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Beginner

Scientific Name
Septoria cucurbitacearum

Identification

• Infections cause circular, white-to-light brown leaf lesions
• Fruit lesions are raised and pimple-like

Often Confused With
Angular leaf spot
Scab
Anthracnose
Alternaria

Period of Activity
Septoria thrives under cooler, humid conditions of 61- 66°F (16- 19°C). 

Scouting Notes
Inspect 10 leaves at each of 20 random locations in the field. Divide the number of infected leaves by 200 to determine the percent infection. Record the average number of lesions per leaf. Inspect developing fruit for small lesions.

Thresholds
Apply a broad-spectrum fungicide at the first sign of foliar infection.

Advanced

Scientific Name
Septoria cucurbitacearum

Septoria occurs only sporadically in Ontario, and it does not usually cause serious economic losses.  Muskmelons, pumpkins and winter squash are all susceptible.  Septoria is more common during unseasonably cool summers or during late fall.

Identification
Septoria leaf lesions are small (1- 2 mm, 1/25- 1/12 in. in diameter), circular and white-to-light-brown in colour with a darker brown border.  As the symptoms progress, the lesion may crack and produce small black pycnidia.

Later season infections may cause small, straw-coloured pimple-like lesions to develop on the fruit.  These lesions are usually superficial and will not develop into fruit rots in storage.

Often Confused With
Angular leaf spot
Scab
Anthracnose
Alternaria

Biology
Septoria overwinters as dormant mycelium on cucurbit residue.  In the spring the mycelia form pycnidia and conidia, which serve as the primary inoculum.  Driving rains and splashing soil spread the conidia from plant-to-plant.

Its spread may be stalled by hot mid-summer conditions, only to return again once temperatures become more moderate in the fall. 

Period of Activity
Septoria thrives under cooler, humid conditions of 16- 19°C (61- 66°F). 

Scouting Notes
Inspect 10 leaves at each of 20 random locations in the field.  Divide the number of infected leaves by 200 to determine the percent infection.  Record the average number of lesions per leaf.  Inspect developing fruit for small lesions.

Thresholds
Apply a broad-spectrum fungicide at the first sign of foliar infection.

Management Notes

• Follow a 3-4 year crop rotation and use certified, treated seed. 
• Fungicides applied at the first onset of Septoria will help reduce the spread of this disease.