Early blight

Alternaria spp.

1. Identification

Alternaria spp. is a group of widely distributed pathogenic fungi responsible for foliar diseases, necrosis, dark lesions and rots in numerous agricultural crops. The most common diseases include Alternaria leaf spot, Alternaria blight and black spot. The most relevant phytopathological species include:

• Alternaria alternata (Fr.) Keissl.
• Alternaria solani Sorauer
• Alternaria brassicae (Berk.) Sacc.
• Alternaria tenuissima (Nees) Wiltshire
• Alternaria dauci J.G. Kühn

These species can cause significant losses in horticultural crops, fruit trees and industrial crops.

2. Causal agent

Fungi of the phylum Ascomycota, order Pleosporales, family Pleosporaceae, genus Alternaria, described by Nees. They are characterised by pigmented multicellular conidia with transverse and longitudinal septa, dark hyphae, high survival capacity in crop residues and rapid dissemination by wind, water splash and agricultural tools.

3. Main hosts

• Solanaceae: tomato, potato, pepper, eggplant
• Brassicaceae: cabbage, broccoli, cauliflower
• Apiaceae: carrot, celery
• Cucurbitaceae: melon, pumpkin, cucumber
• Fruit trees: citrus, pear, apple
• Various ornamentals Weeds and invasive plants

Many species are highly polyphagous, infecting more than 300 plant species.

4. Symptoms

Symptoms caused by Alternaria spp. appear mainly on leaves, stems and fruits, often beginning as small dark spots that expand rapidly. Lesions typically show concentric rings, giving a target‑like appearance, and may enlarge to cause extensive tissue necrosis. On leaves, the disease leads to yellowing and premature drop, reducing photosynthetic area and plant vigour. On fruits, depressed dark lesions develop and may progress to post‑harvest rots, especially in tomato, citrus and carrot. Under favourable conditions, disease progression is rapid and can severely compromise production.

5. Disease cycle

Alternaria spp. survives in infected crop residues, contaminated seeds and soils containing infected organic matter.

Dissemination:

• Conidia transported by wind
• Conidia transported by rain
• Conidia transported by irrigation
• Conidia transported by agricultural tools

Infection:

• Occurs preferentially in weakened or wounded tissues
• Germination favoured by humidity and moderate temperatures

Polycyclic cycle:

• Multiple infections throughout the season, with rapid expansion under favourable conditions

6. Favourable conditions

The development of Alternaria spp. is favoured by humid environments and moderate temperatures, which promote conidial germination and tissue infection. High relative humidity, prolonged leaf wetness and poor ventilation create ideal conditions for disease progression. Plants under water or nutrient stress become more susceptible, as do crops with high planting density. Contaminated crop residues in the soil act as a continuous inoculum source, enabling disease establishment and recurrence across crop cycles.

7. Disease management

Management of Alternaria spp. focuses on reducing favourable conditions for infection and lowering inoculum presence. Preventive measures include improving ventilation, avoiding prolonged leaf wetness and adjusting planting density. Crop rotation and removal of crop residues help limit pathogen survival in soil. The use of certified seeds reduces inoculum introduction. In integrated pest management, the use of less susceptible varieties and regular crop monitoring is recommended. Biological control may be applied with authorised antagonists, while chemical control must follow local regulations and alternate modes of action to avoid resistance.

References

• Agrios, G. N. (2005). Plant Pathology. Elsevier.
• Pscheidt, J. W., & Ocamb, C. M. (2019). Pacific Northwest Plant Disease Management Handbook.
• EPPO – Diagnostic Protocols for Alternaria spp.
• FAO – Guidelines for Integrated Disease Management in Horticultural Crops.
• Thomma, B. P. H. J. (2003). Alternaria spp.: from general saprophyte to specific parasite.