Industrial hemp cultivation restarted in Estonia in mid 2000s and the hemp cultivation area has increased very fast in recent years, but it is unknown to what extent hemp productivity can be curbed by widespread crop pests. Thus, the main aim of this study was to evaluate the presence and damage of hemp pests in northern field conditions. A field trial with the hemp cultivars ‘Finola’ (2021) and ‘Estica’ (2022) was established in Central Estonia (58° N, 26° E), and the plants were grown with different fertilizer types (complex fertilizer, ammonium nitrate), fertilization methods (at sowing/top dressing) and nitrogen rates (0–200 kg ha−1) and their effect on the European corn borer (ECB, Ostrinia nubilalis) damage was assessed. The O. nubilalis infestation increased with every observation date throughout the seasons until the harvest. The O. nubilalis damage was overall greater in 2022 than in 2021, and the seed yield reduction reached to 120 kg ha−1 in 2022, indicating progressive increase in O. nubilalis damage. Fertilization rate strongly increased biomass and seed yields, but weakly reduced O. nubilalis infection. Estonia is one of the most important hemp-cultivation areas in Europe, and in northern conditions, the crop was previously considered to be relatively pest-free. The northward movement of crop infection by O. nubilalis implies that future hemp cultivation might need integrated O. nubilalis control practices adapted to site-specific agro-climatic conditions.
Hinweise
The original online version of this article was revised: The Section Acknowledgments has been corrected:
The study was supported by the European Regional Development Fund (Center of Excellence EcolChange: Ecology of global change: natural and managed ecosystems), the Estonian Research Council (Center of Excellence AgroCropFuture “Agroecology and new crops in future climates”, TK200) and the Estonian University of Life Sciences base funding project P190259PKTT. The study used equipment purchased within the framework of the AnaEE Estonia Project (2014–2020.4.01.20-0285) and the project “Plant Biology Infrastructure-TAIM” (2014–2020.4.01.20-0282) through the EU Regional Development Fund and “Plant Biology Infrastructure – TAIM” funded by the Estonian Research Council (TT5). EÕ was supported by the Estonian Research Council grant PRG741. The authors thank Vineesh Nedumpally (University of Tartu) for his help in the molecular biology lab.
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Introduction
European corn borer (ECB), Ostrinia nubilalis Hübner (Lepidoptera: Crambidae) has become one of the major pests in Europe, Asia and in Americas, damaging mainly maize (Zea mays). It is native to Europe and it has been suggested that wild hop (Humulus lupulus) and wild hemp (Cannabis sativa var. spontanea) were the original host plants (Nagy 1976). It became a major maize pest in the beginning of the 20th century (Calvin et al. 1991; Got et al. 1996; Bereś and Konefał 2010), and currently the overall yield losses because of O. nubilalis are estimated at US$ 1 billion per year (Bažok et al. 2020).
European corn borer is an extremely polyphagous insect and besides maize its host range has more than 200 different plant species belonging to 40 different botanical families, including sorghum (S. bicolor), pepper (Piperaceae), hemp (C. sativa), millet (P. miliaceum), mugwort (A. vulgaris), chrysanthemums (Chrysanthemum) etc. (Gaspers and Schuphan 2010; Leppik 2011; Bažok et al. 2020). It is considered to attack any herbaceous plants with stems large enough for the larva to enter (Bourguet et al. 2000; Leppik 2011). It has been assumed that the O. nubilalis spreads 3–5 km a year (Lisowicz 1995), while in the US the spread has reached 32–42 km yearly (Schurr and Holdaway 1965, as cited in Bereś and Konefał 2010). The spreading speed is determined mostly by the availability of host plants and natural predators and parasites (Trnka et al. 2007). To have time to complete its development in colder climates, O. nubilalis individuals start feeding largely on the whorl leaves, which requires development of certain physiological and behavioural adaptations and is associated with polyphagousness as well as segregation from populations retaining initial trophic connections with dicotyledonous host plants (Zhukovskaya and Frolov 2022). Previous observations suggest that the O. nubilalis prefers larger plants (Small et al. 2007). European corn borers may prefer to bore into larger stems simply because smaller stems are too narrow to accommodate full-grown caterpillars as they begin pupation. Larvae have been found to move up and down the stalk to find more suitable temperature and buffer from outside conditions (Keszthelyi et al. 2021).
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Apart from damage to maize, O. nubilalis is one of the most destructive pests of hemp (Cannabis sativa L.) in North America, and in Europe, and it can cause appreciable hemp crop losses (Grigoryev 1998; Benelli et al. 2018; Park et al. 2019). A heavy infestation on hemp was already reported in 1945–55 in Hungary (Nagy and Csehi 1955, as cited in Nagy 1976) that drew attention to the problem of geographical strains.
Hemp is exceptionally resistant to lodging and usually not affected by ordinary storms, but O. nubilalis tunnels into larger stems and stalks of hemp and has the potential to weaken plants to the extent that they are prone to breakage. Grigoryev (1998) found that under low air humidity conditions the major part of the O. nubilalis damage is to the inflorescences of hemp. Young ovules and immature seeds are gnawed first by the caterpillars. The next instar larvae causes dryness of the inflorescence and stalk through the action of young larvae gnawing out the inside tissues of plant stems. This leads to inflorescence damage and a breaking of hemp stalks in the field with consequent entanglement of plantings, which greatly hampers mechanized harvesting. Adult larvae of this pest survive winter as pupae inside the stalks of hemp plants, in harvested maize stalk residues or in weed stems. Damaged plants are also susceptible to various fungal pathogens (Grigoryev 1998). O. nubilalis causes stalk cankers and break; larvae boring into smaller branches cause wilting of distal plant parts. Under substantial damage entire plants collapse. The damage is more severe in fibre crops, which are grown in tight proximity (Parisi and Ranalli 2000). On the other hand, Grigoryev (1998) studied the relationships of resistant hemp accessions to O. nubilalis and the density of plantings in the field and found the most severe incidences of damage in sparse hemp plantings at 500,000 plants per hectare with a row spacing of 45 cm. In addition, Small et al. (2007) found that within the plots of most hemp cultivar accessions, O. nubilalis-affected plants branched extraordinarily and even appeared to produce more biomass and seeds than normal plants.
Compared to other field crops, hemp is considered relatively pest-tolerant plant because of the potential pest-repellent properties of its cannabinoids and terpenoids (Dambolena et al. 2016). Recent studies have demonstrated that cannabinoid-enriched extracts show pesticidal activity to defend the plants from insect herbivore attacks (Benelli et al. 2018; Park et al. 2019, 2022). However, one of the main cannabinoids—tetrahydrocannabinol (THC) did not have any effect on the infestation and damage by O. nubilalis (Park et al. 2019).
The pheromone polymorphism of the O. nubilalis is the basis for a successful reproductive isolation. Males might use a blend of host volatiles and sex pheromones for mating and females are expected to use host volatiles for finding the host before or after mating (Bengtsson et al. 2006; Sole et al. 2010). Monocots are attacked mainly by Z pherotype and dicots by E pherotype (Ponsard et al. 2004; Leppik 2011). In the case of maize, there are no specific plant volatile compounds that are responsible for attracting O. nubilalis, but probably blends of many volatiles in specific ratios are behaviourally effective (Leppik and Frérot 2012).
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The most important environmental factors affecting O. nubilalis spread and their uni-, bi- and multivoltine nature include ambient temperature, moisture and food resources (Frolov 1998; Zhukovskaya and Frolov 2022). Daily temperatures and precipitation are very important factors for the O. nubilalis population dynamics (Bažok et al. 2020). The increase in the population size and harm caused by this pest is mainly facilitated by simplified crop production methods, monoculture, crop residues left on the field, late ploughing before winter, global warming and especially by the growing area of maize cultivation (Lisowicz 1995; Kocmankova et al. 2011; Kozak et al. 2019; Enos and Kozak 2021). Camerini et al. (2018) studied the potential parasitoid species for O. nubilalis and found that in USA the number of parasitoids decreased from 18 species in 1930s to only 9 species in more recent studies, due to the loss of biodiversity. The possible reasons of such a biodiversity decline include monocultural patterns and overall landscape structure simplification.
In Estonia, O. nubilalis has been registered as a component of natural insect fauna, and it has not been considered as a pest on the crop fields until recently. According to farmers’ reports, in Estonian maize fields, O. nubilalis has been reported only in the last two years. Despite that oil hemp is a widely-grown crop in Estonia, there have been no reports of O. nubilalis damage until a few O. nubilalis-injured hemp plants were observed at the experimental field of Estonian University of Life Sciences in autumn of 2020. Furthermore, previous knowledge of the O. nubilalis damage on the fields of oil hemp in Northern Europe was missing. Motivated by the observation that O. nubilalis could be a significant hemp pest in northern conditions, we started detailed monitoring of the occurrence and damage of O. nubilalis in hemp fields for two subsequent years to assess the spread of the insect and potential damage for hemp cultivation. As it has been shown, the spread of pests can be strongly affected by management practices, particularly the choice of crops in the rotation, the objective of the experiment was to assess the effect of cultivation technologies (fertilizer type, fertilization rate, fertilization method) on the occurrence of O. nubilalis and estimate the possible seed yield losses.
Materials and Methods
Experimental Site and Design
European corn borer (Ostrinia nubilalis) infection was evaluated in 2021 and 2022 on industrial hemp (Cannabis sativa L.) crop plants in an experimental field of the Estonian University of Life Sciences, Tartu County, Estonia (58° 36′ N, 26° 66′ E). The two growing seasons were mostly characterized by usual temperature patterns, compared to the long-term average (Fig. 1). However, in June and July of 2021, the daily average air temperatures were significantly higher and combined with extremely low precipitation values led to moderately severe drought conditions. In August of 2022, the average temperatures were also above the long-term average, reaching over 22 °C in the second decade (Fig. 1). The precipitation was significantly lower in both seasons, compared to long-term average value and was also characterized by high variability (Fig. 1). The precipitation exceeded the long-term average values only in the second decade of August in 2021 and the second decade of July in 2022 (Fig. 1).
Fig. 1
Average air temperature and precipitation on the Eerika (Central Estonia) Cannabis sativa test site in 2021 and 2022 and comparison with long-term (1964–2022) average temperature
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The field experiment included various seeding rates (20–40 kg ha−1), fertilizer types (Type 1: complex fertilizer 17‑4.6‑10 S‑Mg‑B; Type 2: ammonium nitrate), fertilization methods (at sowing/top dressing) and nitrogen addition rates (0–200 kg ha−1). Oil-type hemp cultivars ‘Finola’ (2021) and ‘Estica’ (2022) were used. Plots were sown on 22 April in 2021 and 4 May in 2022. Size of a test plot was 10 m2 and three replications were used for each treatment. The seeds were harvested on 28 September in 2021 and 25 September in 2022. Total above-ground biomass yield (2021) and seed yield (2021, 2022) were measured for the entire plot, and a random sample of plants from 0.25 m2 were used to measure plant height and stalk diameter (2021).
The damage of European corn borer was assessed based on the number of plants damaged per plot. The evaluations were performed weekly when the damage became visible in the field until the harvest (2 assessments in 2021, 4 assessments in 2022). The weather data were obtained from the local weather station next to the sample plots in Eerika, Tartu.
Molecular Identification of the Pest and Pest Damage Assessment
For molecular confirmation of the identification of the pest, genomic DNA of the larva of Ostrinia nubilalis was extracted from the last abdominal segment using DNEasy Blood & Tissue Kit (Quiagen N.V., Venlo, Netherlands) following the manufacturer’s instructions. A standard barcoding fraction (658 bp) of the mitochondrial COI gene (Hebert et al.2004) was sequenced, following the protocol described in Õunap et al. (2021). The identity of the larva was verified using the identification engine of the Barcode of Life Data Systems web portal (http://www.boldsystems.org/index.php).
Data Analysis
Independent samples t-test was conducted to explore the differences between the O. nubilalis damage in two seasons. The differences in the number of O. nubilalis-damaged plants between the observation dates within the given year were assessed by one-way ANOVA followed by Tukey post-hoc test and the effect size ηp2 is presented. Pearson correlation analysis was used to examine the relationships between the levels of N fertilization and seed yield of hemp. Independent samples t-test was used to explore differences between fertilization method and fertilizer type and average biomass and seed yields.
The reductions of seed and biomass yields due to O. nubilalis were calculated based on the total seed and biomass yield of the plot, the number of plants per plot, the number of female individuals and the number of O. nubilalis damaged plants. The effect of harvest dates, fertilizer type and fertilization method on the seed yield reduction due to O. nubilalis was analysed with independent samples t-tests.
Results
Variation in O. nubilalis Damage Among the Two Growing Seasons
The European corn borer larvae were first noticed in industrial hemp trials of Estonian University of Life Sciences (EMU) in 2020, However, the larva were present in low numbers and the visual symptoms of O. nubilalis-infection were rare (less than 0.1% of plants showing visual symptoms of damage) and appeared mostly on fertilized plots. The upper portion of damaged plants looked necrotic and brown (Fig. 2). In addition, the stems became soft, visible borer entrance holes were detected and in some cases the stems broke off, showing larval frass (feces). In previous seasons, no damage on the EMU trial fields was observed nor has there been any report on O. nubilalis occurrence on the commercial fields in Estonia. After first noticing the European corn borer damage in 2020, the potentially harmful pest dispersal was studied in 2021 and 2022. The level of O. nubilalis damage increased from 2021 (average ± SE = 4.45 ± 0.23 number of damaged plants per plot, 10 m2, n = 108 observations) to 2022 (11.3 ± 0.7 per plot, t(131,028) = −9.93, p < 0.001, independent samples t-test for sample populations with unequal variance).
Fig. 2
A typical hemp (Cannabis sativa) plant infested by European corn borer (Ostrinia nubilalis) in hemp field in Estonia in 2021 (a); hole in stem of hemp plant (arrow) caused by O.nubilalis (b). (Photos E. Runno-Paurson)
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The Dynamics of O. nubilalis Damage During Growing Seasons
In 2021, O. nubilalis damage was assessed twice, and in 2022 four times. In 2021, the number of damaged plants per plot (10 m2) was lower in the beginning of September than in the middle of September just before harvest (Fig. 3; for ANOVA, F(1,106) = 12.80, p < 0.001 ηp2 = 0.11). The average number of O. nubilalis damaged plants during the first and the last evaluation were 3.7 ± 0.28 and 5.24 ± 0.34, respectively.
Fig. 3
Number of O.nubilalis damaged plants per plot of cultivar ‘Finola’ in 2021 (left) and ‘Estica’ 2022 (right). Different letters (a, b, c) indicate significant differences between the groups each year (p < 0.001)
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In 2022, there was a progressive increase in infestation during the observation period (Fig. 3; for ANOVA, F(3,102) = 37.93, p < 0.001, ηp2 = 0.53). In this year, the level of infestation did not differ only among the last two sampling dates (29th of August and 5th of September, Fig. 3, Tukey’s post-hoc test). The average number of O. nubilalis damaged plants during the four evaluations were 4.2 ± 0.51, 9.6 ± 1.05, 14.9 ± 0.97 and 16.52 ± 1.02, respectively.
Pest-dependent Yield Reductions
Based on our two-year experiment, the seed yield of oilseed hemp varied from 0.4 to 4.4 t ha−1 in 2021 and from 0.6 to 2.7 t ha−1 in 2022. For data of both cultivars pooled, the seed yield was strongly correlated with N fertilization rate (rP = 0.59, n = 216, p < 0.001). Fertilization at sowing (average ± SE = 1.71 ± 0.11 t ha−1) increased the hemp seed yield, compared to the top dressing method (average ± SE = 1.28 ± 0.04; t(93,368) = 3.76, p < 0.0001). In addition, the complex fertilizer (average ± SE = 1.96 ± 0.06 t ha−1) improved the seed yield, compared to ammonium nitrate fertilizer in 2022 (average ± SE = 1.68 ± 0.06 t ha−1; t(94) = 3.19, p < 0.05).
O. nubilalis damage was negatively correlated with nitrogen fertilization rate and biomass yield, but the relationships were weak (Fig. 4). No correlation was found between the sowing rate (20, 30, 40 kg seeds ha−1), plant size (height and stalk diameter), number of female plants per plot and O. nubilalis damage (Fig. 4). The seed yield reduction remained relatively low, on average (± SE) 12.3 ± 0.15 kg ha−1 (0.77 ± 0.05% of total yield) in 2021 and 68.6 ± 1.07 kg ha−1 (4.1 ± 0.2% of total yield) in 2022 (averages are significantly different at p < 0.001, ANOVA, F(5,215) = 74.5). The highest seed yield reduction reached up to 120 kg ha−1 (7.1 ± 0.4% of total yield; Fig. 5). In 2021, also the biomass yield reduction was calculated (0.92 ± 0.65 t ha−1).
Fig. 4
The relationships (Pearson correlations) between European corn borer (Ostrinia nubilalis) damage of industrial hemp (Cannabis sativa cv. ‘Finola’) plants and fertilization rate and plant characteristics in study year 2021
Fig. 5
Seed yield reduction (kg ha−1) based on the number of O.nubilalis cases observed in 2021 and 2022. Different letters (a, b) indicate significant differences between observation dates (p < 0.001)
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Fertilization rate affected the seed yield reduction in both seasons (in 2021: F(7,100) = 7.234, p < 0.001; in 2022: F(4,103) = 3.565, p < 0.01). In 2021, a few statistically significant effects of N fertilization rates were seen, but there was no linear correlation between fertilization rate, plant biomass and seed yield and yield loss due to O. nubilalis damage. In 2022, the yield loss was greater in non-fertilized treatment (N0) compared to all other nitrogen fertilization rates (N50-200). The plots with fertilization at sowing were more prone to O. nubilalis damage in 2021, compared to top dressing method, 13.6 ± 1.24 and 7.5 ± 0.52 kg ha−1 (t(71.13) = 4.55, p < 0.001). The fertilizer type (complex fertilizer vs. ammonium nitrate) did not affect the seed yield reduction (p > 0.05).
Discussion
European Corn Borer as a Pest in Northern Conditions: General Observations
The hugely polyphagous insect European corn borer (O. nubilalis) is a major pest in maize and hemp cultivation areas, but in Estonia it has been considered as a component of natural flora with no significant impact on crops. After first noticing the pest in our experimental field in 2020, we started monitoring the occurrence dynamics of O. nubilalis on the oil hemp fields in local climatic and weather conditions. Our study demonstrates that the O. nubilalis damage increased through the growing season, and the degree of damage was greater in 2022 than in 2021, suggesting enhanced spread of the insect. Also, in 2022 the average daily temperature was much higher at the end of August and the O. nubilalis damage occurred much earlier, compared to 2021 season. The visual evaluation of the O. nubilalis damage seemed much more harmful as many plants were totally perished and dark-coloured, compared to the number of damaged plants per plot. Based on our investigation it can be stated that the European corn borer could overwinter in the area (in the soil or plant residues) and it found the host hemp plants suitable for oviposition and as a food source.
With both cultivars investigated the seed yield was strongly correlated with N fertilization rate. In case of pest damage it is also important to evaluate the seed yield reduction. For the evaluation of seed yield reduction the combined effect of seed yield, O. nubilalis damage and the number of female plants is considered. Nevertheless, given the diffuse and variable dispersal of the insect (1–30 per plot) the effect of O. nubilalis on the hemp seed yield was relatively low, on average 12.3 kg ha−1 in 2021 and 68.6 kg ha−1 in 2022, i.e. an increase of 5.5 times from 2021 to 2022.
Based on the crop density, size of the plants, stalk diameter etc., hypotheses can be formed about the preference and prevalence of the O. nubilalis to oviposit or the ability of the developing larvae to feed on the plant. Sarajlić et al. (2020) carried out experiments with maize and found the highest O. nubilalis damage on crops with greatest nitrogen fertilization rates. According to our results, although the fertilization rate had an impact on the seed yield reduction in both seasons, the post-hoc test revealed that only the effect of N0 in 2022 was significantly different from all other nitrogen fertilization rates (N50-200). It showed that in a relatively low infestation rate the preference to larger or denser crops was not evident. Hence, the conclusions on the effect of the fertilizer type or application method on the O. nubilalis damage cannot be made. Moreover, Phelan et al. (1995) studied the preference of O. nubilalis moth egg-laying patterns in conventionally and organically managed soils and concluded that in organic soils the egg-laying was uniformly low in all treatments. In addition, the ovipositional preference did not correlate with plant biomass, suggesting that buffering effect of soil biodiversity through soil management practices can significantly affect the susceptibility of crops without compromising the plant productivity (Phelan et al. 1995).
Previous studies have suggested that O. nubilalis prefers larger plants with larger stalk diameter (Small et al. 2007). This can lead to inflorescence damage and breakage of hemp stalks in the field with consequent entanglement of plantings, greatly hampering mechanized harvesting. The stalk diameter of hemp plants had been measured only in 2021 in our experiment, but in case of increasing O. nubilalis damage during upcoming years, it could be considered a relevant parameter for considering the sowing rates and nutrient application. In the case of maize, in addition to stalk diameter, the cell wall thickness, as well as cell wall concentrations of xylose and diferulate esters have shown to be possible crop-based defence mechanisms against the O. nubilalis damage (Barros-Rios et al. 2011).
On its main host—maize—O. nubilalis has shown to reduce the seed yield up to approximately 25% (Tiwari et al. 2009). The magnitude of damage is mainly dependent on the number of O. nubilalis larvae per plant and in case of maize, it is common to assess the 3–5% seed yield reduction per larva per plant. O. nubilalis can also significantly damage the yields of maize cultivated for silage. In our experiment, the overall reduction of biomass and seed yield due to the damage caused by European corn borer has been relatively low so far, but the increasing trend of infestation (by 154% between the years studied) is alarming and demands more attention to integrated pest control measures. The hemp plants seem to be attractive hosts for oviposition of adult females and food source for developing larvae. Moreover, lodging and breakage of damaged stalks during mechanical harvesting would probably add to the losses in seed yield in real cultivation systems.
European Corn Borer as a Range-spreading Pest
A previous study modelling the potential northern shift of O. nubilalis concluded that the northern limit of expansion of O. nubilalis can be attributed to latitude 58°N in Europe (Keszthelyi and Somfalvi-Toth 2020). It is located right on the southern part of Estonia. Although in entomological monitoring the O. nubilalis moths have been noticed in natural landscapes up to latitude 63°N (GBIF.org 2023), no damage of agronomical importance has not been registered.
It is known that for full completion of the life cycle (larva, pupa, moth), 711 degree-days of effective temperatures are required in Russia (Grigoryev 1998). Besides, both uni-, bi- and multivoltine ecotypes of the species have been stably represented in Europe, from which univoltine is the most widely distributed. In the USA, the ecotype change triggered by climatic fluctuation is mostly specific for the southern and middle part of the country. In summary, the northward expansion of the more generations of ecotypes of this serious pest may clearly cause increasing damage in maize and hemp cultivation areas globally (Keszthelyi and Somfalvi-Toth 2020). In our climatic region so far only univoltine ecotypes are considered potentially relevant pests, although no studies on this have been performed.
The fact that the range of O. nubilalis population could expand up to 40 km in a year, the increasing areas of maize cultivated mainly for animal feed and hemp cultivated for seed oil and higher winter temperatures in the northern Baltic region provide a solid basis for the spreading of O. nubilalis further north, although no damage of significant agronomic importance to hemp crop has not been reported by the farmers yet The northern shift of the pest might usually initiate their polyphagous nature and as Estonia has one of the highest hemp cultivation areas in Europe, the crop previously thought as relatively pest-free might need thorough yearly pest status investigation and integrated O. nubilalis control practices adapted to site-specific agro-climatic conditions.
Acknowledgements
The study was supported by the European Regional Development Fund (Center of Excellence EcolChange: Ecology of global change: natural and managed ecosystems), the Estonian Research Council (Center of Excellence AgroCropFuture “Agroecology and new crops in future climates”, TK200) and the Estonian University of Life Sciences base funding project P190259PKTT. The study used equipment purchased within the framework of the AnaEE Estonia Project (2014–2020.4.01.20-0285) and the project “Plant Biology Infrastructure-TAIM” (2014–2020.4.01.20-0282) through the EU Regional Development Fund and “Plant Biology Infrastructure – TAIM” funded by the Estonian Research Council (TT5). EÕ was supported by the Estonian Research Council grant PRG741. The authors thank Vineesh Nedumpally (University of Tartu) for his help in the molecular biology lab.
Conflict of interest
E. Runno-Paurson, E. Mäeorg, O. Kurina, E. Õunap, A. Kännaste, Ü. Niinemets and P. Lääniste declare that they have no competing interests.
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