the amount of the seed potatoes was required for the equal area of the field compared to the whole-tuber sowing, cutting the seed potatoes can still have a strength in terms of yield per the weight of seed potatoes used per unit area, making it an adoptable choice in a special situation where the budget to buy seed potatoes should be primarily considered.

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