Ascochyta leaf spot

Ascochyta rhei

Pathogen: Ascochyta rhei is a fungal pathogen responsible for causing Ascochyta leaf spot, a common disease affecting rhubarb (Rheum spp.).

Symptoms:

  1. Leaf Lesions:

    • Circular to Irregular Lesions: Circular to irregularly shaped lesions on rhubarb leaves.
    • Dark Borders: Lesions often have dark brown to black borders.
    • Tan to Gray Centers: The center of the lesions may become tan to gray.

  2. Stem Lesions:

    • Lesions on Petioles and Stems: Petioles and stems of rhubarb may also develop lesions, impacting overall plant health.

  3. Reduced Plant Vigor:

    • Defoliation: Severe infections can lead to defoliation, affecting the plant's vigor.

Identification:

  1. Circular to Irregular Lesions:
    • Circular to irregular lesions with dark borders and a tan to gray center on leaves and stems.

Conditions Favoring Disease:

  1. Humid Conditions: Ascochyta leaf spot thrives in humid environments.
  2. Rain or Irrigation: Moisture on leaves and stems from rain or irrigation facilitates spore dispersal and infection.

Management:

  1. Resistant Varieties:

    • Plant Resistant Cultivars: Choose rhubarb varieties that exhibit resistance or tolerance to Ascochyta rhei.

  2. Cultural Practices:

    • Sanitation: Remove and destroy infected plant material, including fallen leaves and stems.
    • Proper Spacing: Ensure adequate spacing between rhubarb plants to improve air circulation.

  3. Fungicides:

    • Protective Fungicides: Application of fungicides during the growing season, especially during periods conducive to disease development.

  4. Post-Harvest Practices:

    • Clean Harvesting: Harvest rhubarb using clean tools to minimize physical damage that could provide entry points for the pathogen.

Note: Integrated disease management, combining cultural practices, resistant varieties, and fungicides, is often the most effective approach.