Purple blotch is caused by the fungus Alternaria porri. It can infect all above-ground parts of the plant in addition to the bulb. Initial symptoms appear on older leaves, usually late in the summer as spores are blown from infested debris. Older leaves and plants are more susceptible to infection.
The first symptoms on leaves or seed stalks are small white lesions. Under humid conditions these develop into elliptical purplish areas, spreading to several centimetres long and with a yellowish border. Sporulation in the lesions results in the formation of dark and light concentric zones. Lesions may coalesce and girdle the leaf, causing wilting of distal tissues. After 3-4 weeks the leaves turn yellow and collapse. Under conditions unfavourable for symptom development, lesions may remain as small white flecks.
Lesions may also form on seed stalks and floral parts. In severe cases the seed stalks may be girdled, destroying the stalks before seeds are mature.
Bulbs may also be attacked, mainly at the neck, resulting in the presence of a yellow to reddish watery rot. The scales later become desiccated and dark although often only one or two outer scales are affected.
The disease is most severe when frequent rain or persistent dews occur during moderate (25 to 28°C) temperatures. Almost no infection occurs below 12°C. The fungus is disseminated within and among fields by splashing water and wind, and overwinters in and on infested crop debris. The pathogen may also be seed-borne.