Understanding the two critical conditions for pest disease development, known as the Disease Triangle and Disease Pyramid, is key to effectively managing and preventing the spread of diseases in agricultural systems.
First, let’s explore the Disease Triangle. This concept highlights the three fundamental elements necessary for a disease to thrive: a susceptible host plant, a virulent pathogen, and suitable environmental conditions.
When these three factors converge, the likelihood of disease outbreak increases significantly. Imagine it as a triangle with the host, pathogen, and environment at each point. If any one of these factors is absent or inadequate, the risk of disease development diminishes.
Next, we have the Disease Pyramid, which expands on the Disease Triangle by introducing two additional components: the time factor and the management factor.
The time factor emphasizes the critical window during which the disease is most likely to spread, considering the life cycle of both the host and the pathogen.
The management factor, on the other hand, emphasizes the crucial role of human intervention and control measures in breaking the chain of disease development.
Effective management strategies, including cultural, biological, and chemical controls, can disrupt the progression of the disease and minimize its impact on the agricultural ecosystem.
By grasping the dynamics of these models, farmers and agricultural experts can proactively implement preventive measures. This might involve selecting disease-resistant plant varieties, optimizing cultural practices to create unfavorable conditions for pathogens, and implementing timely and appropriate management strategies.
Understanding the interconnected nature of the Disease Triangle and Disease Pyramid empowers growers to make informed decisions, ultimately fostering a healthier and more sustainable agricultural environment.
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The 2 Conditions for Pest Disease Development
1. Disease Triangle
Disease development is dependent upon three conditions: a susceptible host plant, a favorable environment, and a viable pathogen. All three of these factors must be present for the disease to occur.
The image below presents this concept as a ‘disease triangle.’ Each side of the triangle represents one of these factors: host plant, environment, or pathogen. When all three sides of the triangle are complete, disease occurs.
If one of the conditions is not present (one side of the triangle is missing), then disease does not occur. By altering the susceptibility of host plants, the surrounding environment, and/or the viability of pathogens, the disease triangle can be broken and disease development prevented.
Host plant genetic makeup determines its susceptibility to disease. This susceptibility depends upon various physical and biochemical factors within the plant.
A plant’s stature, growth habit, cuticle thickness (a protective outer layer on plant tissues), and shape of stomata (small openings that allow water, oxygen, and carbon dioxide in and out of plant tissues) are a few physical factors that influence disease development.
Plants may also produce biochemical compounds that limit or prevent colonization or infection. The growth stage and ability to deter pathogens can also impact plant susceptibility to disease. For example, young leaves are often more susceptible to infection than mature leaves.
Environment plays an important role in disease development. Pathogens generally require specific environmental conditions for infection and spread. Most plant pathogens require high humidity and moderate temperatures.
Other pathogens, such as bacteria and water molds, require surface water to spread. In some disease cycles, environmental conditions influence the development of symptoms.
For example, extreme temperatures or drought can cause plant stress; this loss of vigor can increase host susceptibility to both infection and disease development. Other environmental factors affecting disease can include those resulting from planting and maintenance practices.
For example, high-density plantings can have higher relative humidity, while overhead watering increases leaf surface moisture needed by pathogens to infect plant leaves.
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Pathogens: A plant disease is any physiological or structural abnormality that is caused by a living organism. Organisms that cause disease are referred to as ‘pathogens,’ and affected plants are referred to as ‘hosts.’
Many organisms rely on other species for sources of nutrients or as a means of survival but are not always harmful to the host. For example, saprophytic organisms obtain nutrients from dead organic material and are a vital part of many ecosystems.
Plant pathogens, on the other hand, utilize hosts for nutrients and/or reproduction at the hosts’ expense. Disease-causing organisms include fungi, oomycetes (fungus-like organisms called water molds), bacteria, viruses, nematodes, phytoplasmas, and parasitic seed plants.
Once a pathogen infects a host, symptoms often develop. Symptoms are the outward changes in the physical appearance of plants. Symptoms take time to develop, and thus, disease development may be delayed for several days, weeks, months, or even years after the initial infection occurs.
Examples of symptoms include wilt, leaf spots, cankers, rots, and decline. Physical evidence of pathogens (called ‘signs’) may also be observed on diseased tissue. Examples of signs include fungal fruiting bodies, bacterial ooze, nematode cysts, and fungal mycelia. Both symptoms and signs are utilized in making disease diagnoses.
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2. Disease Pyramid
The disease pyramid describes how disease can eventually destroy a plant. It is comprised of the presents of the pathogen that causes the disease, the plant or host, the environmental conditions that set up the pathogen to go after the plant, and time.
It requires all four at the same time to have a disease. So in the case of avocado root rot, it’s necessary to have Phytophthora cinnamomi there with a susceptible avocado root, in warm, moist conditions for a period of time for the disease to express itself.
For a young tree, the disease may show up within a year but for older trees, it may take several years to see the disease symptoms. For avocado crown rot, it may take several years for Phytophthora citricola to appear.
In the case of Huanglongbing, not only is the pathogen required, but also the vector for spreading it, the Asian Citrus Psyllid. Not only time is important, but the timing of sampling and the point of sampling is important.
In the case of avocado root rot, the best sample is from a recently infected root in the hot dry season. If the wrong leaf or root is sampled for the Huanglongbing bacteria, the assay will show a negative response, even though it is in the plant.
And then too, even if a disease is finally diagnosed, it doesn’t mean the tree is going to die. If Phytophthora is caught soon enough, it can be treated with phosphites, but the environment needs to be changed at the same time.
Meaning, the irrigation needs to be adjusted so that the treatment will work. If the irrigation is changed, the trunk canker will disappear.
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