Fireblight infection biology
Open flowers of apple and pear can be infected by Erwinia amylovora when bacteria are present in the orchard surroundings, and the weather conditions are favorable. The model calculates potential infection events during flowering.
- In the model, each day a new flower cohort opens.
- When the conditions are favorable for flower visits by insects (Default setting: 3 hours >=15 °C), we assume contamination with bacteria. A black line starts in the bottom graph, showing the increase of the bacterial population on the surface of the stigma of the flower.
- The bacterial population grows, and at the same time flower gets older and becomes less susceptible. On older flowers more bacteria are needed to cause an infection.
- At high temperature the bacterial growth is faster than the aging of the flower. And critical situations are likely to develop.
- At lower temperature flowers age faster than the bacterial population can grow.
- The black line stops when the balance between potential growth of the bacterial population, and declining susceptibility excludes the possibility of an infection.
- When the bacterial population reaches a critical level at the time the flowers are still susceptible, there is a risk for infection.
- In this situation any form of free water in the flower will permit the bacteria to enter the plant trough the flower cup.
- Low numbers of bacteria penetrating the flower cup, calculated based on lab trials, do not show up in the field. Infection levels over the threshold of 0.2 are likely to cause fireblight in the field.
- After reaching the infection threshold, the history of this flower cohort is turning red to see the day these flowers opened.
- After successful infection, the bacteria move and multiply in the plant tissue. The multiplication rate is depending on temperature and water status of the plant tissue.
- The red line in the upper graph indicates the growth of the bacterial population in the plant. At a predicted level of around log 8 CFU, the plant tissue rupts, discolors and wilts. With that the symptoms of the infection become visible.
Timing of preventive treatments with antagonists
The mode of action of most antagonists is to slow down the development of the bacterial population on the stigma, to prevent growth over a critical level. The product must reach the stigma to be effective. Therefore, the application should be made when the flowers at risk are already open.
When an infection event is forecasted, you can follow the red line down to find the day these flowers opened. The application should be made after this day, and before the infection is reached.