A Venturi Effect Can Help Cure Our Trees

Summary

Compared to the more traditional hole-based methods, most of which require the tree to be drilled, tools with lenticular blades transform the basics of endotherapy easing the closure of the wound and allowing the natural uptake of the solutions.

Abstract

In woody plants, xylem sap moves upwards through the vessels due to a decreasing gradient of water potential from the groundwater to the foliage. According to these factors and their dynamics, small amounts of sap-compatible liquids (i.e. pesticides) can be injected into the xylem system, reaching their target from inside. This endotherapic method, called "trunk injection" or "trunk infusion" (depending on whether the user supplies an external pressure or not), confines the applied chemicals only within the target tree, thereby making it particularly useful in urban situations. The main factors limiting wider use of the traditional drilling methods are related to negative side effects of the holes that must be drilled around the trunk circumference in order to gain access to the xylem vessels beneath the bark.

The University of Padova (Italy) recently developed a manual, drill-free instrument with a small, perforated blade that enters the trunk by separating the woody fibers with minimal friction. Furthermore, the lenticular shaped blade reduces the vessels' cross section, increasing sap velocity and allowing the natural uptake of an external liquid up to the leaves, when transpiration rate is substantial. Ports partially close soon after the removal of the blade due to the natural elasticity and turgidity of the plant tissues, and the cambial activity completes the healing process in few weeks.

Introduction

In recent times, trunk endotherapy has progressively replaced traditional air spray methods in woody plants^1-6, but this is not a recent idea. In the 15^th century Leonardo Da Vinci described in detail how he was able to intoxicate apples by injecting arsenic solution into the trunk of an apple tree through deep holes made with a gimlet⁷. Little has changed since then: chemicals easily available in nature have gradually been replaced with more efficient synthetic active ingredients (insecticides, fungicides, bactericides, fertilizers, plant growth regulators and desiccants). Atmospheric pressure injections evolved into high pressure, and hand-driven gimlets have been replaced by modern battery drills^8-10. Unfortunately, even the sharpest drill bit tears and overheats the cambial tissue responsible for the hole closure. Consequently, wound closure is delayed and large sections of the adjacent woody tissues lose their functionality ("discolored wood") from the injection site to several feet above and below¹¹. Furthermore, unplugged holes can easily be colonized by bacteria and fungi attracted by the bleeding sap and leading to long-term internal decay, with a consequential loss of wood strength and stability^12,13.

Realizing that 1) a group of longitudinal fibers separates according to a lenticular biconvex geometry (Figure 1a), and 2) sap movements into vessels fulfill the Bernoulli's principle on fluid dynamics, in 2011 the University of Padova designed a new¹⁴ drill-free endotherapic instrument with an essential lenticular, biconvex and hollow blade that enters the wood separating its fibers. In this way, inner xylematic vessels are reached with minimal friction (Figure 1b), and the temporary reduction of their section increases sap velocity, accelerating the natural uptake of an external liquid (Figure 1c, Video 1)^15,16.

Protocol

1. Perform treatments preferably between bud break and late summer, in sunny and breezy days, according to general rules in tree endotherapy¹⁷.
2. Wear gloves, protective glasses and any other safety device according to regulations in force.
3. Get familiar with the instrument, disassembling and reassembling it. Its main components are reported in Figure 2.
4. Get familiar with the method, described in the following steps and in Video 2. Start practicing infusion technique using water in an un-pruned tree with smooth bark, fully developed canopy and very broad leaves (i.e. Platanus spp., Quercus spp.).
5. Use only toxic liquids (insecticides, fungicides, desiccants, etc.) authorized for tree injection according to local or national rules, and diluted according to the producer information (see label).
6. Measure the circumference (crf = diam x 3.14) of the tree at breast height and calculate the quantity of solution according to the technical information provided by the liquid's producer (i.e. if cfr is 190 cm and dosage is 0.7 ml /cm, a total amount of 133 ml of diluted liquid will be injected).
7. Calculate the number of ports to be made according to the diameter (diam) or circumference (crf) at breast height, envisaging 1 port/25-30 cm crf and rounding the result to the following unit (i.e. 190 cm crf = 6.3 = 7 ports).
8. Recalculate the distance between ports according to the final number of ports (i.e. 190 cm/7 ports = 27 cm)
9. Calculate the final volume of liquid/port (i.e. 133 cc in total/7 ports = 19 ml/port).
10. When using pre-filled capsules, calculate the number of ports according to the manufacturer' instructions.
11. Select equidistant insertion sites (i.e. 7) along the circumference in the first 150 cm from the ground, preferring lightly convex, smooth locations above root flares.
12. Avoid any part with anomalies able to interfere with sap dynamics, above or below the insertion site (i.e. knots, wood decay, pruning cuts).
13. If necessary, superficially smooth the bark with a knife to allow the external gasket to make a perfect seal with the bark. If the site is too rough or too curved, move a few cm to one side.
14. Choose a blade with a length compatible with both bark thickness and tree diameter and screw it to the body. In broadleaves, at least 2 cm must enter the woody tissues; in conifers and palms, longer blades are preferable to overcome the resin vessels or because of the monocotyledonae anatomy, respectively.
15. Fill a disposable pharmaceutical syringe (the cheapest and most effective solution) of suitable volume with the solution required for a single insertion (i.e. 17.14 ml), keeping the plunger in for at least 3 cm deep (i.e. a 20 ml syringe).
16. As an alternative to the syringe, use any container for tree injections (i.e. drip bags, refillable syringes, pre-filled capsules, external tanks) fitting with the conical hole (female hypodermic needle standard) or the 1/8" thread located on the tool's arm.
17. Hold the body firmly with a hand, directing the blade to the tree's center. The blade's edge must be directed parallel to the fibers (i.e. vertically).
18. With the other hand strike the sliding hammer on the body until the external latex gasket is completely squeezed.
19. Insert the syringe in the conical opening and gently draw out the plunger: air that entered the instrument during the percussion will flow out through the liquid. The resistance of the plunger indicates the perfect insertion of the blade. Note: with different couplings air cannot be drawn out and treatment will be slower.
20. Wait for total uptake.
21. If infusion is quick (i.e. 10 ml/1 min), to decrease the total number of ports refill the container before it's empty and then recalculate the number of ports. Note: to ensure a good distribution of the liquid to the canopy, at least one port /40 cm along the circumference is suggested.
22. If infusion takes too long (more than 1 ml/min; i.e. on cloudy days or in conifers and palms) turn to light injection mode, re-inserting the plunger and applying a low pressure with the thumb, or connecting the tool to a pressurized tank (ca. 1.5 bar; Figures 3a and b).
23. If injection doesn't happen, postpone the treatment until suitable physiological and climatic conditions prevail.
24. When the container is empty, wait for the remaining liquid to exit the blade (8-10 sec), remove the syringe and extract the blade by striking the hammer in the opposite direction.
25. Move to the next port.
26. Use an inert wax or gum if protection of the wound is desirable or compulsory. Note: pruning or grafting gums containing pesticides can be phytotoxic for the cambium, slowing down the wound closure.
27. In case of plants affected by transmittable diseases, disinfect both the blade and the body after treatment. Where locally acceptable, use alcohol or hydrogen peroxide. In case of heat disinfection, remove both the external and two internal rubber gaskets and treat them separately, or substitute them.

Results

Due to its small dimension and unique shape, the lenticular blade does not remove cambial and woody tissues, and when compared with traditional drill holes, the wound is visibly smaller (Figure 4a). Usually, the edges of the hole are slowly compartmentalized by cambial tissues (Figure 4b1), often because of overheating during drilling. Conversely, after treating the same tree with a lenticular blade, the woody fibers revert to their previous shape, and the cambium starts to produce meris...

Discussion

In spite of the acknowledged environmental advantages of tree endotherapy, up to now the main factor limiting a wider spread of this method has been related to the negative side effects of the drill holes used in traditional pressure methods (i.e. delayed wound closure, parasitic infections through the hole, production of inactive discolored wood).

In contrast with other methods, the described one was envisaged to work in compliance with host physiology, considering the delivery rate ...

Materials

Name	Company	Catalog Number	Comments
BITE	Vitzani srl, Perarolo di Cadore, Italy