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Copper nanoparticles for kiwifruit bacterial canker

Kiwifruit
Comparison of the activity of copper nanoparticles on the plate and on the plant
The plate on the left, the plant on the right. The vast laboratory difference does not appear on the twig wounds.

Savvas Pastopoulos, Agronomist MSc, Pastopoulos Agronomics, Neos Mylotopos, Pella, Greece

Kiwifruit bacterial canker, caused by Pseudomonas syringae pv. actinidiae, is managed in practice with copper. A Chinese study published in 2022 prepared three kinds of copper nanomaterials and compared them with a conventional bactericide, first on agar plates and then on plants. On the plate the nanomaterials were at least sixty four times more potent. On the plant the difference disappeared. This text sets out what was measured and why the two results disagree.

1.56 μg/mLthe concentration that kills the bacterium on the plate, for all three nanomaterials
64 timesat least, the gap from the conventional bactericide on the plate
67.1 %the best curative activity on the plant, against 53.7 for the conventional product
Where the figures come from The study was carried out in China, in a climate chamber, on two year old kiwifruit plants of the cultivar Hong Yang Hongxin, with artificial wounds on the twig and assessment after fourteen days. The comparator is thiodiazole copper, a bactericide used widely in China that is not approved in the European Union [3]. What transfers is the mechanism, that is which factor controls what. The percentages do not transfer, because they come from another cultivar, another climate, artificial inoculation and a product that is not on the market here.
Contents
  1. What was compared
  2. On the plate: the difference is vast
  3. On the plant: the difference disappears
  4. Sources

1. What was compared

Three copper nanomaterials were prepared by chemical precipitation and characterised by electron microscopy, X-ray diffraction and spectroscopy [1]. They differ in composition and, above all, in shape and size:

NanomaterialFormSizeCopper content
Copper hydroxide, Cu(OH)2nanowires13 nm diameter, 202 to 823 nm long71.5 per cent
Copper phosphate, Cu3(PO4)2nanosheets93 to 497 nm41.6 per cent
Copper oxychloride, Cu4(OH)6Cl2nanoparticles14 to 51 nm, median 30 nm67.9 per cent

The point worth attention is that the same chemical compounds in ordinary, non nanometric size were almost inactive at the concentration at which the nanomaterials killed the bacterium [1]. The difference therefore is not one of chemistry, it is one of size.

Sources: 1

2. On the plate: the difference is vast

The minimum bactericidal concentration was 1.56 μg/mL for all three nanomaterials. For thiodiazole copper it was above 100 μg/mL, that is at least sixty four times higher [1]. At half that concentration, at 0.78 μg/mL, the three nanomaterials inhibited the bacterium by 88.5, 91.7 and 88.3 per cent, while the comparator was practically inactive.

The mechanism is not simply the release of copper ions. After sixteen hours, the copper that had passed into solution did not exceed 2 per cent of the mass of the particles [1]. Electron microscopy showed that the particles attach to the wall, penetrate it or deform it, and that reactive oxygen species accumulate inside the cells. Thiodiazole copper caused neither damage to the inner membrane nor any rise in reactive oxygen species, which means it acts in an entirely different way.

Sources: 1

3. On the plant: the difference disappears

On the two year old plants, applied at 200 μg/mL to artificial twig wounds, the figures change character. Applied before infection, protective activity was 59.8 per cent for the nanowires and 63.2 for the nanosheets, against 64.4 for thiodiazole copper, while the copper oxychloride nanoparticles stayed at 40.2. Applied after infection, curative activity was 67.1 per cent for the copper oxychloride nanoparticles, 56.1 for the nanosheets, 53.7 for the comparator and 43.9 for the nanowires [1].

What these differences mean statistically Analysis of variance showed that in protective activity only the low value of the copper oxychloride nanoparticles stands apart, and in curative activity only the difference between those and the nanowires. All the remaining pairs do not differ at a significance level of 0.05 [1]. In other words, no nanoform proved clearly better than the conventional product on the plant.

The explanation the authors give rests on size. The larger particles form a dense layer on the surface and block infection, so they work protectively. The smaller ones enter the tissue more easily and travel, so they reach the bacterium that has already established itself and work curatively [1].

Particle size decides whether the activity is protective or curative
The same metal, two roles. The role is set not by the chemistry but by the size and shape of the particle.

Two further findings have practical value. The first is that on leaves the nanoparticles spread out evenly, while the conventional products leave uneven aggregates [1]. The second is that the copper phosphate nanosheets settled out completely within one hour in water, which in a spray tank is a serious drawback, whereas the copper oxychloride nanoparticles remained stable.

How far this is from the orchard The application on the plant was ten microlitres of a 200 μg/mL solution placed into each artificial wound, not a spray over the canopy, and the plants were then held in a chamber with fourteen hours of light at 14 °C and ten hours of darkness at 10 °C, at 85 percent relative humidity. No trial in a real orchard is included. The authors state explicitly that the work is only a beginning and that the distribution, conduction behaviour and toxicity of the particles remain to be studied in the kiwifruit plant [1]. No data are given on rainfastness, on residues, on phytotoxicity to the fruit or on behaviour under orchard spraying conditions.

Sources: 1

4. Sources

The numbers in the text refer to the list below.

Disclaimer This text is informative and does not replace an on site assessment. It recommends no product, no active substance and no technology. The percentages of the study come from a climate chamber in China, on a Chinese cultivar, with artificial inoculation and against a comparator that is not approved in the European Union, and they do not transfer to a Greek kiwifruit orchard. No copper nanoform is authorised as a plant protection product in Greece [2], and the use of an unauthorised product is illegal. Any use of plant protection products follows solely the authorisations of the Greek Ministry of Rural Development and Food and the label, and those authorisations are valid in Greece only and do not concern other countries. Foliar copper sprays on trees carrying fruit can cause spotting on the fruit and scorching of the foliage, so a test on a limited number of trees with the same product and the same rate precedes the application, with five to seven days of waiting, and the spray is applied in the evening or early in the morning, never in a heatwave and never on foliage under water stress. Every intervention is carried out under the responsibility of the user, following an on site assessment by a licensed agronomist. Pastopoulos Agronomics accepts no liability for the use of this information.

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