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Ministry of Agriculture, Food and Rural Affairs

Scab

Scab lesions on fruit Scab lesion on pumpkin Foliar scab lesions Scab lesions on fruit Foliar scab lesions
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Beginner

Scientific Name
Cladosporium cucumerinum

Identification

  • Scab may occur on the leaves, stems, petioles and fruit
  • Leaf lesions are small, numerous and pale yellow-to-white in colour
  • The lesions are somewhat angular in shape
  • Occasionally fruit infections will occur without any apparent leaf lesions
  • Scab fruit infection occurs as large, sunken craters with corky margins 
  • Infected fruit develop small, irregular-shaped cavities
  • The margins of these sunken lesions are often coated with a dried, corky layer 

Often Confused With
Angular leaf spot
Septoria
Anthracnose
Oedema

Period of Activity
The scab fungi prefer cool temperatures of 17- 20°C (62- 68°F) and heavy dews or intermittent showers. Dry days following moist nights allow for significant wind borne spore distribution.

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
Cladosporium cucumerinum

Scab can often be found at low levels in many cucurbit fields, however economic losses occur only sporadically.  It causes the greatest damage when infections occur on the fruit, rendering them unmarketable.

Identification
Scab may occur on the leaves, stems, petioles and fruit. Foliar infections appear as numerous, small, pale yellow-to-white spots.  These lesions are somewhat angular in shape. The center of each lesion may deteriorate, leaving a shot-hole appearance to the leaf. Severely infected plants may also develop shortened internodes and deformed new growth.

Symptoms do not always develop on the leaves.  Occasionally fruit infections will occur without any apparent leaf lesions. 

Infected fruit develop small, irregular-shaped cavities.  The margins of these sunken lesions are often coated with a dried, corky layer. Fruit are most prone to infection while they are green and rapidly expanding, before the rind begins to harden.  When pumpkin and winter squash fruit are infected at an early stage of development, the fruit will continue to expand around the lesions, causing bumpy, misshapen fruit.

Often Confused With
Angular leaf spot
Septoria
Anthracnose
Oedema

Biology
Scab survives in the soil on infected cucurbit residue.  It may also be seed-borne.  The conidia can travel for long distances in moist air currents.  Wide-spread infections can occasionally be traced to a specific storm front.

Scab is present in low amounts every year. A few lesions scattered here and there will not have a big impact on marketability. Unfortunately, if the environmental conditions are ripe and if the scab is left unchecked, it can have a devastating effect on both yield and marketability.

Period of Activity
The scab fungi prefer cool temperatures of 17- 20°C (62- 68°F) and heavy dews or intermittent showers. Dry days following moist nights allow for significant wind borne spore distribution. Scab symptoms on melons may progress in storage at temperatures as low as 8°C (46°F). Symptoms first appear 3- 4 days after the initial infection.  In Ontario fruit infections often occur in late-summer or early fall.

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 scab will help reduce the spread of this disease.