Bacterial blast of citrus—Pseudomonas syringae
Bacterial blast of citrus, also called black pit or citrus blast, kills citrus leaves and twigs. This disease on citrus occurs mainly in the Sacramento Valley where cool, wet, windy conditions that favor development and spread of the bacterium can be common during winter and spring.
Pseudomonas syringae also infects numerous other hosts. In comparison with citrus, some of these other hosts are more seriously damaged by this bacterium that causes various diseases including bacterial blight and bacterial canker.
Identification
Citrus infections caused by P. syringae usually start as small black or water-soaked lesions in the leaf petiole that spread into the leaf axil. Once the petiole is girdled, leaves curl and turn brown and may drop prematurely. Entire twigs can be killed. Diseased areas on twigs are reddish brown and can become callused or scabby.
Dieback is generally scattered throughout the canopy, creating the blast appearance for which the disease is named. Bacterial blast damage may be confused with symptoms from frost, wind, or bot gummosis formerly called Dothiorella blight or Dothiorella gummosis.
When citrus fruit are damaged by P. syringae, the disease is called black pit. Numerous circular, dark lesions develop in the rind due to infection by the bacterium.
Life cycle
The bacterium infects through small injuries, including those from insect feeding, thorn punctures, and wind abrasion. Bacterial blast damage is most severe when winters have abundant rain. Most damage occurs on the south side of the tree where it is most exposed to the winter, wind-driven rains.
Damage
Bacterial blast kills citrus leaves and twigs. It also can cause dark lesions in fruit rinds. Leaves and twigs of orange and grapefruit and the fruit of lemons are the most susceptible to infection.
Solutions
To reduce the spread of bacterial blast, prune out and dispose of dead or diseased twigs in spring once the rainy season appears to be over. Where the problem has been common, planting other species of dense-canopy trees on the south side of citrus to act as a wind break can reduce the abundance of bacterial blast.
Fertilize and prune citrus during spring or early summer instead of later in the growing season; this reduces the fall flush of leaves, which are especially susceptible to infection by the bacterium. Where the problem has been common, to reduce the abundance of bacterial blast Bordeaux mixture or another copper fungicide can be sprayed to thoroughly cover the plant before the first fall rain.
Adapted from Integrated Pest Management for Citrus, Pest Management Guidelines: Citrus, and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM). |
