Bacterial diseases are caused by parasitic bacteria that attack plant cells. They can cause a wide range of different symptoms, depending on the type of bacteria involved and the plant and plant part(s) infected and affected.
These are the most common and most serious bacterial diseases that affect garden plants.
Bacterial canker (Pseudomonas syringae pv morsprunorum and P. syringae pv syringae)
Attacks: Prunus, species, especially cherries and plums, but also apricots, peaches and ornamental varieties.
Symptoms: Sunken areas of dead, sunken bark and cankers, often accompanied by an amber, gummy ooze. Affected branches die rapidly if the infection spreads all round the branch. Small brown spots appear in the leaf, which later fall out to leave holes, looking like the leaf has been hit by shotgun pellets, hence the common name of shothole. The disease can be fatal, killing even large trees.
Clematis slime flux (various bacteria)
Attacks: Mainly Clematis species.
Symptoms: Affected stems wilt, die back and produce a foul-smelling exudate. The disease can be fatal. It shouldn’t be confused with clematis wilt.
Fireblight (Erwinia amylovora)
Attacks: Cotoneaster, Crataegus, Photinia, Pyracantha, Sorbus, apples, pears.
Symptoms: Leaves turn a dark brown colour and may drop. Cankers (areas of dead, sunken bark) appear on branches, and the wood is stained a brown colour when the bark is peeled back. Blossom wilts and dies after flowering. Fruits become discoloured and wrinkled A slimy white liquid may exude from infections in wet weather. Fireblight is now relatively unimportant in gardens, but epidemics still occur depending on the weather.
Horse chestnut bleeding canker (Pseudomonas syringae pv aesculi)
Attacks: Horse chestnuts (Aesculus hippocastanum and A. carnea).
Symptoms: Bark infection producing cankers that bleed a dark, sticky fluid. Dead bark falls away on older cankers, exposing the wood. Rapidly spreading infections cause branches to die and even death of part or all of the tree. Horse chestnut bleeding canker is also caused by some species of Phytophthora.
Crown galls (Agrobacterium tumefaciens)
Attacks: Many plants.
Symptoms: Knobbly, swollen galls on stems, branches and roots. Plant growth may be adversely affected, but often there is little apparent damage and infections go unnoticed.
Rhubarb crown rot (various bacteria)
Symptoms: Plants don’t thrive, fail to grow well and rot away at the crown. It can spread to stems and foliage causing total dieback.
Iris rhizome rot (Pectobacterium carotovorum)
Attacks: Bearded irises.
Symptoms: Rhizomes develop a slimy, often foul-smelling rot and die.