Etiology and control of purple blotch in onions (Alliunl Cepa l) caused by Alternaria Porrl
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Investigations on J\lternaria porri pathogen with respect to the effects of culture media. temperatures and light regimes on growth and sporulation commenced after collection of the isolates from Kabet e field station. University of Nairobi. Njabini Farmer's Training Station (South Kinangop) and Kenya Agricultural Research Institute (KARl) experimental station at Oljorok (Nyandarua district). Onion seeds obtained from different locations were assessed for infection levels of the seed borne Alternaria porri and the incidence of the infection on seed germination and losses. Seed drcssrnu chemicals (Thiram and Captan) and foliar fungicides (dithane M45. c1aconil 2787. ridomil MZ and kocide 101) were tested for control or the disease. A. porti was found to grow well in most media rich in nutrients although best growth occured in host decoction agar. Generally growth of the pa thogen was not affected by light. while temperature did. with best growth occuring at 240C. During growth. characteristic violacious pigmentation was evident. The pathogen could not sporulate easily in artificlal conditions. Nevertheless. the host decoctions (onion decoctions) favoured the best growth and sporulation. The pathogen required low temperatures (below 270C) for maximum sporulation. Light was also essential during coniclial formation at the incubation period of 10 to 16 days with maximum conidial production occuring at the 13th day. During seed health testing. the level of seed infection was influenced by the variety. location from where the seeds were grown or obtained and also by the testing (incubation) method used. Bombay Red recorded the highest percentage infection while I\:ON 3 the least infection irrespective of the location. Seeds obtained from KalJete and Marigat locations had high levels of infection compared to seeds from Njabini location which had low infection level. Both freezing and agar plate methods proved to be suitable incubation procedures favouring sporulation and easier identification of fungus in seeds respectively. Seeds from Njabtnl location recorded the highest germination percentage compared to the other locations. Exotic cultivars compared to local checks were superior in glasshouse germination and emergence although the difference with local check Red Creole was small. Kabete and Martga t locations produced seeds which gave the lowest percentage glass house germination and emergence. Thrrarn proved to be better than cap tan in reduction of the seedborne inoculum. although both were effective and improved yields considerably. Dithane M45 and daconil 2787 effectively controlled onion purple blotch while kocide 101 and ridomil MZ proved to be less effective. In conclusion, A. POTTi pathogen should be cultured on onion decoction agar at 180C to 240C under 12h. light/12h dark for 10 - 13 days. Seed dressing prior to planting with th iram or captan and subsequent foliar spraying with clithane M45 or daconil 2787 at the same rate of 50 g/20 litres at weekly intervals would greatly improve the bulb yield.