Evaluation of resistance inducer effect as a substitution of fungicide for downy mildew control in onion genotypes

Using plant resistance inducers leads to the reduction of disease incidence and severity. The effect of several resistance inducer compounds on onion downy mildew disease was tested in a factorial experiment with a randomized complete block design comprising of 10 treatments in 3 replications at Behbahan Agricultural Research Station. Treatments consisted of chemical indicers in five levels of 50mμ dipotassium phosphate, 80mμ nicotinic acid and 3mμ salicylic acid, and ridomil mancozeb fungicide (3g/L) and control that treated with water. Treatments were applied 5 times from mid-February when the plants were in 3 to 4 leaf stage until late April every two weeks. Genotypes were in two levels including modified onion of Behbahan and Early White Texas. Disease severity was calculated by evaluating 40 plants in each treatment when the disease index in the control treatment was more than 75% in two middle rows of each plot in each treatment. The highest percentage of disease severity (75.50%) was observed in the control treatment with no spraying and the lowest rate of disease severity was observed in the application of Ridomil with 14.17%. The severity of the disease was 28.33%, 33.33% and 23.33% in dipotassium phosphate, salicylic acid and nicotinic acid treatments, respectively. Therefore, the use of resistance inducers can be a substitute of fungicides.