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Garlic & Onion Diseases | Onion Anthracnose

Onion Anthracnose

Symptoms

Bulb
Lesions normally formed in concentric rings (approximately 1 cm in diameter) on the bulb. If environmental conditions are conducive, the fungus produces cushion-like structures, known as acervuli, containing cream-colored conidia. The acervuli contain black setae, which can be seen with a hand lens.  Colletotrichum circinans usually first attacks the bulbs.

Foliar
On the neck and leaves of the onion plants, dry, oval, salmon-colored lesions can be observed. Yellow halos may appear around the lesions.  In general, yellowing, curling, twisting, chlorosis, and death of leaves can be associated with this disease.  In severely infected fields, dieback and neck elongation of onion plants can be observed.  Colletotrichum gloeosporioides and C. coccodes have been reported to infect mainly leaves and neck, which is different from C. circinans, which first infects the bulbs.

Signs

The lesions on the scales are first characterized by the dark green stromata.  As the disease progresses, the stromata darken to black dots. Stromata (singular: stroma) are dense masses of mycelia, which give rise to reproductive fungal structures.  In the case of this disease, the fungus produces cushion-like structures, known as acervuli, containing conidiophores with cream-colored conidia.  The conidia are unicellular, aseptate, cylindrical to fusiform and hyaline to moderately . Conidia (3-6 X 14-30 µm) can be slightly curved and obtuse at the apex.  In addition, the acervuli contain black, thick-walled setae (80 – 315 µm long) containing one to three septa, which can be seen with a hand lens. Colletotrichum gloesporioides and C. coccodes are reported to have straight conidia and C. circinans to have falcate conidia.

Often Confused With

  • Onion smut also produces dark lesions, which break open exposing black fungal growth.
  • Purple blotch produces similar lesions under very similar environmental conditions; however, the lesions tend to be darker in color. This disease may also be present with anthracnose.

Isolation Media

Rapid Diagnostic Tests