Alternaria porri

Fungal Disease

Alternaria porri

Fungal Disease

Alternaria porri

Fungal Disease

Alternaria porri

Fungal Disease

Alternaria porri

Fungal Disease

Purple blotch of onion, caused by Alternaria porri, is a common and destructive fungal disease that can lead to significant crop losses. The disease primarily affects the leaves, leading to weakened plants and reduced yields, with potential losses ranging from 30% to 50%, and in severe cases, even up to 100%.

Symptoms of Purple Blotch

The early symptoms of purple blotch begin as small, elliptical lesions or spots that turn purplish-brown and are surrounded by a chlorotic (yellow) margin. As the disease progresses, the spots enlarge, and the chlorotic margin extends both above and below the lesion. The lesions often girdle the leaves, causing them to collapse and fall over.

The infection typically starts at the tips of older leaves as whitish minute dots. As the disease advances, circular to oblong concentric black velvety rings develop in the chlorotic areas. These lesions spread quickly down the leaf, gradually covering the entire area and causing the leaf to die from the tip downward.

Disease Development and Spread

The pathogen overwinters in crop residue on or near the soil surface. During warm (25-29°C) and humid conditions, spores are produced and spread by wind, splashing rain, or irrigation. Symptoms appear 1 to 4 days after infection, with new spores being produced by fresh lesions within 5 days. These spores are released during the early morning as humidity decreases, favoring further spread.

Leaf wetness duration plays a significant role in disease development, with typical lesions forming when leaves remain wet for more than 16 hours. In some cases, purple blotch may co-occur with Stemphylium leaf blight, further complicating disease management.

Impact on Yield

The rapid spread of the disease can lead to significant yield loss both in the field and during storage. As the leaves die off, the plant's ability to photosynthesize diminishes, leading to smaller bulbs and poor-quality harvests.

Managing Purple Blotch

Effective management of purple blotch includes practices such as removing infected crop debris, monitoring irrigation to prevent prolonged leaf wetness, and applying appropriate fungicides. By implementing these measures, farmers can reduce the spread and impact of this devastating disease.