Bacterial spot of citrus is a common disease caused by the bacterium Xanthomonas citri subsp. citri. It primarily affects citrus trees, including oranges, lemons, limes, grapefruits, and other citrus varieties, and can lead to significant economic losses in citrus-producing regions.

Symptoms:

1. Leaf lesions: The most characteristic symptom of bacterial spot of citrus is the development of water-soaked lesions on the leaves. These lesions initially appear as small, dark spots that may be surrounded by a yellow halo. As the disease progresses, the lesions may enlarge and coalesce, leading to extensive leaf damage.
2. Fruit lesions: Bacterial spot can also affect citrus fruit, causing small, raised lesions on the surface. These lesions are typically dark in color and may become corky or scabby as they mature. In severe cases, fruit lesions can lead to premature fruit drop and reduced marketability.
3. Twig dieback: Infected citrus trees may exhibit dieback of twigs and branches, particularly in areas with heavy disease pressure. This can result in canopy thinning and reduced fruit production over time.
4. Leaf and fruit drop: Severe infections of bacterial spot can cause defoliation and premature fruit drop, leading to yield losses and reduced tree vigor.

Causes:

Bacterial spot of citrus is caused by the bacterium Xanthomonas citri subsp. citri, which can survive and overwinter in infected plant debris, as well as in asymptomatic citrus trees. The disease is primarily spread through wind-driven rain, irrigation water, and mechanical means, such as pruning tools and equipment. Once established in an orchard or citrus grove, bacterial spot can persist for multiple growing seasons, especially under favorable environmental conditions.

Recommendations and Preventive Measures:

1. Planting disease-free stock: Start with healthy, disease-free citrus trees from reputable nurseries or certified sources to minimize the risk of introducing bacterial spot into orchards or groves.
2. Sanitation: Practice good sanitation by removing and destroying infected plant debris, fallen leaves, and affected fruit from the orchard floor. This helps reduce the inoculum source and limit the spread of bacterial spot within the citrus trees.
3. Water management: Avoid overhead irrigation, especially during periods of high humidity or rainfall, as this can promote the spread of bacterial spot. Use drip irrigation or other methods that deliver water directly to the soil without wetting the foliage.
4. Pruning: Prune citrus trees to improve air circulation and reduce canopy density, which can help minimize leaf wetness and create less favorable conditions for bacterial spot development.
5. Copper sprays: Apply copper-based bactericides according to label instructions and recommended timing to suppress bacterial populations and reduce the severity of bacterial spot infections. Copper sprays are most effective when applied preventively or at the first signs of disease.
6. Resistant varieties: Consider planting citrus varieties that are less susceptible to bacterial spot, although complete resistance may not be available. Some citrus cultivars exhibit varying degrees of tolerance or resistance to the disease.
7. Avoiding mechanical injury: Handle citrus trees with care to avoid causing wounds or injuries that can provide entry points for bacterial spot infection. Disinfect pruning tools between cuts to prevent cross-contamination.
8. Monitoring: Regularly scout citrus trees for symptoms of bacterial spot, especially during periods of high disease pressure. Early detection allows for prompt intervention and management to limit the spread of the disease.

By implementing these preventive measures and practicing integrated pest management, citrus growers can effectively manage bacterial spot and minimize its impact on citrus production. Combining cultural practices, sanitation, chemical treatments, and resistant varieties can help reduce disease pressure and maintain healthy citrus trees and fruit.