Angular leaf spot

Angular Leaf Spot (ALS), also known as bacterial blight, is a plant disease caused by the bacterium Pseudomonas syringae pv. lachrymans. It affects a wide range of crops, including cucumber, melon, squash, pumpkin, and other members of the Cucurbitaceae family. This disease is characterized by the appearance of water-soaked lesions, typically angular in shape, bounded by the veins on the leaves. These lesions can become necrotic over time, leading to significant damage to the foliage and, occasionally, the fruit, which can result in reduced yield and quality of the affected crops.

Symptoms

The initial symptoms of Angular Leaf Spot include water-soaked spots on the leaves that are usually angular in shape because they are limited by the leaf veins. As the disease progresses, these spots turn brown or straw-colored and may become necrotic. In conditions of high humidity, a white to cream-colored ooze can be observed, which dries to form a crusty surface on the lesions. In severe cases, the disease can lead to defoliation of the plant, exposing the fruit to sunburn and further reducing the crop yield.

Causative Agent

Angular Leaf Spot is caused by Pseudomonas syringae pv. lachrymans, a bacterium that thrives in cool, wet conditions. The pathogen can be spread through infected seed, water splash, and handling of plants during wet conditions. It can also survive on plant debris and weeds related to the host crops, making it a challenging disease to manage in environments conducive to its development.

Management

Management of Angular Leaf Spot involves an integrated approach combining cultural, chemical, and biological methods:

• Cultural Practices: Implementing crop rotation with non-host crops for 2-3 years, removing plant debris, and avoiding overhead irrigation can help reduce the incidence of the disease.
• Resistant Varieties: Planting resistant varieties, when available, is one of the most effective ways to manage ALS.
• Chemical Control: Application of copper-based fungicides may help manage the disease, although it is crucial to follow label recommendations to avoid phytotoxicity and resistance development.
• Sanitation: Sanitizing equipment and tools can prevent the spread of the pathogen from infected to healthy plants.

Economic Importance

Angular Leaf Spot can significantly impact the yield and quality of susceptible crops. In severe infections, the loss of foliage can lead to reduced fruit size and quality, directly affecting the economic returns for growers. Moreover, managing the disease requires additional inputs in terms of resistant varieties, chemical treatments, and labor, further increasing the production costs.