Angular Leaf Spot
Beginner
Scientific Name
Pseudomonas syringae pv. Lachrymans
Identification
- Small, water-soaked lesions develop on infected leaves
- Lesions will expand in size until they are confined by the veins, resulting in an angular appearance
- Affected areas turn brown, and the centres fall out, leaving small holes in the leaf
- Infections on the fruit appear as 1- 3 mm (1/25- 1/8 in.), circular, tan coloured lesions
Often Confused With
Downy mildew
Septoria leaf spot
Scab
Period of Activity
Angular leaf spot is a warm weather disease. It is most active at temperatures of 24 to 27°C (75- 80°F). Seed borne infections will usually appear shortly after emergence in the spring. Soil borne infections may appear at any time.
Scouting Notes
Inspect 10 leaves at each of 20 random locations in the field. Observe both the older and newer leaf growth. Record the percentage of leaves infected and the average number of lesions per leaf.
Thresholds
None established. Apply copper fungicides at the first sign of infection.
Advanced
Scientific Name
Pseudomonas syringae pv. Lachrymans
Angular leaf spot may infect all cucurbit crops, however, it
is more prevalent in pumpkins and squash. Many cucumber
varieties are resistant to angular leaf spot.
Identification
Small, water-soaked lesions develop on infected leaves. These lesions will expand in size until they are confined by the secondary veins, resulting in an angular appearance. In humid conditions angular leaf spot lesions may develop a milky exudate on the lower surface of the lesion.
As the disease progresses, the lesions turn brown and the centres fall out, leaving small holes in the leaf. Angular leaf spot lesions on the fruit appear as 1-3mm (0.04- 0.12 in.), circular, tan coloured lesions. Infected fruit often develop secondary infections such as bacterial soft rot, further reducing the marketability and storage life of the crop.
Plants can tolerate low levels of angular leaf spot without serious yield loss, as long as the new growth remains uninfected. Angular leaf spot resistant varieties may develop symptoms, however, the lesions are usually smaller and the disease progress is considerably slower.
Often Confused With
Downy mildew
Septoria leaf spot
Scab
Biology
Angular leaf spot is spread by infected seed and in soil containing infected crop residue. Angular leaf spot bacteria may survive in the soil for 2- 3 years. Infections are accelerated by extended periods of leaf wetness due to rain, high humidity or heavy dewfall.
Angular leaf spot bacteria enter the leaf via the stomates, hydathodes and small abrasions. Infections are spread throughout the field by splashing rain, insects, wind erosion and during regular field operations such as tillage, hoeing and picking.
Period of Activity
Angular leaf spot is a warm weather disease. It is most active at temperatures of 24 to 27°C (75- 80°F). Seed borne infections will usually appear shortly after emergence in the spring. Soil borne infections may appear at any time.
Scouting Notes
Inspect 10 leaves at each of 20 random locations in the field. Observe both the older and newer leaf growth. Record the percentage of leaves infected and the average number of lesions per leaf.
Thresholds
None established. Apply copper fungicides at the first sign of infection.
Management Notes
- Purchase treated seed that has been certified pathogen-free.
- Do not save seed from infected fields.
- A 3 to 4 year rotation away from all cucurbit crops will effectively eliminate soil borne infections.
- Avoid working in the field when the foliage is wet, especially early in the growing season.
