The plant as a living entity

In the previous step, we explored how the search for a living and balanced soil is of utmost importance to improve soil fertility, understood as chemical, biological and physical fertility, and consequently to have a better chance of success in crop development.

In this sense, it must also be taken into account that plants are living beings, and therefore, their susceptibility to pests and diseases can vary according to their nutritional and physiological state, which is strongly influenced by soil fertility. In this step, we’ll discuss the trophobiosis theory, which attempts to explain the incidence of pests and diseases in relation to the internal state of plants.

Nowadays, many farmers have a very reasoned approach to pest and disease control, prioritising biological control, the use of pheromones, or even other plant protection products allowed by the organic production regulations, in place to reduce the use of synthetic plant protection products as much as possible.

However, these products have collateral effects on the agro-ecosystem, creating imbalances that end up negatively affecting crops, such as:

• the increased incidence of a pest that was previously controlled; or
• the result obtained in the control of the pest being not the same depending on the plot; and
• a decrease of the nutritional content of the plants that are being treated.

The trophobiosis theory

Francis Chaboussou was a French researcher who did a lot of work on the relationship between plant and parasite. The conclusion of all his studies was that “Every vital process is closely dependent on the satisfaction of the nutritional needs of the pest or disease”._[1]

In other words, if the pest or disease finds the right nutrient substrate in abundance to be able to carry out its lifecycle, its incidence will be higher. An example could be an excess of nitrogen, which generally leads to a higher incidence of aphids, as they need nutrients rich in this element.

Therefore, growers must try to seek this complex balance, avoiding as far as possible agricultural practices that break this theoretical equilibrium.

With the Green Revolution and the appearance of synthetic phytosanitary products, pest control has been simplified to the choice of the product that is theoretically most effective, often forgetting traditional agricultural practices that indirectly helped control pests.

In this context, the trophobiosis theory helps to move towards a more rational agriculture, taking into account the repercussions that certain agricultural practices such as fertilisation, irrigation, pruning, choice of plant material, etc. can have on plant physiology, and which can be key to maintaining a good health status of the crop.

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Choice of plant material

The correct choice of plant material according to the environmental conditions of the growing site is key to its success. It’ll have a great impact on the health of the crop, and it is also a decision that will not involve major changes in the investment to be made. However, it will have a direct influence on increasing cultivation costs, if the initial choice of plant material is not the right one. In that case, farmers will have to take action to solve the problem that they have created.

Generally speaking, a variety, or rootstock in the case of woody crops, that are well-adapted to the environmental conditions (soil-climate-orography) of the growing plot, will be less susceptible to pests and diseases.

Plants possess several defence mechanisms, such as the presence of antimicrobial substances, genetic resistance, and the absence of nutrients necessary for the pest/disease cycle. The trophobiosis theory states that plants will be less susceptible if they have lower contents of simple, more easily transported compounds such as amino acids and sugars, which are used to create more complex structures such as proteins or starch. Therefore, they should not be lacking, but neither should they be stored in the plant in excess, as they will make it more appealing to pests and diseases.

In many cases, when a pest or disease is not able to develop in a particular variety, it is because this variety does not supply it in sufficient quantities with the nutrients it needs for its development.

As in humans, much depends on the internal metabolism of the plants. For example, foreign varieties, with which the breeding and evolution process have been developed to specific environmental conditions, are generally not adapted to environmental conditions in another region. In that sense, local and traditional varieties will be better adapted, and less susceptible to pests and diseases.

For example, Chaboussou did a lot of work on vine cultivation, studying various varieties of rootstock combinations, and the differences in their sensitivity to pests. He observed that depending on the rootstock on which a variety was grafted, the amino acid content could vary. Combinations with a higher percentage of amino acids in the leaf structure were shown to be more susceptible to downy mildew* and red spider mite._[1]

_{*Downy mildew: a widespread, serious disease of grapevines. Initial leaf symptoms are light green to yellow spots, called “oil spots” because they appear greasy. Under humid conditions, white, downy spore masses can be seen on the lower leaf surface. These spores are wind dispersed.}

_{Photo by Alejandro Barrón on Pexels}

Effects of fertilisation on plant health

High doses of mineral fertiliser often create imbalances in crops, as does excessive organic fertilisation, thus affecting plant resistance. For example, an excessive supply of nitrogen leads to an overproduction of amino acids, with all the subsequent negative consequences that this entails, as mentioned above.

Potassium also plays a fundamental role, interacting in an important way in the plant-parasite relationship, with both positive and negative effects._[2] On the one hand, it stimulates protein synthesis, i.e. less free amino acid content and less susceptibility.

On the other hand, it increases the osmotic pressure* of the processed sap, making it less easily exploitable by fungi or sucking insects as there is less free water. However it is easier for pathogens to enter by destroying cell membranes.

It should also be noted that soluble nitrogen levels and transport are increased by potassium deficiency, which accelerates leaf ageing, causing protein hydrolysis._[3] In short, neither potassium excesses nor potassium deficiencies are good, and both cause imbalances in plant metabolism.

In addition, there is a tendency to simplify fertilisation to macro nutrients (N, P, K, Ca, Mg and S), ignoring other trace elements such as boron or manganese, which play a very important role in plant metabolism, and therefore in resistance to pests and diseases. In fact, some studies have shown that plants that were provided with trace elements produced a greater quantity of protective substances of phenolic* origin, compared to plants that were only provided with macroelements._[4]

Just as plant health affects the incidence of pests and diseases, it also affects viruses, bacteria and phytoplasmas*. On the one hand, it has a direct effect on insect vectors of viruses but on the other hand, virus multiplication depends on simple forms of nitrogen and cannot use the more complex plant proteins._[1]

_{*Osmosis: the process in plants by which a liquid moves gradually from one part of the plant to another through a membrane (cell covering).}

_{*Phenols: a family of organic compounds characterised by a hydroxyl (―OH) group attached to a carbon atom. Phenols are similar to alcohols but form stronger hydrogen bonds. Thus, they are more soluble in water than alcohols and have higher boiling points. They occur either as colourless liquids or white solids at room temperature and may be highly toxic and caustic.}

_{*Phytoplasma: a group of bacteria that are related to mycoplasmas, cause plant diseases by infecting phloem tissue and are transmitted especially by homopteran insect vectors. For almost half a century, plant pathologists thought phytoplasmas were viruses. To this day, the inability to grow these bacteria outside plants or insects has hindered efforts to get a handle on their biology and genomes.}