First of all, we need to know about the plant to be treated first hand. Therefore, we must differentiate between a tree and a palm tree. A very important feature that distinguishes them is their growth; trees have primary and secondary growth, while palm trees (monocot angiosperms) lack secondary growth.

On the other hand, we need to distinguish between two groups of trees, those belonging to the gymnosperms, which include the group of conifers (pines, firs, cedars, etc.) and those belonging to the dicot angiosperms.

This last group of trees is divided into two subgroups according to the porosity of their wood: diffuse porosity trees (banana, lime tree, poplar, etc.), which have the conductive vessel elements uniformly distributed along the xylem, and annular porosity trees (holm oak, oak, cork oak, catalpa, elm, etc.), which have large vessels in the youngest part of the trunk (in the outermost xylem).

These characteristics will determine the behaviour of the product in the plant’s vascular system and the compartmentalisation or mineralisation of wounds.

Then, we must study the aetiology (the origin) of the pathogen causing the damage. First, we must define whether it is an insect, a microorganism, physiological damage, nutritional deficiency or a combination of more than one of these pathogens. However, a large part of the problems are caused by a type of insect or mite. In this case, we need to properly assess its biological cycle; which family it belongs to, its stage of development and in what season of the year the damage takes place. This way, we will know how, when and with what to carry out the treatment.

In the case of diseases, we must identify the microorganism causing the problem. Most times, this will be a more complicated task than that of identifying an insect, since lab tests are required more than often. Nutritional deficiencies are usually more evident, especially chlorosis due to a lack of iron. However, there are other chlorosis types due to the lack of some other micronutrients. This may also require lab testing to determine which of them is insufficient.

Plant endotherapy plays a fundamental role in IPM (Integrated Pest Management) control strategies for pests, diseases or physiopathies.
Therefore, aetiology responds to how, when and with what product we need to perform the treatment.

The applying technician must know firsthand what system they have in their hands to carry out the treatment. Of course, it must be an equipment approved by the European Union to avoid risks, both for the person who applies it and for the plant and other people.

There are two very important features that must be taken into account to achieve good control without causing damage to the plant:

• The working pressure: the range of pressures can vary depending on the type of wood of the plant to be treated.
• The volume that will be injected: some equipment works with macro-infusion and others with micro-infusion, depending on the volume that will be injected.

Therefore, the more precise the system used in terms of pressure and volume control, the more the error on the part of the person applying it is minimised, and the safer and more efficient the treatment will be.

This section, together with the next one, carrier substance, determines the dosage of the treatment, that is, the dosage of the product to be injected. To do so, we need to know the nature of the plant protection product or nutrient. In the case of plant protection product (insecticides and/or fungicides), we need to know the concentration of the active substance, its chemical family and how it acts on the insect or micro-organism.

By using plant endotherapy (ENDOterapia Vegetal), small but highly-concentrated doses of the plant protection product are injected. This is why it is very important to use products that give the maximum possible quality guarantees. There are products with a low active substance percentage, which means that the rest are adjuvants that can significantly modify the qualities of a product or another with the same active substance. This fact that can also affect the final result of the treatment.

Knowing the chemical family and the mode of action is also important. We need to know if they act either by affecting the nervous system, such as neonicotinoids (agonists of the nicotinic acetylcholine receptor) or by affecting the nervous and the muscular system, such as avermectins (chloride channel activators).

In the case of nutrients, their content of macro and micronutrients is also very decisive, since an excess of some of them can cause phytotoxicity.

Carrier substance or emulsifier is understood as the product that stabilises an emulsion with a plant protection product so that it properly spreads inside the plant, favouring its distribution and persistence.

Its function is to balance pH, electrical conductivity and density, in addition to achieving a completely homogeneous mixture over time. This avoids the precipitation of the active substance or any other component of the plant protection product.

Precipitation can cause phytotoxicity problems due to an excess of concentration of the plant protection product. The precipitate can collapse the conductive vessel elements and consequently cause the non-translocation of the product in the whole plant.

Because plant endotherapy is a technique that works through the sap flow of the plant, which is determined by respiration and photosynthesis, the application season and the climatic and environmental conditions will be very important for these two factors to be fully operational.

The season of application should coincide with the time of active growth of the plant, which is the period of maximum sap flow. Transpiration through the leaves creates a negative pressure in the xylem (less than 1 atm), which allows the “raw sap” to flow upward. The raw sap transports water, minerals and other elements in solution, such as the injected product.

Climatic conditions intervene in the transpiration process. Therefore, having optimum light, warm temperatures and relative humidity is very important. These factors affect the opening of the stomata, which are the main actors in charge of plant transpiration. Solar energy causes an increase in temperature that accelerates the transpiration rate. A high relative humidity causes slower water loss because the air is saturated with water vapour and the plant closes its stomata. Wind also has an influence on this process. It increases the gradient of water vapour concentration between the inside of the leaf and the surrounding air, dragging the water vapour from the leaf surface.

Light and temperature are also decisive for the photosynthetic process in which the plant elaborates its food. It transforms raw sap into sap made from atmospheric CO2 (carbon dioxide), water and minerals with the help of sunlight. The sap it makes mainly contains sugars, besides water, plant growth regulators and dissolved minerals. It is transported by the phloem from the leaves and the green stems to the roots, passing through the whole plant.