Whenever San Jose, California gets much needed rain, we should be on the lookout for a disease called citrus blast. Also known as bacterial blast or black pit, citrus blast is a bacterial infection that becomes visible after periods of wind-driven rain.
Symptoms of citrus blast
The bacteria, Pseudomonas syringae, enters through thorn punctures, wind damage, and insect feeding sites. This disease is more likely to appear on the south-facing side of your citrus tree. The first symptom of citrus blast is dark lesions on the leaf petiole. The petiole is the little stem that connects a leaf to a twig.
Once the bacteria enters through these lesions, it travels down the leaf axil, or stem, and into the rest of the tree. Infected leaves wilt quickly, curl up and dry on the tree before breaking off. Usually, the petiole is left attached, creating a larger wound for further infection. Diseased areas may also show a reddish brown scabbing and entire twigs may die. Left to progress, citrus blast will leave black spots on fruit.
New growth is especially vulnerable to citrus blast, so it is a good idea to minimize fall and winter feeding and pruning to reduce winter growth in areas prone to citrus blast.
Preventing citrus blast
Applying fixed copper or Bordeaux sprays before the first rain can reduce the likelihood of bacterial infection. These treatments can also be used against fireblight, walnut blight, black spot on roses, peacock spot on olives, downy mildews and powdery mildew on grapes, and leaf curl, and shot hole on nectarines and peaches.
Bordeaux sprays consist of copper sulfate, lime, and water. You can make your own Bordeaux spray by combining 3-1/3 tablespoons of copper sulfate and 10 tablespoons of dry hydrated lime (calcium hydroxide) in one gallon of water. These materials are available at most garden centers. Avoid using slaked lime (calcium oxide) as it can burn your skin and eyes. Visit UC Davis Integrated Pest Management (IPM) for more detailed information.
Diseased twigs should be removed and thrown in the trash. As you prune out diseased areas, be sure to sanitize your tools with a household cleaner, such as Lysol, to kill the pathogen and avoid spreading it to healthy plant tissue.
To prevent citrus blast, be sure to plant citrus trees with some wind protection and remove any dead or diseased twigs as soon as they are seen.
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