Conditions contributing to increased risk from Fire-Blight

Fire blight is a disease caused by a bacterium Erwinia Amylovora, which can kill blossoms, fruits, shoots, limbs and even an entire tree. The bacterium gets entry into trees through the tips of branches and travel down into stems, resulting in death of tree parts. The disease is more prevalent during cool, wet springs with the pathogen growth and development aided by moisture which is present in orchards through precipitation events.

Most importantly, fire blight is often influenced by seasonal weather. This is now more difficult to predict and manage the disease. We experience late springs and early summers which was never the case in the past. These are periods with warm moist weather conditions. However, the bacteria growth slows down in late summer, fall or winter. This is because the conditions during these periods do not encourage spread of bacteria, since there are fewer precipitation events towards the end of the growing season.

The conditions which contribute to increased risk from fire blight on any orchard are as follows:

• Bacteria multiplies in spring
• Rainfall, high relative humidity and dew allow bacteria to travel
• Applications of excess nitrogen should be avoided as it also contributes to blight infection
• Different cultivars have varying degrees of disease susceptibility. While cultivars like Jona and Gala are highly susceptible, recently developed cultivars like ambrosia and honeycrisp are moderately susceptible to the disease.
• Temperatures above 65F make it easy for disease propagation. At optimum temperature, bacteria can double volume in 20 minutes
• Bloom period is a very important time in ensuring orchards do not become infected. This is the phase when tissues are soft and new tree parts are developing and pathogen can gain entry into trees
• High relative humidity (HRH) during bloom come with precipitation events. Although, it’s important to mention that duration of HRH contributing to disease infection is still unknown
• Disease vulnerability is very high for young trees, especially in warm temperatures and humid conditions

Early detection is very key to managing disease on an orchard. Yet, pruning affected parts with unsanitized tools can contribute to disease spreading especially if done during growing season. With growers now trying to maximize yield by growing more trees in high density, results of infection can be more devastating. In managing fire blight, growers need to stay informed about micro climatic conditions on their orchards and nearby environments to enable them make plan for effective decision making.

In conclusion, there are many ways to manage risk of fire blight to orchards and some of these are highlighted below:

• Practice good sanitation on orchards
• Time spray applications accordingly and strategically
• Avoid excess applications of nitrogen fertilizer
• Make use of fire blight decision support systems (DSS) to predict risk days ahead of infection. The best support systems are those which make predictions based on tree phenology and weather conditions to determine high risk periods