Beginner

Scientific Name
Colletotrichum obriculare

Identification

• Anthracnose leaf spots are dry and reddish brown, often with a lighter yellow border
• They can reach up to 12 mm (½ in.) in size
• Severely infected leaves become distorted, dry, cracked and ragged-looking
• Infected fruit develop sunken lesions, varying in colour from red-brown to black
• Under humid conditions, these lesions may develop masses of salmon pink spores
• Anthracnose infections may also develop on fruit in storage

Often Confused With
Alternaria
Septoria leaf spot
Scab

Period of Activity
Anthracnose is a late-season disease.  It prefers warm temperatures of 22– 27˚C (72– 81˚F) and high relative humidity (100% for 24 hours).

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.

Advanced

Scientific Name
Colletotrichum obriculare

Muskmelon and watermelon are most susceptible to anthracnose.  Infections in pumpkins and squash are less common.

Identification
Dry, reddish-brown, circular lesions appear on the leaves.  These lesions often have a lighter yellow border.  Black fruiting bodies (called acervuli) may be visible on the under surface of older lesions.  The new growth on infected plants may become twisted and distorted.  As the disease progresses, the dead tissue may fall out of the centre of the lesion, giving the leaf a ragged appearance.

Infected fruit develop sunken lesions, 2- 5 mm (1/12- 1/5 in.) in diameter (or larger).  Salmon pink spores may develop on the lesions under moist conditions.  Anthracnose lesions may appear on infected muskmelons after harvest, as well as during storage and shipping.

Often Confused With
Alternaria
Septoria leaf spot
Scab

Biology
Crop residue is the primary source of inoculum.  Rainfall from heavy storms often results in the deposition of spore-laden soil on the leaf surface.  Overhead irrigation may also increase the spread of this disease from plant-to-plant.

Symptoms develop 4-5 days after infection.  Anthracnose infections often spread faster on the older leaves.

At least seven different races of anthracnose have been identified in cucurbit crops.  Each race varies in its ability to infect different cucurbits.  Cucurbit breeders have developed varieties with resistance to multiple races of this disease.

Period of Activity
Anthracnose is a late-season disease.  It prefers warm temperatures of 22- 27°C (72- 81°F) and high relative humidity (100% for 24 hours).

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 preventative fungicides at row closure or at the first sign of disease.

Management Notes

• A 3- 4 year rotation away from all cucurbit crops will effectively eliminate soil-borne infection.
• Avoid heavy fertilizer applications. 
• An unnecessarily dense, heavy canopy will prolong humidity on the leaf surface. 
• Avoid working in the field when the foliage is wet, and workers and equipment may transfer spores from infected plants to clean ones.