Perhaps the best illustration of how plant nutrition and health are associated is with excess nitrogen (N) resulting in the production of succulent plant tissue that makes for easy invasion by insects and disease organisms.
There exists in the environment some level of disease and insect forms awaiting those conditions that will favor their growth and reproduction. These conditions may be environmental, such as temperature and humidity, but in addition, there are certain plant conditions that will also serve to provide the host for their development. Succulent plants provide the ideal condition for invasion and reproductive growth.
Anyone who has grown greenhouse tomatoes is well aware of the white fly, a pesky insect that is difficult to control. What is that part of the plant that the white fly thrives on? That would be the emerging leaf tissue, the most succulent part.
The home tomato gardener also has the hornworm to worry about. Hornworms are not easily seen due to their green color, and in a few short hours do considerable damage, munching on newly emerging plant tissue. Then, there is powdery mildew—a common occurring fungus that affects a wide variety of plants, particularly cucumber when grown in the greenhouse. A grower found that as his cucumber plants began to set fruit, the mildew infestation would occur even though he took protective measures.
It was also found that when the plants began to set fruit, the magnesium (Mg) concentration in the plant dropped. As he adjusted the nutrient solution formulation so as to increase the Mg plant level, it was found that the intensity of the infection was markedly reduced, and in addition, the applied preventative measures seemed to be more effective.
More recently, the role silicon (Si) has in making plant tissue—primarily cell walls—more difficult for invasion by sucking insects and disease organism hyphae has been observed. It is now recommended that some form of soluble Si, either as silicic acid, or potassium or sodium silicate, be included in a nutrient solution formulation with the recommended Si concentration being at least 140 ppm.
For plants grown in soil, or some types of soilless media, the inclusion of Si in an applied fertilizer solution is probably not needed. Both soil and soilless medium can be tested to assess Si availability.
For those hydroponic growing systems in which an applied nutrient solution is recovered and recirculated, root disease occurrence is not uncommon. A recovered nutrient solution is an ideal "soup" for organisms to thrive in. Therefore, the recommendation is to filter the nutrient solution to remove the suspended organic matter, and then sterilize before recirculation.
I am reminded of a time back when, for many months, a tomato grower was producing excellent yields of quality tomatoes with plants rooted in gravel in a series of self-designed and constructed individual flood-and-drain hydroponic growing units. Each unit was being supplied with nutrient solution from a central storage tank.
With frequent replacement of the nutrient solution, there was little opportunity for any disease organism to multiply in the circulating nutrient solution. However, experimenting with a nutrient flow technique (NFT) system, the grower found that he could use the same nutrient solution for a longer period of time, which turned out to be a fatal mistake.
Pythium, a common root disease organism, invaded the nutrient solution and delivery system and in less than 10 days wiped out his entire tomato crop in both hydroponic growing systems. Unable to sterilize the entire growing system, which included the gravel beds, he lost his entire business.
Filtering and sterilizing the nutrient solution would have prevented this failure. Also, I had recommended the inclusion of milled composted pinebark as a component in the gravel bed since pinebark has been found to prevent the growth of pythium. Unfortunately, the grower was unable to cover from this loss and start again.
There are also important mechanical procedures that will minimize the potential for insect and disease infestations, such as maintaining air movement up through the plant canopy that keeps the foliage dry and makes it difficult for insects to fly into the canopy.
Air movement over plant leaves sustains natural transpiration that is the mechanism for supporting the upward movement of the transpiration stream that carries nutrient elements up into the plant, thereby maintaining nutrient element sufficiency.
In addition, keeping the relative humidity of the air surrounding the plant at less than 50% is desirable since most organisms require atmospheric moisture to survive and reproduce. Proper sanitation practices, such as keeping the plant area clean and free from plant debris that can be a harbor for disease organisms, are equally essential.
Since disease organisms and insects are both opportunistic and take advantage of those conditions that suit their development, they can be kept under control by using a combination of preventative measures that includes maintaining plants in a nutrient-element sufficient state.
As you can see from the examples above, there are many ways plant nutrition correlates with disease and infestations. The key is understanding why your plant's are in trouble and resolve their issues before it's too late.