Bacterial blossom blast & leaf spotting—Pseudomonas syringae pv syringae (Pss) occurs naturally on a wide range of plant species.
It overwinters in orchards on bud scales and leaf scars, then in spring the populations increase and are spread to developing plant tissues by rain splash, overhead irrigation and insects.
Pss is the causal organism for bacterial blossom blast (Figure 14) in pears and more recently spring leaf spotting of apples (Figure 15) and cherries.
Pss is also the cause of bacterial canker in stone fruit.
A number of copper-based fungicides are registered for the control of bacterial canker (bacterial gummosis) in apricots and cherries. In both these crops, applications are timed to coincide with first bud movement in spring and again postharvest to coincide with leaf fall.
There are currently no chemicals registered for the control of Pss in apples and pears.
Copper fungicides applied for the management of scab during the dormant phase and at green tip might have a supplementary effect on Pss.
Non-pesticide management
Plant tissues that are injured by frost are more exposed to infection by Pss as the injury sites are readily colonised by the bacteria.
Therefore, any measure taken to avoid or prevent frost damage will also reduce the potential for infections.
When planning new orchards, avoid planting susceptible species in frost prone areas, such as poorly drained gullies and known frost pockets.
Overhead irrigation systems and wind machines have been shown to be effective in reducing the risk of frost. Maintaining a weed-free strip of bare soil under the trees also reduces frost potential.
Bacterial canker
Bacterial canker is favoured by wet, windy conditions in autumn and early winter before and during leaf fall.
Damage to trees and limbs from pruning, hail or wind during the early dormancy period increases the risk of the disease.
Rain during the growing season ensures the disease is spread throughout the orchard.
Non-pesticide management
In severely infected blocks, avoid pruning in winter; prune after harvest or as close to budburst as possible.
Prune areas of the orchard with canker problems last and paint large pruning wounds with white acrylic paint.
As canker can be particularly severe on young plantings it is important to maintain a complete disease control schedule.
Young plantings seldom recover full vigour after severe infections and can be a source of inoculum for older, more productive trees.
Consider removal and replanting in severe cases. Avoid damage to trees, particularly during winter. Control wildlife such as rabbits, hares and macropods which will chew young green bark, creating disease entry sites.
Download the Orchard plant protection guide
See this article in Tree Fruit May 2019
