Tomato Alternaria, also known as Alternaria leaf spot or early blight, is a fungal disease caused by the pathogen Alternaria solani. It affects tomato plants and can lead to significant damage to foliage, stems, and fruit. Here’s an overview of its symptoms, recommendations, causes, and preventive measures:

Symptoms:

1. Leaf Symptoms:
   • Small, dark brown to black spots develop on lower leaves, typically starting near the base of the plant.
   • Spots may have a concentric ring pattern, giving them a target-like appearance.
   • Lesions gradually enlarge and may coalesce, causing extensive browning and necrosis of affected leaves.
   • Infected leaves may eventually yellow, wither, and drop prematurely, leading to defoliation.
2. Stem Symptoms:
   • Lesions similar to those on leaves may also develop on stems, particularly near leaf attachment points.
   • Stem lesions can lead to girdling and weakening of stems, potentially causing plant collapse.
3. Fruit Symptoms:
   • In severe cases, fruit may become infected with Alternaria, leading to the development of sunken, dark-colored lesions.
   • Lesions may spread, causing fruit to rot and become unsuitable for consumption.

Recommendations:

1. Cultural Practices:
   • Crop rotation: Rotate tomato crops with non-related plants to break the disease cycle and reduce inoculum levels in the soil.
   • Proper spacing: Maintain adequate spacing between tomato plants to promote air circulation and reduce humidity, which can minimize disease development.
   • Mulching: Apply mulch around tomato plants to prevent soil splash onto foliage, which can spread fungal spores.
   • Pruning: Remove and destroy infected leaves and stems to reduce disease spread within the plant canopy.
2. Fungicidal Control:
   • Apply fungicides labeled for use against Alternaria leaf spot according to recommended schedules, particularly during periods of high disease pressure.
   • Rotate between fungicide groups with different modes of action to reduce the risk of resistance development in Alternaria populations.

Causes:

1. Fungal Pathogen: Tomato Alternaria is caused by the fungus Alternaria solani.
2. Environmental Conditions: Alternaria solani thrives in warm, humid conditions. Rain, dew, and high humidity create favorable conditions for spore germination and disease development.
3. Overhead Irrigation: Excessive overhead irrigation or frequent rainfall can promote the spread of Alternaria spores from infected plant debris to healthy plants.

Preventive Measures:

1. Sanitation: Remove and destroy infected plant debris, including infected leaves, stems, and fruit, to reduce the source of inoculum for future infections.
2. Air Circulation: Prune tomato plants to improve air circulation within the canopy and reduce humidity around plants.
3. Resistant Varieties: Select tomato varieties that are resistant or less susceptible to Alternaria leaf spot, if available.
4. Integrated Pest Management (IPM): Implement an IPM approach that combines cultural practices, fungicidal control, and resistant varieties to effectively manage Alternaria leaf spot while minimizing environmental impact.

By implementing these preventive measures and adopting an integrated approach to disease management, tomato growers can minimize the impact of Alternaria leaf spot and maintain healthy tomato production.