- Author: Steven A. Tjosvold
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?
- Author: Steven A. Tjosvold
The majority of fungi, aerial nematodes and bacteria that cause plant diseases require liquid “free” water on the plant surfaces before they can infect the plant. Free water, sufficient for disease infection, commonly occurs in the form of rain, fog, and dew. Or it can even occur after a sprinkler irrigation , syringing , or pesticide spray. Moreover, many fungi need high relative humidity to produce spores, and abundant free water can contribute to high relative humidity. Dew formation is triggered when the surface temperature of the plant surface drops below the dew point temperature of the surrounding air (See previous blog post). This typically occurs at night in greenhouses that are not ventilated and heated properly, or outside on calm clear nights. Plant pathogens have different leaf wetness requirements for infection to occur (as seen below).
Many diseases will get worse with longer leaf wetness periods. In the table below you can see that the severity of gray mold (Botrytis cinerea) on rose increases with longer wet periods.
Commercial disease prediction models exist for apple scab, cedar apple rust, potato late blight, tomato early blight, strawberry anthracnose, botrytis fruit rot, citrus brown spot, lettuce downy mildew, grape powdery mildew, among others. Leaf wetness sensors help quantify leaf wetness periods, and models predict disease risk These systems can reduce the number of sprays that are needed for disease control. It has been recently suggested that disease models instead use leaf wetness data based on a simple empirical model using relative humidity. Relative humidity sensors can be standardized and calibrated more easily than leaf wetness sensors.
Many current greenhouse control systems can help collect and organize data from leaf wetness, relative humidity and temperature sensors. Alternatively, a simple environmental monitoring system can be pieced together for an outdoor nursery or greenhouse using readily available sensors and data loggers from various companies (e.g. , Campbell Scientific Inc (Logan, UT), Onset (Bourne, MA), Spectrum Technologies Inc. (Aurora, IL). Most disease risk models have not been tested in ornamental crops but there is no reason why they cannot be wholly or partly used for disease risk monitoring in ornamental crops. Botrytis models have been intensively studied in other crops and should be one of the first to try in ornamental crops. Empirical evaluation of these models in the field is a first step to confirm their usefulness. Models that predict high disease risk could improve scouting efficiency by targeting more intensive scouting during these periods, help reduce fungicide applications by predicting optimal timing of fungicides before infection occurs, and target periods when de-humidification cycles are needed in greenhouses.