Abstract The yield formation of sweet cherries occurs in a relatively short time and has been influenced by various factors. Shortly after blooming, the incompletely fructified fruits drop off the trees. Later, the second fruit drop is mostly is related to resource availability and sink-source relations in the trees. The aim of the study was to evaluate the initial fruit density, fruit drop and final fruit density for eleven sweet cherry cultivars and the impact of rootstocks, year-to-year differences and growing systems. Fruit density on a sample branches was detected in 2022 – 2024 after the initial fruit drop and the second fruit drop. Generally, the number of fruits reduced by 24 – 86% during their development and it was affected by growing season, cultivar and rootstock. The lowest level of fruit drop was shown by the cultivars ‘Paula’ in growing system with VOEN cover and drip irrigation. Fruit drop was significantly less for the trees grown on the rootstock ‘Latvijas Zemais’ ( Prunus cerasus L. ) comparing to the trees on the rootstocks P7 and Mahaleb cherry ( Prunus mahaleb L.).

Agriculture 5.0 incorporates autonomous decision-making systems in order to make agriculture more productive. Our study is related to the development of the autonomous orchard monitoring system using unnamed aerial vehicles for automatic fruiting assessment and yield forecasting. Respectively, artificial intelligence must be developed to count fruits in an orchard. The modern solutions are mainly data-based. Therefore, we collected and annotated cherry dataset with natural images (CherryBBCH81) for neural network training. The goal of the experiment was to select the optimal “You Look Only Once” (YOLO) model for the rapid development of fruit detection. Our experiment showed that YOLOv5m provided better results for CherryBBCH81 – mean average precision (mAP) at 0.5 0.886 in comparison with YOLOv8m mAP@0.5 0.870. However, additional tests with dataset Pear640 showed that YOLOv8m can outperform YOLOv5m: 0.951 vs 0.943 (mAP@0.5).

The climate in Latvia is influenced both by maritime and continental air intrusions. The cherries in continental regions used to be under the threat of prolonged low temperatures but in the maritime regions - of temperature fluctuations. The interest has arisen to grow local sweet cherries in the agroforestry system, and after initial evaluation, eight accessions with a large and healthy trunk, dominant apex, sparse branching, and stable fruit production were selected. The research aimed to evaluate the cold tolerance of the selected accessions compared to the cultivars 'Bryanskaya Rozovaya' (Russia) and 'Meelika' (Estonia). The annual and biannual shoots, spurs and flower buds were evaluated visually in the natural conditions and after artificial freezing at -20, -25 and -30 degrees C in the middle of winter dormancy; and at 10, -15 and -20 degrees C at the end of winter dormancy. They were evaluated visually on a scale from 0 (no visible changes of cross-section) to 5 (cross-section is completely brown); the percentage of damaged flower buds was calculated. Additionally, the analysis of relative electrolyte leakage (REL) was done for annual shoots. The accessions differed in their response to low temperatures and the results indicated various possible patterns of the acclimation and re-acclimation. In general, the accessions from Latvian continental regions had significantly less damaged flower buds, less browned spurs and lower REL of annual shoots during the middle of winter dormancy. One of them, 'Smiltenes 9' stands out with high cold tolerance of the shoots and spurs and acceptable intact flower bud proportion after the freezing at -30 degrees C. In the March, at the end of dormancy, shoots of the accessions from Latvian continental regions still showed lower REL in natural conditions indicating the slower overcoming of dormancy. The accessions from maritime regions had a decrease of REL after the freezing at -15 or -20 degrees C indicating re-acclimation to low temperature.

Orchard management can benefit greatly from the use of modern technology to reach higher yields, decrease costs and achieve more sustainable farming. Implementation or such a smart farming approach into orchard management can be realised via application of unmanned aerial vehicles (UAV) for data collection and artificial intelligence (AI) for yield estimation and forecasting. On top of that, a digital twin of the orchard can be implemented to represent the physical system of the orchard in the digital format allowing implement modern data-driven decision-making based on fruit-growing automation.The aim of this study is to present a digital twin based on application of UAV and AI for orchard management that is being developed as part of a research project lzp-2021/1-0134. At this moment, we are developing a user-centred design which is oriented to satisfy horticulture specialists’ needs for an autonomous monitoring system and to help them in decision-making. Within the framework of this study an enterprise model of orchard management is designed, which supports the digital twin concept and provides autonomous orchard monitoring. The study is scoped with subjects: apples, pears and cherries, and yield management based on orchard monitoring using UAV.

Winter-hardiness of cultivars and rootstocks is a critical issue in Latvia's climatic conditions. Prunus mahaleb is the most popular rootstock for sweet cherries in Latvia. However, this rootstock is unsuitable for intensive sweet cherry orchards due to its vigorous habit. Therefore, it is necessary to find new rootstocks adapted to the local climate. A trial was established in 2014 in the Institute of Horticulture (LatHort), where five clonal rootstocks: 'PiKu 1', PHL-A', 'GiSelA (R) 5', GiSelA (R) 6', 'VSL 1', and P. mahaleb (control) were evaluated. A sweet cherry 'Arthur' obtained from the Estonian breeding program was grafted on the 6 rootstocks. Trees were planted at a distance of 5x3.5 m with three replications per plot. The intensity of flowering, productivity, tree growth, and tree viability after winter was evaluated. During the first five investigation years, the cultivar 'Arthur' had the best viability of trees in combination with 'GiSelA (R) 5'. The trees on P. mahaleb and 'GiSelA (R) 5' showed the highest flowering intensity. More dead trees were observed in combination with 'PiKu 1'.

Abstract The aim of the study was to characterise the variability of flower bud frost tolerance, self-pollination and fruit quality in Latvian local sour cherries; and to select the most promising aces-sions. Data on fruit set after self-pollination, frost tolerance of flower buds, fruit and stone weight, stem retention force and soluble solid content were collected for 29 accessions of local cherries in 2016–2019. Sour cherry cv. ‘Latvijas Zemais’ clones no. 64, D7, and 4 were the most promising for fruit processing; clones no. 62, 65, Matkules, and Lietuvas were promising as pollinators due to tolerance to spring frosts and high level of fruit set after self-pollination, and cv. ‘Latvijas Zemais’ clones Eglīša, Piïkas 3, Matkules, and Dumbrava had relatively high fruit weight and soluble solid content for fresh consumption. ‘Latvijas Zemais’ clone no. 62 was superior with stable fruit set after self-pollination (21.54%), cold resistant flower buds (89.5%) and high proportion of fruit flesh (93.8 %), clone no. 52 had high soluble solid content (18.88 Brix) and clone Eglīša had large fruit (5.56 g). The highest contribution to variability was provided by fruit weight, stone weight, and soluble solid content, which explained 79.90% of total variability.

Abstract The distribution of sweet cherry (Prunus avium L. syn. Cerasus avium Moench.) in natural woodlands of Latvia was studied. The growth, vitality and stem quality of the first-generation plantations established in Latvia in different agricultural soils using the planting stock of local origin was assessed. In woodlands, sweet cherry is found in 11 Forest Resource Monitoring plots in the administrative regions of Madona, Kuldīga, Alsunga, Saldus, Cīrava, and Kazdanga. In the last decade, interest in sweet cherry as a wood producer has increased and plantations with a total area of 21.92 ha were established in 22 localities (Priekule, Kazdanga, Durbe, Talsi, Ventspils, Kuldīga, Tukums, Saldus, Dobele, Bauska, Cēsis, Pārgauja, and Krāslava). Survey was made of sweet cherry plantations established between 1997 and 2019, using both local and foreign origin clonal stock (Danish clone Truust and Swedish clones nos. 9 and 10) in different agricultural soils (typic sod-calcareous soil; sod-podzolic soil; stagnogley sod-podzolic soil) in the districts of Priekule, Kuldīga, Dobele, Bauska, Ķegums, and Smiltene. In plantation forests with optimal growing conditions (typical sod-calcareous soil; pH 6 to 7), 22-year-old sweet cherries of local origin had height (H) of 15.1 m, the diameter at breast hight (DBH) — 16.4 cm and stem volume 157.7 dm3 on average. The maximum H and DBH observed for the largest trees reached 17.6 m and 25.3 cm, respectively. The Danish clone Truust and the Swedish clones nos. 9 and 10 of sweet cherries were promising for growing in Latvian climatic conditions. Selection of perspective clones for establishment of second-generation plantations was made for sweet cherries of local origin.

Sour cherry ‘Latvijas Zemais’ ( Prunus cerasus ) is a promising dwarfing rootstock for sweet cherries in Latvia, but low growing rate of newly propagated sour cherry plants limits their use. Plum rootstock PU 20651 was selected from hybrids of P. cerasifera and P. salicina ssp . usuriensis recently. The aim of our study was to estimate the effect of arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis MUCL 41833 (Ri) on the growth and P content of rooted green cuttings of sour cherry ‘Latvijas Zemais’ 52, cherry rootstock Gisela 5 ( P. cerasus x P. canescens ), and plum rootstock PU 20651. The cuttings were planted in peat substrate and grown in plastic tunnel equipped with automatic air moistening system and controllable floor heating . We determined growth parameters and P content in shoots and roots, intensity of mycorrhizal colonization, and frequency of mycorrhiza in root system. In all three Prunus sp. genotypes, the inoculation with Ri resulted in the root colonization. PU 20651 had highest level of colonization at all arbuscular mycorrhiza (AM) estimation parameters. A common effect of colonization was increased diameter of root collar and positive correlation between the leaf number and AM parameters. The effect of Ri on plant biomass and P content depended on the genotype. Beneficial effect of AM on root mass and root P content was detected in ‘Latvijas Zemais’ 52. However, no AM effects on P content were found in Gisela 5, but little negative impact of AM on shoot mass and shoot P content—in PU 20651.

Abstract Fruit set is a crucial stage in the process of yield formation, which is influenced by environmental factors, growing technologies and peculiarities of genotype. The aim of the study was to evaluate the quality of pollen (viability and germination capacity) and the effect of weather before and during flowering on fruit set in sour cherry cultivars ‘Latvijas Zemais’, ‘Zentenes’, ‘Bulatnikovskaya’, and ‘Orlica’. The research was carried out in Institute of Horticulture (Latvia University of Agriculture) in 2009-2016. Good pollen viability and germination was found for cultivars ‘Latvijas Zemais’ and ‘Bulatnikovskaya’. Negative effects of increasing air temperature (in the range of 7.7 to 17.5 °C) and relative humidity (in the range of 51.4 to 88.5%) was observed for all cultivars during flowering. The effects of diurnal temperature fluctuations, wind and the amount of days with precipitation differed depending on sour cherry cultivar.

The content of tocochromanols and carotenoids in kernels recovered from fruit pits of six sour cherry (Prunus cerasus L.) cultivars was studied. The highest concentration was noted for gamma-T (23.50-52.76 mg/100 g dry weight basis (dw)). Considerably, lower amounts compared to gamma-T were detected for alpha-T, delta-T and alpha-T3 (5.74-13.33, 2.74-4.54 and 0.30-1.16 mg/100 g dw, respectively). The beta-T and gamma-T3 were quantified only in minor levels. The total amount of tocochromanols was in the range 41.69-63.48 mg/100 g dw. The levels of total carotenoids ranged between 0.17 and 0.39 mg/100 dw. The concentration of tocochromanols and carotenoids in kernels of different sour cherries is cultivar-dependent. A significant correlation (r = 0.985, p < 0.001) has been observed between the total content of tocochromanols in kernels and scavenging of free radicals DPPH by native lipophilic antioxidants in the by-products of sour cherry. [GRAPHICS] .

Nitrogen is an essential nutrition element which influences the growth, productivity and fruit quality of sour cherries. The aim of the research was to estimate chlorophyll and nitrogen status in sour cherries (Prunus cerasus L.) grown with various soil moisture treatments. The investigation was done in 2011-2012 at the Latvia State Institute of Fruit-Growing, with the cultivar 'Latvijas Zemais' (rootstock: seedlings of Prunus mahaleb L.). Three different treatments of soil moisture were compared: drip irrigation, woodchip mulch and control. Inter-row strips were covered by grass vegetation. Relative chlorophyll content was detected using chlorophyll meter Opti-Sciences CCM 200. Nitrogen content was determined by Kjeldahl method. Pearson's correlation was calculated. The amount of chlorophyll and nitrogen varied among the years and soil moisture variants. Positive relationship was recognized between relative chlorophyll content and nitrogen content in cherry leaves: R-2=0.75 (P < 0.01) in 2011, R-2=0.72 ( P < 0.05) in 2012; and regression model was built. Therefore, chlorophyll meter Opti-Sciences CCM 200 was appropriate for the estimation of nitrogen content in sour cherry leaves.

Sweet cherry (Prunus avium L.) cultivars with large and firm fruit are susceptible to rain induced fruit cracking during fruit ripening in wet summers. Cracked fruit are susceptible to the entry of fungal pathogens. To obtain qualitative sweet cherry yield for fresh market, polyethylene rain cover was used in the trial for the first time in Latvia. Four cultivars of sweet cherries on Prunus mahaleb L. were grown under VOEN type plastic cover. Trees were covered at the beginning of June (at time of green fruit) and throughout the harvest for three years (2010-2012) at the orchard of Latvia State Institute of Fruit-Growing. Fruit cracking and rots depending on cultivar and season was reduced significantly by using rain cover. Outcome of marketable fruit varied from 56.9 to 99.0% under rain cover. Fruit under cover can be harvested in optimal ripening period also in rainy season.

The aim of this research was to determine the movement of nutrients (nitrogen, phosphorus, potassium) in sour cherry leaves depending on soil properties - soil moisture treatment (woodchip mulch and drip irrigation), the content of phosphorus and the C/N ratio in the soil. The content of phosphorus in the roots was investigated as well in the order to detect the differences of its uptake. The research was carried out in 2014, in Latvia State institute of Fruit-Growing. In a sour cherry orchard different soil moisture treatments: drip irrigation, woodchip mulch and control variants, were arranged in the spring of 2007. The cultivars used in the study were 'Latvijas Zemais', 'Tamaris', 'Bulatnikovskaya', 'Shokoladnica', 'Orlica', using as rootstock seedlings of mahaleb cherry (Prunus mahaleb). The fertilization was done yearly accordingly to results of soil analysis. The volumetric soil moisture was measured once in 7-14 days during vegetation period. Soil moisture varied from 14 to 28% depending on the date and treatment. The samples for soil and root analysis were collected in May of 2014, but for leaves analysis was done in August. The content of phosphorus was from 130 to 270 mg kg(-1) in the soil and from 0.20 to 0.27% in the roots. In sour cherry leaves the nitrogen content was from 1.9 to 2.4%, phosphorus content from 0.2 to 0.3%, but potassium content from 0.8 to 1.3%. The results varied depending both on the soil moisture treatment and sour cherry cultivar.

Winter cold is one of the most important risk factors in sour cherry growing in Latvia. Winter hardiness of sour cherries depends on the features of cultivars as well as on growing conditions. The research of flower bud and tree winter hardiness of seven sour cherry cultivars grown with drip irrigation and woodchip mulch was conducted in Latvia State Institute of Fruit Growing. The observations were made during four years. Winter hardiness of sour cherry trees was evaluated in the orchard. The air temperature dropped down below -25 degrees C, reaching even -28 degrees C after thaw in the second part of winter during some observation periods. The branches with flower buds were collected after the last winter frosts. Buds after swelling at room temperature were cut, evaluated visually and classified as partially damaged, completely damaged or undamaged; the percentage was calculated. Tree survival rate was determined in spring. The results showed that differences of bud winter hardiness among cherry cultivars were more pronounced than among soil moisture treatments. Sufficient winter hardiness of flower buds was detected for the cultivars 'Bulatnikovskaya', 'Latvijas Zemais', 'Desertnaya Morozovoi', 'Zentenes', 'Orlica'. The cultivar 'Tamaris' had lower winter hardiness of flower buds. The tendency for higher tree survival rate was observed in drip irrigation treatment. The cultivars 'Desertnaya Morozovoi' and 'Shokoladnica' had significantly lower tree survival rate than the other cultivars.

The aim of this study was to evaluate the growth, productivity, and tree condition of sour cherries, depending on the cultivar and soil-moisture treatment. The research was carried out at the Institute of Horticulture, Latvia University of Agriculture. Five sour cherry cultivars were investigated using drip irrigation and woodchip mulch. The following data were collected and processed: trunk cross-section area (TCSA), trunk growth intensity (TGI), fruit set, yield tree(-1) and yield efficiency as a ratio of TCSA, photosynthesis performance index, and the level of defoliation caused by cherry leaf spot after natural infection in the field. Woodchip mulch preserved soil moisture at optimal levels or higher during most vegetation seasons. Sour cherry cultivars differed significantly in terms of growth, productivity, and tree condition, and in the response to soil-moisture treatment. Most of the sour cherry cultivars responded to drip irrigation and woodchip mulch by increasing the growth and productivity of trees; however, a negative (in 'Orlica') or minimal (in 'Bulatnikovskaya') effect of mulch was observed in a few cultivars. Our study highlights the variability of sour cherry cultivars and the necessity of further research before recommendations can be made to growers.

Optimal soil moisture which provides sufficient and balanced vegetative growth for sour cherries is a precondition of high yields. In order to investigate the influence of drip irrigation and woodchip mulch on sour cherries, the trial was established at the Latvia State Institute of Fruit-Growing in 2007. The cultivars 'Bulatnikovskaya', 'Desertnaya Morozovoi', 'Latvijas Zemais', 'Orlica', 'Shokoladnica', 'Tamaris', and 'Zentenes' were planted in the plots with drip irrigation and control. Trunk circumference, canopy volume, the amount of spurs and yield per tree were measured. The data were summarized from 2007 to 2011. Drip irrigation and woodchip mulch advanced vegetative growth. Trunk growth increased significantly and a similar tendency was observed for canopy growth. Spur proportion was not changed by soil moisture treatments. Yield of the cultivars 'Bulatnikovskaya', 'Orlica', 'Tamaris' and 'Zentenes' was more responsive to drip irrigation. The yield in drip irrigation treatment was significantly higher for these cultivars in 2011. The cultivars 'Bulatnikovskaya' and 'Tamaris' had higher yield efficiency than the other cultivars. There was a significant correlation between canopy volume and yield per tree for the cultivars 'Bulatnikovskaya' and 'Tamaris'.

A trial to investigate cultivar/rootstock compatibility in cherry was established at the Latvia State Institute of Fruit-Growing in Dobele in 1998 using one year old plant material. Cultivars 'Iputj' ('Iput') and 'Krupnoplodnaya', on rootstocks Gisela 4, Gisela 5, Weiroot 154 and F 12/1, were planted at 2.8 x 4 m. After eleven years, rootstock had a significant effect on trunk cross-sectional area. The trunks were twice as thick on F 12/1 as on the other rootstocks. Trees on Gisela 4 had the narrowest trunks and smallest crowns. Both cultivars yielded in the fifth year. The yield in two years (2003 and 2007) was considerably reduced by winter cold and spring frost, especially for 'Krupnoplodnaya'. However, good yields were obtained four times during the seven years after the trees started cropping. Despite no yield in two years, 'Krupnoplodnaya' cropped more heavily than 'Iput'. Both cultivars cropped best on the rootstocks Gisela 5 and Weiroot 154. Yield efficiency was the lowest for both cultivars on the rootstock F 12/1. The highest yield efficiency was found on trees on Gisela 5. Yield efficiency for 'Krupnoplodnaya' was also high on Weiroot 154 and for 'Iput' on Gisela 4. The biggest fruits of 'Iput' were obtained on Weiroot 154, and for 'Krupnoplodnaya' on F 12/1. Considering both growth and productivity, the rootstocks Gisela 5 and Weiroot 154 were found to be promising for conditions in Dobele, Latvia.

Horticultural production in Latvia has always been subject to numerous and diverse risks similar to other branches of crop plant production in Latvia. On the other hand, the climate of Latvia is favourable for plant crop growing but the value of production depends on climatic condition as well as field management and competence of the farmers. The markets of inputs used in strawberry, raspberry, and cherry farming have a direct impact on risks through unexpected rise in prices. Similarly, returns from horticultural outputs are affected by high volatility of fresh farm produce markets. Besides that, growers face the inevitable yield risks induced by adverse weather conditions, pests, and diseases. There are a few systems used in production of strawberries, raspberries, and cherries - extensive and intensive growing both in open and covered areas. These methods vary by the level of risks and necessary investments. The production of berries and stone fruit in areas covered by polyethylene tunnels is expanding. The tunnel method of production provides better climatic conditions and reduces the damage by pests and diseases, thus, contributing to a longer and more predictable shelf life of the fruit. Production in tunnels extend the harvest season. High tunnels, in turn, can advance harvest dates earlier. Beyond the normal season, there is less competition and producer prices can be set higher. The aim of the study is the assessment of general risks in strawberry, raspberry, and cherry production, risks in production with various methods at farm level and evaluation of the tradeoffs among farming risks and expected returns.