|An onion bulb showing a purple blotch lesion|
| Alternaria alii Nolla
Macrosporium porri Ellis (1879)
Purple blotch (Alternaria porri) is a fungal pathogen of Alliums.
Purple blotch lesions are oval, with a well-delimited margin between dry infected tissue and the healthy part of the leaf or scape. With pigment-producing isolates, the central part of the spot is purple. The fungus sporulates there, appearing as a tenuous black mould. Since conidia are very large, they can be seen with a strong magnifying glass. When several spot appear on a leaf, its apical part becomes yellow and withers. Hollow scapes can be broken at the level of a large lesion.
A three- to four-year crop rotation plays an important part in reducing the incidence of infested onion debris and of alternaria pathogens during non-host cycles. Proper sanitation of onion debris, especially culled onions, is very important. Incorporate this material into the soil immediately after harvest. No exposed culls should be present anywhere in the growing region when the next crop is planted. Dispose of culls and trash from storage sheds at landfills by covering them with soil or bury them in deep trenches before spring. Onion culls and other debris can be sources of plant pathogens and insect pests if improperly handled. These can disseminate throughout production areas each year to the detriment of nearby and distant producers and processors.
Plant high-quality onion seed and transplants free of contamination. Follow fertility recommendations carefully and avoid excess (greater than 200 pounds per acre) or late (after July 15) applications of nitrogen. Split nitrogen applications are recommended. Follow good weed management practices. Do not irrigate within 10 to 14 days of lifting. Late season applications of labeled fungicides such as chloro-tha-lonil, mancozeb, metalaxyl, Acrobat, Switch, Endura, Quadris, Pristine, Cabrio, or Rovral may provide some foliage protection and reduce neck contamina-tion, especially after fall rains.
Use care during lifting and processing to minimize bruising or cutting of bulbs. Allow onions to cure properly before topping.
Discard thick-necked onions, scallions, rots, doubles, splits, bruised, sunburned or frozen bulbs. If additional curing or drying of bulk or crated onions is required, circulate 2 cubic feet of ambient or warm air (90 to 95 degrees) per minute for five to 10 days or more. Storage decay is reduced by exposing freshly harvested onions to infrared irradiation for six minutes (6-inch distance from lamp to onion). After curing properly, maintain the storage temperature at 32 to 40 degrees, and maintain humidity at 65 to 70 percent. Onions will freeze at -1°C (30°F). Monitor storage temperatures regularly.
Promote air circulation by leaving space between crates or bulked onions and outer walls of the shed. Do not stack onion bins in direct sunlight before storing or shipping, because translucent scales may occur or moisture may accumulate at the necks of bulbs.
The following plants are susceptible to purple blotch:
- Diekmann, M. (1997). Allium spp.]. Food and Agriculture Organization of the United Nations. International Plant Genetic Resources Institute. p 35. ISBN 92-9043-346-9.
- Schwartz, H.F. (2004). Botrytis, Downy Mildew and Purple Blotch of Onion. Colorado State University Extension. Retrieved: 2010-08-09.