Pseudomonas syringae

This disease affects stonefruit trees and is also known as Pseudomonas syringae, gummosis, sour sap, bacterial canker or, simply, canker.

Plum, prune plum, apricot and cherry trees are most susceptible, though peach and nectarines trees can also be at risk.

Minor infections in branches can be eliminated by pruning, but severe trunk infections will often kill a tree.

Bacterial/blossom blast/canker is a serious disease in apricots, sweet cherries and plums, but impacts peaches and sour cherries to a lesser extent. This pathogen infects blossoms, leaves, fruit and branches. This disease is also known as gummosis, bacterial canker or, simply, canker. Minor infections in branches can be controlled by pruning out infected wood, but major trunk infections will often kill a tree.

Site selection, management, variety selection, and copper application are primary control strategies. 

Symptoms

Cankers on trunk, limbs, and branches exude gum during spring and summer
Leaves turn yellow and wilt on affected branches
Leaf and fruit infection can occur when severe, as evidenced by necrotic spots, leaf yellowing, and/or water-soaked necrotic tissue on fruits
Infected flower buds can fail to open in spring or result in rapid die-back browning of flowers/fruit/foliage
 

The bacteria can infect trees in autumn, often if there is a sharp, early cold period. Sometimes this can cause dieback on branches that is not visible (because the infection is beneath the bark). The bacteria then overwinter in cankers, in buds and within the tree. During wet patches in spring, the bacteria multiply, ooze from the branch cankers and are spread by rain. 

The bacteria enter the plant through natural openings such as emerging buds and through pruning and other wounds, which can be as small as leaf scars. 

Favourable conditions for bacteria spread and infections are frequent periods of rain, cool temperatures and strong wind. 

The disease stops further development when temperatures increase in late spring and throughout summer. 

Infections of bacterial blast can be most conspicuous in spring. Symptoms of the disease include branch dieback, rough, irregularly shaped cankers and amber-colored gum. In the case of blossom infection, blossoms turn brown and cling to the tree. Infected leaves develop dark brown spots that drop out, and fruitlets show sunken spots.

When ooze is present, it can be a sign of disease or just normal sap leaking.  If it has no aroma, it's probably not an infection but normal ooze that occurs from minor stress.  When the gum smells sweet, it is most likely the disease. Sour sap is another term associated with this disease named for when gum or ooze exudes a sour odor from the fermentation of sugars by yeasts under the bark of the damaged area.  The oozing gum is filled with bacteria and is a source of spreading infection.

Disease cycle

• Bacteria over-winter on tree in canker margins, healthy buds, and in the vascular system within wood
• Bacteria multiply in spring and spread during rainfall
• Wet, cool springs with frost or storms that damage blossom and leaf tissue favor outbreaks
• Bacteria can survive symptomless on leaves from bloom through leaf fall, whereupon infects fresh leaf scars and overwinters 

Management

• Apply a high-rate copper in spring just before bud-break and in fall after leaf drop
• Avoid pruning in spring during wet weather; apply copper immediately after pruning
• Prune in summer after harvest
• Discard nursery stock at planting if any sign of bacterial canker present

For infections that are limited to just a few branches, remove by pruning at a point about 4 inches below the infection or at a point where there is no brown staining in the wood.  Remove prunings from the orchard. Clean pruners afterwards. Prune trees during dry weather, preferably in late summer after harvest when wet weather is less likely. In late spring, around the time of bud break is another less risky time to prune stone fruit.  Pruning in winter can reduce tree hardiness at a time when severely cold is still likely.