Bacterial blight on pomegranate — symptoms and treatment

Severity: high

What is Bacterial blight

Bacterial blight of pomegranate is caused by Xanthomonas axonopodis pv. punicae. It is the most serious disease of the crop, and it earns that reputation by attacking everything at once: leaves, fruit and wood. Being bacterial rather than fungal matters in practice — no curative spray clears an established infection, and the bacteria move in water. Left unmanaged it takes the fruit, then works back into the branches and kills them, hence the high severity.

Symptoms

The disease announces itself with water-soaked spots on the leaves — small, translucent, as though the tissue had been soaked through. As they age they darken to dark brown, and heavily spotted leaves fall. On the fruit the damage is worse: infected pomegranates crack open and rot, and the splits frequently ooze a gummy exudate that dries on the skin. Cracked fruit is not salvageable. In advanced infections the bacteria move into the wood and branches die back, carrying the infection into the next season.

• Early: water-soaked spots on leaves, later turning dark brown; leaf fall.
• Advanced: cracking and rotting of fruit; gummy exudate on lesions; branch dieback.
• Confusable with: fungal fruit spotting — but the water-soaked stage on leaves and the gummy ooze are bacterial signatures.

Causes and conditions

The bacteria need water to move and a way in to infect. Rain splash and overhead irrigation throw them from lesion to leaf and fruit to fruit, so wet, humid weather and any watering that wets the canopy drive the disease hard. They enter through wounds and natural openings, which is why pruning cuts, insect damage and cracks in the fruit skin all become infection points. The pathogen carries over in infected branches and debris left on the tree or the ground, so untreated infected wood is next spring's outbreak. Dense canopies that stay wet after rain make everything worse.

Treatment

Pruning and sanitation — cultural

Timing: during dry weather. Remove and destroy infected branches. Prune for air circulation. Avoid overhead irrigation. Prune in dry conditions and not in the wet — cutting while the canopy is wet spreads the bacteria on your tools and through the splash you create. Removing infected wood takes out the reservoir the disease overwinters in, and opening the canopy lets leaves and fruit dry faster after rain.

Copper + Streptomycin — chemical

Timing: preventive. Apply copper-based sprays preventively. Streptomycin where permitted for severe outbreaks. Preventive is the operative word: copper protects healthy tissue from infection but will not cure spots and cracks that have already formed. Pre-harvest interval: 21 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Water at the base with drip or under-canopy irrigation; never wet the foliage and fruit from above.
• Prune for an open canopy so leaves and fruit dry quickly after rain.
• Do all pruning in dry weather, and sterilise tools between cuts when working near infected wood.
• Remove and destroy infected branches, fallen leaves and rotted fruit instead of leaving them in the orchard.
• Start protective sprays before symptoms appear — a bacterial infection cannot be sprayed out once it is established.

Frequently asked questions

Can I eat pomegranates from an infected tree? Cracked, rotting fruit with gummy ooze is not worth eating. Clean, undamaged fruit from the same tree can be used, but observe the pre-harvest interval of 21 days after any copper or streptomycin spray.

Why do my fruit keep splitting and rotting? Cracking and rot with a gummy exudate is the classic fruit stage of this disease. Once a fruit has cracked the bacteria are inside it — remove and destroy it rather than leaving it hanging to infect the rest.

When should I spray? Preventively, before symptoms show. Copper works by protecting tissue that is still healthy, so a spray applied after the leaves are spotted and the fruit is cracked is too late to help those parts.