Early Blight (of Solanaceous crops)

Causal Organism: Alternaria solani

Crops Affected: potato, tomato, pepper, eggplant, solanaceous weeds

Disease Cycle:

• This is a fungal pathogen
  • It persists in crop residues, soil, on infected tubers or on other alternate host plants (e.g. weeds)
• Spores are produced and are the primary means of infection and spread
  • Spore production and germination is favoured by cool, moist, or dewy nights or alternating wet and dry conditions (e.g. irrigated situations)
• Spores can be introduced onto leaves through contact with the soil, blown on the wind, or through water splash
  • Secondary infections can occur on the plant, resulting in a more rapid increase of symptoms
• Water is generally required for germination to occur
  • Infection occurs directly through the leaf tissue
  • Wounds are not required for leaf infection
• This is typically a disease of older, dying plants (but not always)
  • It can be a top killer for early maturing crops
• Tubers are infected through contact with spore-infested soil, typically in conditions that favour significant foliar spore production
  • Spores do not move into the soil
  • Tubers are not infected prior to harvest
  • Wounds are required for tuber infection

Symptoms:

Leaves:

• Initial symptoms are small brown, pinhead-like lesions on older leaves
• Circular lesions increase in size, becoming 3-10mm across
  • Lesions consist of a series of concentric rings of dead tissue, creating a lesion that resembles a bullseye pattern
    • Tissues may collapse, leaving a hole in the middle
  • Lesions are delimited (contained) by large leaf veins and are usually bordered by yellow chlorotic margins
• Heavy infections can result in significant loss of leaf area, which can affect yields
• Leaves do not usually fall off
• Stressed or older tissues are more susceptible

Tubers:

• Tubers can have dark, sunken, circular areas surrounded by raised borders
  • Sunken areas may increase in size during storage
• Tuber lesions typically remain superficial and are brown and leathery

Management:

• Ensure that debris and crop residues are worked under to encourage rapid breakdown
• Ensure plants are not stressed or weakened through insufficient or excessive fertility, drought, or other stressors
• Maintain a rotation of at least 2-3 years
  • This includes other solanaceous crops
• Control solanaceous weeds to prevent alternate hosts
• Apply protective applications of registered fungicides when first early lesions are noted on lower leaflets

Tuber Infection

• Vine killing of later crops can reduce spore populations prior to harvest, which may reduce tuber infection
• Harvest only mature tubers (if possible)
• Avoid injury or wounding of tubers during harvest and post-harvest handling
• Avoid handling tubers in wet soil
• Ensure that tubers have good skin set and have time to cure prior to long term storage
  • This reduces infection and the development of lesions on tubers
• Cool conditions in long-term storage will slow disease development