Effect of partially UV-blocking films on purple blotch of onion

The purple blotch of onion is the most important disease in Bangladesh as well as worldwide. UV-blocking films and pathogens were used in this experiment to testify the ability of UV radiations to suppress the purple blotch of onion disease development. Disease incidence and disease severity were measured from the pathogens, temperature, relative humidity, partially UV-blocking conditions. The mini houses were constructed and covered with four types of polyolefin films that have the ability to block solar UV-irradiation shorter than UV-A of (<400 nm), (<360 nm), (<350 nm) and (<340 nm) and the results were compared with UV transmitting and outdoors. Different pathogenic combination of Alternaria porri + Stemphylium vasicarium, A. porri and S. vasicarium was also inoculated to find out the pathogenic effect. The lowest disease incidence (43.2%) and severity (2.4%) was recorded in T6 (outdoors) and highest disease incidence (83.6%) and severity (41.8%) was recorded in T1 (<400 nm) under different UV treatments. The maximum incidence (80.5%) and severity (39.2%) was recorded in P1 (A. porri + S. vasicarium) and the minimum incidence (48.9%) and severity (4.7%) was recorded in P3 (S. vasicarium) under pathogen treatment. The highest incidence (91.6%) and severity (44.3%) was recorded in treatment T1P1 at 21 days after incubation (DAI) and the lowest incidence (34.7%) and severity (1.3%) was recorded in treatment T6P3 under combination of UV and pathogen treatments. Disease incidence and severity gradually decreased as the UV blocking rates decreased. Solar radiation which contains combination of different UV radiation intensity may be detrimental for multiplication of the pathogens