The Effect of Growth Regulators and a Biostimulator on the Health Status, Yield and Yield Components of Potatoes (Solanum tuberosum L.)

A micro-plot experiment with a randomized sub-block design and three replications was conducted in Tomaszkowo near Olsztyn (53°41′N, 20°24′E) in 2013–2015. Five cultivars of edible potatoes were cultivated, including two cultivars with cream- and yellow-colored flesh—Irga (Poland) and Satina (Germany), two cultivars with purple-colored flesh—Valfi (Czech Republic) and Blaue St. Galler (Switzerland), and one cultivar with red-colored flesh—Highland Burgundy Red (France and Great Britain). Potato cultivars with cream- and yellow-colored flesh (Irga and Satina) are listed in the National Register of Plant Varieties, and they are characterized by high susceptibility to Phytophthora infestans (severity score of 2 to 3 points). The experiment was carried out on podzolic soil with the granulometric composition of light loam, characterized by high suitability for the cultivation of rye (suitability complex 4) and quality class IIIb (WRB 2014). Potatoes were grown in soil with the following parameters: pH in KCl 4.04–4.54 (PN ISO 10390:1997); levels of available minerals: P—85.6–111.8 mg kg⁻¹ soil (PN-R-04023:1996), K—104.2–204.2 mg kg⁻¹ soil (PN-R-04022:1996 + Az1:2002), Mg—38.0–42.0 mg kg⁻¹ soil (PN-R-04020:1994 + Az1:2004); organic carbon content—10.4–10.8 g kg⁻¹ and nitrogen total—0.71–0.76 g kg⁻¹ (PB 29 ed. 4 27.11.2014). The content of exchangeable bases in mg kg⁻¹ soil was determined by flame photometry at: K—130.0–244.0; Ca—350.0–517.0; Na—120.0–140.0 and Mg—50.0–55.0 (Chemical and Agricultural Station in Olsztyn). Cereals were the preceding crops: winter rye in 2013, winter triticale in 2014 and oats in 2015. Farmyard manure (FYM) was applied in autumn at 25 t ha⁻¹, and mineral fertilizers were applied in spring at 40 kg N ha⁻¹ (46% urea), 60 kg P ha⁻¹ (17.45% P, superphosphate) and 100 kg K ha⁻¹ (50% K, potash salt). Potato tubers were planted in heated soil, 40 cm apart, with inter-row spacing of 67.5 cm, on 30 April 2013, 28 April 2014 and 23 April 2015.

A biostimulator and growth regulators were applied in doses recommended by the manufacturers at 10–14 day intervals during the growing season (beginning in stage BBCH 39—crop cover complete). The experiment involved the following treatments:

Plots where growth regulators were not applied were the control.

All plots were subjected to identical cultivation treatments (according to the recommendations of the Institute of Soil Science and Plant Cultivation in Puławy), and weeds were controlled mechanically. Pests were controlled (according to the recommendations of the Institute of Plant Protection in Poznań) with thiacloprid at 0.1 dm³ ha⁻¹, acetamipirid at 0.08 kg ha⁻¹, lambda-cyhalothrin at 0.16 dm³ ha⁻¹; and pathogens were controlled in stage BBCH 20–89 with propamocarb hydrochloride and fluopicolide at 1.4 dm³ ha⁻¹, propamocarb hydrochloride and fenamidone at 2 dm³ ha⁻¹, dimethomorph and mancozeb at 2 kg ha⁻¹, and mancozeb and cymoxanil at 2 kg ha⁻¹. Potato plants were naturally infected by pathogens. Tubers were harvested on 26 August 2013 and 2014, 28 August 2015.

The severity of late blight (Phytophthora infestans) and early blight (Alternaria solani, A. alternata) was evaluated three times during the experiment on a 9-point scale (Pietkiewicz 1985; 1—no symptoms, 9—all leaves damaged, shoots partially or completely dry). The results were expressed as a percentage by calculating the infection index Ii according to the formula proposed by McKinney (Łacicowa 1970):where

Potato tuber yield was calculated in three size fractions: horizontal tuber diameter <35 mm, 35–50 mm and >50 mm.

The average monthly temperatures between May and August in 2013–2015 were similar to the long-term average. In the analyzed months, total precipitation was consistent with the long-term average in the first year of the experiment and was 24% below the long-term average in the second year. Weather conditions in the first two growing seasons were conducive to the spread of Phytophthora infestans infections. The second year of the study was characterized by low temperatures and low precipitation levels in May and more equally distributed precipitation in the summer months, and it was less favorable for potato cultivation than the first year. Weather conditions in 2014 were also more conducive to the spread of late blight. In the growing season of 2015, precipitation reached 155.4 mm (43% lower than the long-term average), which contributed to the spread of early blight (Table 1).

Late blight was most prevalent (≥50%) in 2014 in cv. Valfi and Blaue St. Galler treated with Bio-Algeen S‑90 and Kelpak SL growth regulators, and in cv. HB Red treated with Trifender WP (Table 2). The disease was less prevalent in the remaining years of the experiment, and the highest values of the infection index were noted in untreated potatoes of cv. Valfi (44.1% in 2013 and 36.0% in 2015). An analysis of average values of the infection index revealed significant differences in the prevalence of late blight between cultivars and years (Fig. 1). HB Red was the least infected cultivar in the first and last year of the study, and its infection index values differed significantly in comparison with the remaining cultivars. Potato cvs. Irga and Satina (with cream- and yellow-colored flesh) were healthier than Valfi and Blaue St. Galler (with purple-colored flesh), in the analyzed period. Tierno and Ruiz de Galarreta (2016) reported that HB Red with red-colored flesh is susceptible to infections caused by Phytophthora infestans, whereas Valfi with purple-colored flesh is moderately sensitive to this pathogen. Hamouz et al. (2010) demonstrated that the content of phenolic compounds in potato tubers of cv. Valfi was 2.46 to 3.18-fold higher than in cultivars with yellow-colored flesh. Kröner et al. (2012) and Brazinskiene et al. (2014) did notobserve significant correlations between the phenolic acid content of potato tubers and the spread of P. infestans. According to the cited authors, the incidence of disease is more likely to be influenced by the production system than the phenolic acid content of tubers.

In all experimental years, the applied growth regulators and biostimulator reduced the severity of late blight in traditional cultivars Irga and Satina, but the observed differences were not significant relative to the control treatment (excluding Irga treated with Kelpak in 2013). In 2013, infection severity was significantly reduced in cv. Valfi treated with Bio-Algeen S‑90, Kelpak SL and Trifender WP, and in cv. Blaue St. Galler treated with Trifender WP. In an in vitro study by Stephan et al. (2005), ELOT-Vis (growth regulator containing plant extracts, available on the German market) effectively inhibited the growth of Phytophthora infestans. ELOT-Vis combined with a 5% aqueous extract of Rheum rhabarbarum completely inhibited mycelial growth, which was stimulated by the addition of an aqueous extract of Solidago canadensis. In a study exploring alternative solutions to copper-based compounds, products containing Bacillus subtilis and Trichoderma spp. influenced the severity of leaf infections with Phytophthora infestans, subject to the date of application (Stephan et al. 2005). Kowalska and Remlein-Starosta (2012) demonstrated that one soil treatment and four foliar treatments with the Trichoderma asperellum fungus were as effective in reducing the severity of late blight in potatoes of cv. Impala as two applications of the copper-based fungicide Miedzian 50 WP. In the potato cv. Tajfun subjected to ten foliar applications of T. asperellum, the severity of the disease was reduced by around 30% relative to unprotected plants. Sawicka (2003) demonstrated that the combined application of the Asahi SL biostimulator and foliar fertilizers delayed P. infestans infections in a potato farm and prolonged plant growth by 2 to 14 days, subject to cultivar. In a study by Sawicka and Skiba (2009), the Asahi SL delayed the first symptoms of P. infestans infection by 3 to 7 days. Cwalina-Ambroziak et al. (2015) reported a decrease in the severity of late blight in potatoes of cv. Irga and Satina treated with Bio-Algeen S‑90 and Kelpak SL growth regulators. In a study by Uromova et al. (2016), greenhouse-grown potatoes of cv. Udacha were characterized by higher resistance to late blight after the application of the Agat-25K growth regulator.

In the present experiment, the prevalence of Alternaria spp. was lowest in 2014 when weather conditions were more conducive to the spread of late blight. The lowest value of the infection index (6.8%) was noted in HB Red plants treated with Trifender WP, and it differed significantly from the values noted in cv. Irga (excluding the Kelpak SL treatment) and cv. Satina (Table 3). The highest value of the infection index (29.9%) was observed in untreated potatoes of cv. Irga in the last year of the study (2015). HB Red was also least susceptible to early blight in the remaining years of the experiment, and its index values differed significantly relative to the remaining cultivars in 2013, and relative to Irga and Satina in 2015. In plots were growth regulators and the biostimulator were applied, the severity of disease was significantly reduced in cv. Irga in 2015. An analysis of average values for the entire experiment revealed that HB Red was significantly less infected by Alternaria spp. (Fig. 2). Infections caused by A. alternata have been shown to increase the content of phenolic acids in tomatoes (López-Gresa et al. 2011; Wojciechowska et al. 2014). Plants rapidly accumulate phenolic compounds in response to a pathogenic infection (Gogoi et al. 2001). According to Sawicka and Krochmal-Marczak (2008), Asahi SL is highly effective in reducing the spread of early blight in potatoes. This biostimulator is particularly useful under adverse weather conditions. Sawicka and Skiba (2009) demonstrated that in plants treated with Asahi SL, the infection of 50% of the leaf area by P. infestans was delayed by 22 days. Cwalina-Ambroziak et al. (2015) informed a decrease the severity of early blight in potatoes of cv. Irga and Satina in the treatments where Asahi SL, Bio-Algeen S‑90 and Kelpak SL growth regulators were applied. In a study by Chowdappa et al. (2013), isolates of Trichoderma harzianum OTPB3 inhibited mycelial growth of A. solani and P. infestans in vitro. In tomato seedlings, the fungal spore suspension enhanced systemic resistance through induction of growth hormones (indole-3-acetic acid, IAA and gibberellic acid, GA3) and defense-related enzymes (peroxidase, polyphenol oxidase and superoxide dismutase). Hernández-Herrera et al. (2014) demonstrated that application of extracts from green (Ulva lactuca, Caulerpa sertularioides) and brown alga (P. gymnospora, Sargassum liebmannii) have suppressive effects on Alternaria solani infecting tomato.

An analysis of average values revealed that the yield of Irga and Satina tubers in all treatments significantly exceeded the yield of remaining potato cultivars in all experimental years (Table 4, Fig 3a). The above cultivars produced the highest yields in the first year of the study, and Irga potatoes treated with Klepak SL was characterized by the heaviest tubers (524.67 dt ha⁻¹). In the remaining two years, traditional cultivars were characterized by similar yields, and the highest yields were noted in 2014 in cv. Satina treated with Trifender WP (around 290 dt ha⁻¹). The yield of potato tubers treated with growth regulators increased in Irga and Satina cultivars in the first two years of the study, in cv. HB Red in the first year of the study. The tested growth regulators Kelpak SL and Bio-Algeen S‑90 induced a significant increase in the tuber yield of cvs. Irga and HB Red, respectively, in the first year of the experiment, and Trifender WP increased the tuber yield of cv. Satina in the second year. In general, the growth regulators and biostimulator increased potato yields in 2013 and 2014 (Fig. 3b). Czeczko and Mikos-Bielak (2004) demonstrated that the yield of potato tubers increased by around 14% already after one foliar application of Asahi SL. In a study by Kowalska and Remlein-Starosta (2012), the yield of potato tubers treated with Trifender WP was nearly two-fold higher than the yield of untreated plants. According to Wierzbowska et al. (2015), Bio-Algeen S‑90 and Kelpak SL growth regulators increased the tuber yield of Irga by 8.9% and 15.6%, respectively, and the tuber yield of Satina by 14.7% and 18.3%, respectively. The cited authors also reported an increase in the nitrate (V) content of tubers harvested from potato plants treated with Kelpak SL. Haider et al. (2012) evaluated an organic biostimulator containing algae extracts which was applied to the leaves of potato plants of cv. Sante. The evaluated product increased plant growth parameters, including plant height, number of stems, tuber yield and tuber quality (higher content of dry matter, protein, N, P and K). In a study by Uromova et al. (2016), the HB-101 growth regulator increased the tuber yield of Udacha potatoes grown in a greenhouse.

In the first two years of the study, large and medium-sized tubers accounted for approximately 84% and 77% in cv. Irga and Satina, respectively (with a 50% share of medium-sized tubers), which was reflected in tuber yield (Table 5). In the first year of the study, the applied growth regulators and biostimulator significantly increased the percentage of medium-sized tubers of cv. Blaue St. Galler, and Bio-Algeen S‑90 increased the percentage of medium-sized tubers of cv. HB Red. In potato cvs. Valfi, Blaue St. Galler and HB Red (with colored flesh), large tubers were noted sporadically in all years of the experiment. The highest percentage of medium-sized tubers in the total yield was observed in cv. Valfi (58%) in the first year of the study, and in cv. Valfi and Blaue St. Galler (50%) in the second year of the experiment. In the last year of the study, potato cultivars Valfi, Blaue St. Galler and HB Red (with purple- and red-colored flesh) were characterized by predominance (76–92%) of small tubers. According to Bohl et al. (2011), growth regulators decrease the percentage of medium-sized potato tubers. Blauer et al. (2013) demonstrated that the application of gibberellic acid (GA) decreased the percentage of tubers weighing 198 g and more and increased the share of smaller tubers in red-skinned potatoes of cv. Chieftain. The tuber yield of cv. Chieftain and Satina differed subject to GA concentration. Thornton et al. (2014) observed that ethephon (2-chloroethylphosphonic acid) can decrease the average size of potato tubers. Foliar application of ethephon to red-skinned potatoes of cv. LaSoda increased the percentage of small tubers, but did not decrease marketable yield (Buhrig et al. 2015).

In this experiment, potato tuber yield was influenced by the severity of P. infestans infections during the growing season. Tuber yield decreased with a rise in the prevalence of late blight in the analyzed period of 2013–2015, as revealed by the calculated coefficients of correlation (Pearson’s r): r = −0.576, r = −0.387, r = −0.182, respectively (Fig. 4). In contrast, tuber yield was not correlated with the prevalence of Alternaria spp. infections during the study. Kowalska (2016) demonstrated that yield is not directly correlated with the severity of disease, and a real drop in yield is noted only when more than 60% of plants are infected with P. infestans and Alternaria spp. The products tested in the cited study (Bioilsa Fertil NC, EM Farma Plus, UGmax) delayed the onset of disease, prolonged the growing season (in particular when disease severity was low) and protected crops. Runno-Paurson et al. (2014) reported higher tuber yields in treatments where intercropping and manure fertilizers were used. Potato leaves were more severely infected with Alternaria spp. in the above treatments than in intercropped treatments where manure fertilizers were not applied. According to the above authors, potato tuber yield is less influenced by early blight than by manure application.