Citrus Blast (sometimes called bacterial blast) is caused by the bacteria Pseudomonas syringae. Symptoms of citrus blast first become evident after a period of wet, windy weather, usually in the winter or early spring. Orange, lemon and mandarin trees show the worst symptoms.
Plant Parts: Leaves, stems and fruit.
Season: Spring - Autumn
Symptoms: The first symptoms are dark, reddish-black spots on the petiole—the short stem running between leaf and stem. Eventually trees develop large areas of reddish-black scabbing, and the disease spreads to the fruit, where it causes black spotting.
Disease progression. Initially, the disease symptoms are the presence of water-soaked lesions at the base of the leaf blade and black areas on the petiole. Later, these lesions extend to the mid-vein of leaves and to the twigs surrounding the base of the petiole. Later on, the leaves dry and curl, but still, remain firmly attached to the branch. Eventually, they fall, usually without petioles.
Twigs: The necrotic areas on twigs can further enlarge and the twigs may be eventually killed if they become completely girdled.
The symptoms are less severe or may even revert with the onset of warm or dry weather.
Fruit infection is seen occasionally in oranges in the form of small black pits on the skin.
Control: Prune out dead or diseased twigs in spring to reduce the spread of the disease. Spray your tree with copper before the spring rainy season to reduce the likelihood of bacterial or fungal infections. These sprays have shown some effectiveness in preventing or reducing the severity of citrus blast.
Prevention: Protect your trees from strong winds by planting windbreaks. Fertilise your trees during spring and early summer (rather than autumn), as this will minimise excessive new fall growth, which is particularly susceptible to blast infection.
If an outbreak of citrus blast occurs, any diseased leaves or twigs should be pruned away and removed from orchards immediately. Do not allow diseased tissue to remain on the ground. Pruning tools should be sanitized with alcohol or Lysol between prunings to avoid spreading the disease.
References: Plantix, University of California Integrated Pest Management, Fusion 360, Researchgate, and the FST Team.
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