Plants everywhere, including the ones we like to grow, are food for a large number of organisms.  When these organisms try to consume the plants we care for, we call those organisms pests, pathogens or diseases.  And we try very hard to stop them from consuming our cherished plants before we do.  The fight against pests and diseases can sometimes seem very challenging, especially when conditions favour a particular pest or disease, and the challenge can also be just trying to eliminate the pests without eliminating our plants or ourselves.

The relationship between plants and the organisms that attack them is complex. To increase our ability to combat these living threats it’s helpful to have at least some understanding of these complexities.

So why do pests and diseases attack some plants and not others? When you see how a plant can be devastated by some pest or disease, you’d wonder how plants ever survive on their own and without our help. Well, the good news is that plants have very effective defense mechanisms that allow them to fight back against their attackers. The reverse side of that, of course, is that the attackers are always evolving methods to bypass these defenses. These defence mechanisms are quite complex and depend on the relationship between the plant and the particular organism.

With pests such as caterpillars, some plants have evolved to emit a chemical signal when they are attacked. This signal attracts a parasitic wasp which homes in on this signal to find the caterpillar. The parasitic wasp then lays its eggs in the caterpillar’s body, the eggs hatch and eat the caterpillar from the inside out.  In other cases, the plant will produce toxic chemicals which inhibit the caterpillar’s attack by reducing either the palatability or digestibility of the plant. In this way, even though the caterpillar may eat enough to reach adulthood, the plant is also able to retain enough structure to flower and produce seed, which, after all, is its main purpose in its life. Sometimes, plants will enlist the help of birds by timing the production of leaves to the springtime hatching of birds’ eggs, the consequent requirement for food means the birds will need to collect as many caterpillars as the can to feed their young. In this way, plants have cleverly used their environment as a protective mechanism.
Diseases such as fungi, bacteria and viruses pose another problem for plants.  Usually, these organisms get inside the plant and attack at a cellular level. They either invade the plant’s DNA in the case of viruses, or actually feed on the contents of the plant cells. These attacks produce both short and long term responses.

In the short term, at the first sign of an attack by a pathogen (disease causing organism), the plant produces a group of chemicals known collectively as phytoalexins. These phytoalexins are the plant’s defence system against pathogens. They are usually toxic to the pathogens, destroying or inhibiting them in some way. For instance, the plant is able to produce enzymes or free radicals which directly attack the pathogen .It can even instruct its own cells surrounding the area of attack to commit suicide in order to create a physical barrier to prevent the advance of the pathogen. In the longer term, the pathogen’s attack will have induced chemical signals which will continue to flow around the plant ready to respond to another attack. This is called Systemic Acquired Resistance.

Of course, in all of this, the pests and pathogens are not helpless. They, in their turn, are always evolving ways to get around a plant’s defences. In addition, the environment plays its part in the overall struggle. When the environment is suitable for the pathogen, it is the pathogen which may win the battle. The opposite also applies. Sometimes, the environment so favours the plant that a pest or pathogen can really make no headway at all.  When these conditions continue for prolonged periods of time, and they sometimes do, some species of pathogens will become extinct and the same thing can happen to plants too.
Thrown into this mix is the variability displayed by both plants and their attackers. Some pathogens are so weak that they are hardly able to cause any damage while some are so virulent that no plant can withstand their attack. In humans, the common cold is a good example of variable virulence. Some stains are mild, others are severe.  Then there are the plant factors. Some plants are genetically more able to withstand attack than others.

So, what does this all mean to the average grower? The main benefit arising from the discoveries relating to the interaction between plants and their parasites (diseases and pests) is the isolating of many of the chemicals the plants use to defend themselves. These chemicals, once isolated, can be used in a concentrated manner against these pests and diseases.
It is now possible for the average grower to buy and use these phytochemicals (plant-derived chemicals) as a method of plant protection rather than using toxic poisons.

The relationship between plants and pathogens and pests is a fascinating one and is very much more complex in its detail than has been described here. An article like this can only just provide an overview of the subject. Ongoing research around the world is producing deeper and more thorough understanding of this relationship.