The first factor in the disease triangle to evaluate is the host plant. Know what a healthy plant looks like-- both its aboveground portions and roots-- so that abnormalities can be recognized. Identify the plant species or cultivars that the unknown disease is occurring on. Look for patterns where abnormalities are occurring. Are there certain species or many species? Are there patterns that could suggest abnormalities caused by abiotic factors such as excesses in temperature, light, fertilizer, or pesticide application?
The second factor is the pathogen. Identifying biotic diseases is sometimes facilitated if the pathogen is visible. Particularly with some fungi, distinctive mycelium and spores can be visible on diseased tissue. Sometimes the pathogen can be seen with the naked eye but sometimes a hand lens can aid in seeing them. Gray mold (Botrytis), powdery mildews, and rusts are good examples of diseases that have distinctive spores and mycelium that are usually quite visible. Often, however, diseased tissue must be cultured in the laboratory to get the pathogen to produce mycelium, spores, or spore-producing structures that could aid identification. A light microscope can help the plant pathologist see and identify the specific pathogen in the laboratory. With viruses or other similar microscopic pathogens an electron microscope is needed to see and identify the pathogen. Sometimes evidence of a secondary fungus, bacteria or insect is visible and not directly related to the primary cause of disease.
The third factor is the environment. The diagnostician can collect information on the environmental conditions associated with the disease. Does irrigation frequency, dew, greenhouse condensation support disease occurrence? Do temperatures support the pathogen? Have temperatures been excessively high or low for plant growth? Have there been excesses applied of fertilizer, soil amendments, or pesticides that have been associated with the occurrence of disease?
References that aid in the diagnostic process are often organized so that they use and describe these three factors. To use these references, you need to first identify the host plant. After host identification, you can often go to a section where the host plant's common diseases are listed. The disease symptoms are described along with a description of the pathogen's mycelium, spores, or spore producing structures. There usually is a description of the specific disease's life cycle and any environmental conditions that support infection and development of a disease. Together, the information helps to focus the diagnostic process and formulate a likely diagnosis of an unknown plant disease.
Next: Suggested references that aid in identifying diseases and pests of ornamental plants.
For a more comprehensive look at the diagnostic process see attachment below:
The disease triangle is a fundamental principle illustrating the factors involved in the occurrence and severity of plant disease. Disease caused by a living agent requires the interaction of a susceptible host, a virulent pathogen, all in the context of a favorable environment. Plant disease is prevented when any one of these three components are eliminated. The three factors can be manipulated to reduce the risk of disease development (See previous blog).
Some plant pathologists have expanded the usefulness of the disease triangle concept by adding one or more factors such as “human activity” or “vectors” to represent special-case applications. Another useful factor is “time” and it can be represented by one of the vertices of a three-dimensional pyramid (here with the top of the pyramid pointing out at you).
The dimension of time can convey that disease occurrence and intensity are affected by the duration that the three primary factors are aligned. Disease may not occur instantly once the three parameters are favorably aligned but will occur after some time.
For example, the initiation of gray mold disease on rose petals (caused by Botrytis cinerea) can occur in just 4 to 6 hours. With free moisture on rose petals and cool temperatures, Botrytis spores germinate, penetrate the petal's surface, and its hyphae infect and develop within the plants cells.
Figure to the left: Botrytis spores formed in clusters (upper left), germinate (upper center) and penetrate the petal's surface , and its hyphae infect and develop within the plants cells (lower two figures).
The physical manifestation of gray mold is first seen as pinpoint-like lesions surrounded by a reddish border. In very favorable conditions, the hyphae develop quickly through the petal and abundant spores are formed on the surface in a distinctive gray mat in about 24 hours.
Figure to the left: Once infection is initiated, symptoms begin to manifest themselves. Gray mold on rose petals is first seen as pinpoint lesions with reddish borders (left), but later the lesions coalesce and form a gray mat composed of thousands of spores (right and close up below).
Gray mold diseases exhibit symptoms relatively quickly after initiation of Botrytis infection. Other diseases such as those caused by Phytophthora cinnamomi may take much longer. A young avocado tree, may take up to a year to exhibit symptoms after its roots are infected. An older avocado tree may take several years to exhibit symptoms.
This time lag or latency of disease expression leads to many diseased nursery stock plants to be inadvertently sold and moved in the nursery trade. An infamous example of this was in 2004 when hundreds of infected, but symptomless, camellia plants were inadvertently shipped with Sudden Oak Death (caused by Phytophthora ramorum) from a southern California nursery to many parts of the United States and Canada.
Management factors such as fungicide applications are influenced by the time factor. Most fungicides are preventative, they should be applied before infection occurs. But some other fungicides have the ability to slow or stop infection and/or sporulation. The urgency of application will depend on the disease and the fungicide tools available to control it.
Next: Using the disease triangle in plant disease diagnosis
Plant diseases– their occurrence and severity– result from the impact of three factors: the host plant, the pathogen, and the environmental conditions. This is represented with the disease triangle.
If any one of the three factors is missing, the triangle is not complete, no disease will occur. Simply, plant disease will not occur if there is no viable pathogen, or no susceptible host plant, or the environmental conditions are not favorable. The severity of disease depends on the favorable level of each factor. How susceptible is the plant? How virulent is the pathogen? How conducive are the existing environmental conditions in supporting disease and pathogen spread?
The triangle also helps illustrate that the 3 factors are interacting with each other. The clearest example of this is how the environment factor interacts with the pathogen and host factors. Previous blogs illustrate the importance of leaf wetness on pathogen infection and disease severity. Long durations of free water on a susceptible plant can increase pathogen infection and disease severity. At the same time, the low- sunlight conditions, when these wet periods could occur (e.g. winter), could also be stressful to the plant, and the plant is less likely to mount defensive reactions to fend off infection.
Learn about the biology for any disease you are managing. Consider the disease triangle and the three interacting factors, and how management practices might help weaken or break the triangle's bond. For example, could you grow non-susceptible plant varieties or species? Could you eliminate the pathogen through judicious sanitation practices? Could you manage leaf wetness and relative humidity to create unfavorable environmental conditions for disease?
Could you grow non-susceptible plant varieties or species?
Although these three plant hosts all have rust diseases, each host is infected with a unique plant pathogen that attacks its specific host. In managing these diseases, a strategy of host resistance might be employed, where crops of non-susceptible hosts are rotated into the infested area. Conceivably these three hosts could be rotated.
Sanitation is a key management practice that employs the strategy of eliminating the pathogen from the growing area. Here, sanitizing shoes before entering the greenhouse. Cleaning soil from tools and sanitizing them before use. Properly covering and disposing of rogued plants or cut flowers (not shown here!).
Could you manage leaf wetness and relative humidity to create unfavorable environmental conditions for disease?