Purple blotch

Alternaria porri:

Alternaria porri is a fungal pathogen that causes leaf blight in various Allium species, particularly affecting onions and garlic. Here's an informative overview:

Hosts:

1. Onions (Allium cepa): Onions are one of the primary hosts affected by Alternaria porri.
2. Garlic (Allium sativum): Garlic is also susceptible to leaf blight caused by this pathogen.
3. Other Allium Species: Alternaria porri can affect other Allium species to varying degrees.

Symptoms:

1. Leaf Lesions: Initial symptoms include the appearance of small, water-soaked lesions on leaves.
2. Lesion Expansion: Lesions enlarge and become tan to brown with characteristic concentric rings, leading to a bull's-eye appearance.
3. Leaf Yellowing: Infected leaves may exhibit yellowing and wilting.
4. Bulb Infection: In severe cases, the pathogen can infect bulbs, causing decay and reducing quality.

Identification:

1. Leaf Lesions: Identify Alternaria porri by the presence of characteristic lesions with concentric rings on leaves.
2. Bull's-Eye Appearance: The bull's-eye appearance of the lesions is a distinctive feature.

Management:

Cultural Practices:

1. Crop Rotation: Practice crop rotation to reduce the risk of disease buildup in the soil.
2. Proper Spacing: Ensure proper plant spacing for good air circulation, reducing humidity around plants.
3. Avoid Overhead Irrigation: Minimize leaf wetness by using drip irrigation or watering at the base of plants.

Fungicide Application:

1. Copper-Based Fungicides: Copper-based fungicides may provide some control; however, their effectiveness can be limited.
2. Fungicides with Alternaria Activity: Consider using fungicides specifically labeled for controlling Alternaria diseases.

Resistant Varieties:

1. Plant Resistant Varieties: Utilize onion and garlic varieties that are bred for resistance or tolerance to Alternaria porri.

Sanitation:

1. Remove Infected Residues: Collect and destroy infected plant residues, including leaves and bulbs, to reduce the overwintering source of the pathogen.

Regular Monitoring:

1. Scouting for Symptoms: Regularly monitor onion and garlic crops for early symptoms of leaf blight, especially during periods conducive to disease development.

Post-Harvest Measures:

1. Proper Storage: Ensure proper storage conditions for harvested onions and garlic to minimize the risk of post-harvest infections.