Bonitet assessment of graft-rootstock combinations of apple tree varieties in the conditions of the Central Chernozem region

The study and analysis of the condition of garden plantings is the most important stage in assessing their profitability and determining the feasibility of further production. Bonitet assessment and inventory are one of the options for such an assessment. The paper presents various methods of assessing graft-rootstock combinations of apple trees in the conditions of the Central Chernozem region. The bonitet class of garden plantings has been determined, the growth characteristics of trees and their relationship with the sub-tree inventory and bonitet assessment of graft-rootstock combinations have been analyzed.


Introduction
The use of digital technologies in the cultivation of agricultural crops in terms of the use of automated control systems and decision support systems allows for monitoring about 60% of crop losses. The use of precision farming technologies that are used at all stages of crop cultivation are one of the key elements of this control system [1].
Studies in the field of intensive horticulture determine the need to optimize the amount of solar radiation received by fruit trees during their growth. Besides, timely allocation of the area of infection reduces the loss of production and the area of distribution. Detection of defective poorly developing rootstocks in the early stages will allow a specialist to choose a less expensive way of solving the problem, for example, increasing the supply of nutrients, treatment with growth regulators, etc. These factors determine the need for research on the formation of a fruit tree crown in the process of its growth and development.
Currently, planting fruit trees in Russia has been actively promoted due to the active import substitution. This requires a large amount of planting material, since planting is carried out using intensive technologies with a high density of trees per hectare (more than 1000 trees). In addition, there is a high need for garden-friendly land, since fruit trees grow in one place for more than ten years, and during this time they are exposed to all kinds of stress factors, both during the growing season and in winter. Minimizing the aftereffects of stress and preserving the yield and its high quality is possible through the use of highly adaptive varieties and rootstocks, the most favorable zones for their cultivation, as well as various agrotechnological techniques [1][2][3].
It is believed that the methodology and categories of this scientific approach have been successfully tested for the intensification of field cultivation, but are not yet fully implemented in horticulture [3].
The existing methods for diagnosing the stability and productivity of productive garden agrocenoses are labor-intensive and long-term, although it is the state of plantings that allows us to assess the plots  [4,5].
Data on the condition of plants and plantings in general have so far been obtained as a result of expeditions involving surveys of large areas. A quarterly survey of all plantings is carried out in farms. Previously unused areas are studied on the basis of historical data using expeditionary biological methods, and the assessment is based on the presence and condition of existing wild fruit crops [5]. A large amount of data is paper-based and requires subsequent manual processing [4,6].
The purpose of our research is to carry out the bonitet assessment of the stability of graft-rootstock combinations of scab-immune apple varieties growing in the Michurinsky district of the Tambov region. We have also set the task of developing methodology for using quadrocopters in the inventory of garden agrocenoses and to determine the compliance of the bonitet classes established during expeditions and using digital technologies.

Results and Discussion
The acute shortage of fresh fruits and the need for the fastest possible return of the crop by plantings on the one hand, and the need to obtain environmentally safe products on the other hand, create the need to use different varieties in plantings of different types. A comprehensive description of plantings in specific soil and climatic conditions is necessary in order to recommend certain varieties and designs to producers.
The inventory and bonitet assessment of plantings of scab-immune varieties was carried out during their entry into fruiting, when the features of growth processes and biometric indicators become the most typical for a variety.
An expedition survey of the plantings showed that the plants are in excellent condition, as evidenced by the high class of the bonitet (Table 1). of trees was often associated not with the biological characteristics of a variety, but with mechanical damage.
In our research we have also analyzed the growth characteristics of trees and their relationship with the sub-tree inventory and bonitet assessment of graft-rootstock combinations (Figure 1). By the fourth year of life, the average height of trees makes 170 cm with a crown width of 110 cm. The same compact crown is characteristic of the Bylina variety, which forms the most elongated, oval crown. The crown of the Fregat variety is 110 cm wide, but it is more spreading than that of the Bylina variety, an angle of branch union is almost 90 0 , the shoots are thick. Trees of the Flagman variety form an oval, elongated crown. Such biological and varietal characteristics should be taken into account when using drones and drawing conclusions about the development of plants. A high bonitet class indicates a high stability of the combinations in specific cultivation conditions, while the growth characteristics can vary greatly.
The results of application of this method provide quite informative indicators that assess the state of the garden, but its use requires significant time and labor. To reduce these costs, it is planned to use a quadrocopter that takes pictures of the object from a distance of up to 500 m (in some cases, the distance to the object can be reduced to the minimum allowed). In addition, video recording can be performed with the help of quadrocopters. The flight duration is from 18 minutes under normal weather conditions [8][9][10].
The video signal is transmitted over a digital channel in HD format and stored on a removable media. To obtain reliable data determined by the research purpose, the quadrocopter should fly over the garden at an extremely low altitude -from 2 m, depending on the age of the garden, while the flight speed should not exceed 1 m/s. Images of the garden and trees are obtained using the DJI Zenmuse X3 camera.  Based on the obtained images, it becomes possible to visually evaluate the tree and compare it with the standard. However, even in this case, the human factor is of great importance, since it can lead to errors.
To eliminate the human factor, it is proposed to select reference trees of certain graft-rootstock combinations with different estimates of the bonitet, and get their images. They will form the basis of the program for recognizing images. In other words, the program compares the photos obtained during the garden flyover with the reference database, which at the initial stage will have images of groups of trees with different inventory scores: 100, 90, 80...10 points. In the future, the capabilities of this software can be expanded by detailing the inventory, as well as adding new parameters, such as volume of the crown, state of the leaves, etc.
Thus, the development and implementation of technologies and software that allows for quick assessment of the condition of trees in the garden and exclusion of the subjective human factor will significantly speed up the process of evaluating the garden and determining measures for its further development.
Besides, the use of modern tools allows researchers to obtain a large amount of data on parametric characteristics of the crown, depending on the variety, rootstock, planting scheme, and even the influence of landscape features on the condition of trees.

Conclusion
The conducted research and comparison of the methods confirms the need for different approaches to assessing the state of plants in the garden and the stability of garden agrocenoses in general. The use of drones will allow improving the process of inventory and bonitet assessment and to get a large amount of data on the state of specific graft-rootstock combinations in real time.