Bacterial blast, blight, and canker—Pseudomonas syringae
Pseudomonas syringae commonly kills blossoms or causes dark lesions on petals and occasionally on fruits. Damage otherwise depends on the host plant and strain of P. syringae bacterium and whether infection occurred through blossoms, buds, or wounds in other tissues. Severe damage usually occurs only on lilacs (Syringa spp.) and on stone fruits and other Prunus spp.
Lilac leaves and sometimes shoots and stems discolor (usually brown or reddish), shrivel, and die and the damage is called bacterial blight. Stone fruits and other Prunus spp. develop elongated lesions and brownish ooze on branches and twigs, called bacterial canker. Lesions commonly progress to branch cankers and shoot dieback due to phloem damage, which is visible under bark as brown to reddish discoloration or streaks in the outer wood. Trees can be seriously damaged or killed if major limbs or the trunk become infected.
On apple, citrus, and pear, disease is relatively minor and is called bacterial blast. On apple and pear, usually only flower clusters and a few adjacent leaves are killed. On citrus, small twigs and several leaves in a group are killed scattered throughout mostly the south (wind-exposed) side of the canopy.
On oleander and many herbaceous ornamentals, damage includes blossom and shoot tip dieback, leaf spots and vein blackening, and stem cankers. Pseudomonas-infected oleander and olive also develop swellings on leaves and stems, diseases called olive knot and oleander gall, or knot.
Pseudomonas syringae survives on plant surfaces. The bacterium is spread by splashing rain. Disease development is favored by high moisture and low temperatures in spring. Affected limbs may fail to leaf out in the spring or may produce new growth, which dies soon after temperatures increase in the summer.
Determine whether resistant species or cultivars are available and plant or replace plants using these. Many Syringa vulgaris cultivars are highly susceptible to bacterial blight and canker, while Syringa josikaea, S. komarowii, S. microphylla, S. pekinensis, and S. reflexa are generally less susceptible.
Prunus spp. susceptibility to P. syringae is greatly affected by the rootstock. Less-susceptible or resistant rootstocks are available for cherry, peach, and plum. Because rootstocks that are less susceptible to certain pathogens are more susceptible to others, consult publications such as Integrated Pest Management for Stone Fruits to identify the best cultivars for the common problems in your planting situation.
Prune and dispose of infected twigs and branches during the dry season; new infections are less likely to occur then. Do not wet foliage with overhead irrigation. Provide good cultural care, especially appropriate irrigation. Keep roots healthy, such as by planting on a broad mound of soil where drainage is poor. Bactericide applications have not been found to give reliable control and spraying for P. syringae is not recommended.
Pseudomonas sp. on lilac
Apple blossoms and shoots killed by blight
Bacterial ooze and infected (brown) phloem
Red flecks in wood beneath the main canker