Fire blight

Severe fire blight infection on apples

Fire blight (Erwinia amylovora) of pear

Fire blight, caused by the bacterium Erwinia amylovora, is a devastating plant disease that primarily affects apples (Malus domestica), pears (Pyrus spp.), and some other members of the family Rosaceae. This disease is known for the damage it can cause to entire orchards and the significant economic losses it can incur. Fire blight is characterized by the rapid wilting, blackening, and death of flowers, shoots, and branches, which appear as if scorched by fire, hence the name.

Symptoms and Diagnosis

The first signs of fire blight can be observed in spring when temperatures rise above 60°F (15°C). Symptoms include the wilting and blackening of blossoms, which eventually leads to the death of the flower cluster. As the disease progresses, it infects the shoots, causing them to bend over into a shepherd's crook shape. The bark on infected branches may crack and ooze a watery, bacterial ooze that turns dark upon exposure to air. This ooze can spread the bacteria to other parts of the plant or nearby plants.

Diagnosis of fire blight involves observing the characteristic symptoms and may include laboratory tests to confirm the presence of Erwinia amylovora.

Transmission and Spread

Fire blight bacteria overwinter in cankers on infected trees and are spread primarily through wet conditions in spring, such as rain or heavy dew. Insects, birds, and gardening tools can also spread the bacteria. The disease is highly contagious, especially in warm, humid weather, which facilitates the growth and spread of the bacteria.

Management and Control

Management of fire blight involves an integrated approach combining cultural practices, chemical treatments, and, where available, the use of resistant cultivars. Important cultural practices include the removal and destruction of infected plant material, avoiding overhead irrigation, and implementing proper pruning techniques to improve air circulation within the canopy. Chemical control measures may include the application of antibiotics like streptomycin or copper-based products during susceptible periods to prevent infection. However, the use of antibiotics is subject to regulatory approval and may not be available in all regions due to concerns about antibiotic resistance.

Breeding for resistance to fire blight is an ongoing area of research, with some success in developing resistant apple and pear varieties. However, resistance does not mean immunity, and even resistant varieties may require management practices to control the disease.

Economic Impact

Fire blight poses a significant threat to the fruit industry, particularly in regions where apples and pears are major crops. The disease can cause substantial yield losses and, in severe cases, the death of trees. The economic impact includes not only the loss of fruit production but also the cost of disease management and the replacement of dead or severely affected trees.

Prevention

Prevention of fire blight involves selecting resistant varieties when available, implementing good orchard hygiene practices, and monitoring for symptoms of the disease. Early detection and prompt removal of infected material can help prevent the spread of fire blight.