Alternaria brown spot

Alternaria sp.

Alternaria brown spot is a fungal disease caused by various species of the Alternaria genus. In citrus, it is often associated with Alternaria alternata.

Hosts: Citrus species, particularly mandarins and tangerines, are commonly affected by Alternaria brown spot.

Symptoms:

  1. Lesions:

    • Small Brown Spots: The disease begins with the appearance of small, round, brown spots on leaves, fruit, and sometimes stems.
    • Concentric Rings: Lesions often have a characteristic concentric ring pattern.

  2. Leaf Symptoms:

    • Leaf Lesions: On leaves, the lesions may expand, coalesce, and cause defoliation if severe.
    • Yellowing: Yellowing of the tissue surrounding the lesions can occur.

  3. Fruit Symptoms:

    • Fruit Lesions: Brown spots on fruit can lead to surface blemishes and, in severe cases, affect the internal quality.
    • Premature Fruit Drop: Infected fruit may drop prematurely.

  4. Spore Production:

    • Dark Spore Structures: As the disease progresses, dark structures containing spores (conidia) may form in the center of lesions.

Identification:

  1. Distinctive Lesions: The small brown spots with concentric rings on leaves and fruit are characteristic.
  2. Spore Structures: The presence of dark spore structures within lesions helps differentiate it from other diseases.

Conditions Favoring Disease:

  1. Warm and Humid Conditions: Alternaria species thrive in warm and humid environments, promoting spore production and disease development.
  2. Wounds and Injuries: Injuries to leaves or fruit provide entry points for the fungus.

Management:

  1. Fungicides:

    • Protective Sprays: Regular application of fungicides during the growing season can help prevent and manage Alternaria brown spot.
    • Rotation of Fungicides: Rotate different classes of fungicides to reduce the risk of resistance development.

  2. Cultural Practices:

    • Pruning: Prune to improve air circulation within the canopy and reduce humidity.
    • Sanitation: Remove and destroy infected plant material to reduce inoculum.
    • Avoid Overhead Irrigation: Minimize prolonged leaf wetness by using drip or furrow irrigation.

  3. Resistant Varieties:

    • Plant Resistant Varieties: Consider planting citrus varieties that show resistance or tolerance to Alternaria brown spot.

  4. Monitoring:

    • Regular Inspection: Monitor orchards regularly for early signs of the disease.
    • Weather Monitoring: Be particularly vigilant during warm and humid weather conditions.

Note: Integrated disease management, combining chemical control with cultural practices, is often the most effective approach.

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