Healthy plants are generally more able to withstand pathogens pests and abiotic stresses.
The disease of a plant is considered a reaction of the plant disease triangle where the components are plant host pathogen and the environment.
Classic studies from University of Wisconsin show that Fusarium graminearum a pothogen of both maize and wheat is prevalent in maiz a sub optimum temperature for the plant and in wheat on too high temperature for the plant.
Unfavorable temperature for the plant was more important than the role of optimkized temperature for the pathogen.
When soil condtions are optimized the environment for root pathogens is minimized. The excess of the water with low oxygen is an environment which is unhealthy and predisposes the plant to Pythium pathogen.
When the soil pH and nutritional levels are adequate a vibrnat beneficial mycoflora can inhibit many root pathogens.
In the case of soil organic matter the humic acid components have been demonstrated to stimulate a series of biochemical pathways which allow the effective plant reaction to reduce the onslaught of pathogens, pests and abiotic environmental threats such as heat and drought.
That's because both the biotic and abiotic factors surrounding the healthy plant would be optimized enough to prevent or at least minimize pest attack. Let's take this analogy: suppose you are eating healthy or following a healthy lifestyle, as such keeping yourself healthier would mean boosting your immune system. As a result, your body would keep itself healthier through resisting disease-causing microbes. A healthy plant has a potent ability to defend itself against outsiders. What you need to do is make sure you use healthy seed or planting materials, optimized watering, fertilization, temperature and humidity conditions.