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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 (Nolla, 1927; Ellis and Holliday, 1970; Miller and Lacy, 1995).
Optimal timing of sowing or transplanting can minimize purple blotch attack by A. porri, depending on the local environmental conditions (Awad et al., 1978; Sandhu et al., 1982a; Gupta and Pathak, 1987; Martinez-Reyes et al., 1987).
Increased ploughing between seasons reduced the disease, probably by reducing the amount of soilborne inoculum (Gupta and Pathak, 1987). Increased spacing between plants also reduced disease development (Awad et al., 1978; Sandhu et al., 1982a).
Higher doses of nitrogen and phosphate increased the number of leaves and decreased the amount of disease (Sandhu et al., 1982a; Mondal et al., 1989).
Sandhu et al. (1982a) noted a lower disease incidence in onions grown from seeds than from sets.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Disease caused by A. porri is most severe in areas with hot, humid climates (Miller and Lacy, 1995). In south Texas, USA, purple blotch causes major foliage losses in onion (Miller, 1983). Sandhu et al. (1981) reported more than 50% yield loss in onion seed crops in Punjab, India. In field trials, Gupta and Pathak (1988c) obtained 59.1% reduction of seed yield and 504% reduction of bulb yield after artificial inoculation.