Citrus black spot

Guignardia citricarpa

Guignardia citricarpa is a fungal pathogen that causes a disease known as citrus black spot (CBS). This disease primarily affects citrus trees and can lead to significant economic losses in citrus-producing regions. Here is an overview of Guignardia citricarpa, including its appearance, identification, and management:

1. Appearance:

  • Lesions: The characteristic symptoms of citrus black spot include the development of lesions on citrus fruit, leaves, and twigs. These lesions initially appear as small, dark spots with a raised black center and a yellow halo. As the disease progresses, the lesions may become larger and more pronounced.

  • Fruit Symptoms: On citrus fruit, the black spots often have a distinct ring pattern, giving them a target-like appearance. Severe infections can lead to fruit distortion and premature fruit drop.

  • Leaf Symptoms: Infected leaves may also exhibit lesions with a dark center and a surrounding yellow halo. In some cases, the lesions on leaves may coalesce, leading to defoliation.

2. Identification:

  • Microscopic Features: Laboratory identification of Guignardia citricarpa involves examining spore-producing structures (pycnidia) and spores (conidia) under a microscope. The characteristic spores are dark, curved, and have a distinctive shape.

  • Symptom Recognition: Field identification is based on recognizing the characteristic symptoms described above. The presence of lesions with a dark center on citrus fruit and leaves, especially with a target-like appearance on fruit, is indicative of citrus black spot.

3. Management:

  • Cultural Practices: Implementing good cultural practices is essential for managing citrus black spot. These include proper sanitation, pruning to improve air circulation, and avoiding overhead irrigation that can promote disease spread.

  • Fungicide Applications: Fungicides are commonly used to manage citrus black spot. Copper-based fungicides and other fungicides with activity against Guignardia citricarpa are applied during critical periods, especially during fruit development. Timing and frequency of fungicide applications are crucial.

  • Fruit Removal: Infected fruit should be promptly removed and destroyed to reduce the source of inoculum. This is an important cultural practice for disease management.

  • Quarantine Measures: In some regions, quarantine measures may be implemented to restrict the movement of citrus fruit and plant material from areas with citrus black spot.

  • Resistant Varieties: Research efforts may focus on developing citrus varieties that exhibit some level of resistance to Guignardia citricarpa.