What Is It? | Facts in Depth | For the Professional Diagnostician
Garlic & Onion Diseases | Onion Anthracnose
Onion Anthracnose
Identification
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.
Pathogen Biology
The pathogen Colletotrichum sp. survives in the soil or saprophytically in plant debris. Infection can occur by direct penetration via a germ tube through the cuticle or indirectly by wounds and natural openings. Mycelia colonize the plant and form stromata. Acervuli bearing conidia rise from the stroma 72 to 96 hours after inoculation. The conidia germinate and produce a short germ tube with an appressoria. Cutin-degrading enzymes are produced to penetrate new healthy tissue and hosts. After 96 hours, the pathogen shifts to a necrotrophic phase, causing intracellular and intercellular damage, which results in a water-soaked appearance (Panday et al., 2012). In optimal environmental conditions, the pathogen can complete its life cycle in a few days (Schwartz & Mohan, 2008) and can completely destroy the host within 7 days.
Favorable Environmental Conditions
Spore germination occurs at 55°F – 77°F and disease development at 55°F – 90°F. At 82°F, conidia can germinate and produce a short germ tube after 6 hours. High humidity and 12+ hours of continuous wetness aid in disease progression.
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.
Scouting Notes
Periodically monitor for any foliar symptoms including yellowing, curling, twisting, chlorosis, or death after periods of warm/hot temperatures with high humidity or continuous wetness. Also, salmon- to cream-colored masses in lesions are an indication of onion anthracnose.
Thresholds
Currently, there is no threshold information for onion anthracnose.
Management Notes
- Use protectant fungicides – A 5-day interval spray program with protectant fungicides are effective in managing and significantly reducing disease incidence, especially under warm/hot and humid conditions. Mancozeb (e.g., Dithane or Manzate) and chlorothalanil (Bravo, Equus, Echo, etc) fungicides are effective in controlling the disease. Strobilurins are effective but should be used in rotation with a protectant fungicide. (Fungicide field trials: http://veggies.msu.edu/Research/GLOnion2012.pdf)
- Use sound cultural practices – Start with certified disease-free seeds in a site with adequate drainage. Rotating with non-host crops every 2 to 3 years and removing unharvested bulbs and plant material will also help manage this disease from season to season in the field.
- Use tolerant onion varieties – It has been observed that colored bulbs are more tolerant than white cultivars, possibly because they contain phenolic substances in the outer scales that inhibit the growth of the pathogen. Although symptoms were found in yellow-scaled cultivars and red-colored cultivars, the symptoms were much less severe. Here is a report of onion cultivars tested for susceptibility to anthracnose in an inoculated field trial: Michigan Onion Disease Update (http://veggies.msu.edu/Research/GLOnion2013.pdf).
