Brenneria is a genus of bacteria that causes two types of bark canker in walnut trees. Shallow Bark Canker – caused by Brenneria nigrifluens, which is typically mild in its severity. And Deep Bark Canker – caused by Brenneria rubrifaciens, which can lead to significant damage and yield loss in severe cases. Deep bark canker can lead to serious damage and even tree death if left unmanaged.
Symptoms and Diagnosis
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Shallow Bark Canker
This disease develops slowly and does not lead to significant economic losses, but it can weaken the trees over time. The main symptoms include:
- Brownish-black spots on the trunk and lower branches.
- Dark, sometimes gummy exudates that dry and form sunken, discolored lesions on the bark.
- Primarily affects older trees, but rarely causes their death.
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Deep Bark Canker
This is a more aggressive form of the disease that can lead to serious economic losses. The main symptoms are:
- The leakage of dark brown or reddish fluid from infected areas ("bleeding" bark).
- Black discoloration of the internal tissues beneath the affected areas.
- Spread of the infection into the deeper layers of the bark, which can significantly weaken the trees.
- In severe cases, trees may die if the infection spreads uncontrollably.
Diagnosis of the diseases is performed through visual observation and laboratory analysis of bark samples, using molecular and microbiological techniques to identify the pathogens.
Causes and Mechanisms of Spread
Bacteria from the Brenneria genus spread through various mechanisms that facilitate the infection of new trees:
- Wounds on trees – Mechanical injuries from pruning, hail, insects, or shaking of trees during harvest can serve as entry points for the bacteria.
- Spread through rain splash – water droplets can move the bacteria from infected to healthy trees.
- Infected seedlings – Young trees from infected nurseries may harbor latent infections that become apparent after planting, especially under stress conditions.
Management and Control Methods
Given the lack of effective treatment for advanced stages of the disease, integrated prevention and control strategies must be applied:
- Maintaining Tree Health
- Optimal irrigation – both excessive moisture and drought should be avoided, as they can weaken the trees and make them more susceptible to infections.
- Fertilization – the use of balanced fertilizers helps enhance the overall resistance of the trees to diseases.
- Avoiding mechanical injuries – this is especially important during mechanized harvesting.
- Controlling the Spread
- Disinfection of tools – Pruning shears and saws should be sterilized with a bleach (10%) or alcohol solution after working with each tree.
- Avoid pruning during wet weather – wet conditions favor the spread of bacteria.
- Removal of infected branches – infected parts should be burned or removed far from the plantations.
- Chemical and Biological Control
- Copper-based bactericides – reduce surface bacterial populations but have limited efficacy once the infection is established in internal tissues.
- Biological agents - Some studies suggest that beneficial microbes may help suppress Brenneria through competition or antagonism.
- Antibiotic treatments – generally not permitted in commercial orchards due to regulatory restrictions and the risk of resistance development.
Brenneria (formerly Erwinia) represents a serious threat to walnut plantations. Although there is no completely effective treatment for advanced cases, proper management of the plantations and the strict application of agro-technical measures can significantly reduce the risk of infection and limit the spread of the disease. Regular monitoring of trees and the use of integrated control methods are recommended to reduce production losses and maintain the health of walnut orchards.
