Role of the auxin, Naphthalene Acetic Acid (NAA) in the pathogenicity of Sclerotinia sclerotiorum, causative organism of white mold disease

The effect of Naphthalene Acetic Acid (NAA) on white mold disease was investigated by measuring the mycelium growth rate, and sclerotia weight and number formed by the disease. For this, causative organism Sclerotinia sclerotiorum was grown on Potato Dextrose Agar (PDA) amended with different concentrations of NAA. This effect was also investigated on white mold lesion growth rate on bean and cucumber detached leaves, and the devlopment of disease severity on bean and cucumber plants. Results showed that NAA has an inhibition effect on the fungus in vitro and it has reduced the weight and the number of sclerotia formation. Consequently, at 200 g/ml, the mean of mycelium growth rate of three isolate was reduced to 0.4 cm/day (=2.9% of mycelium growth rate of control). NAA significantly (P<0.05) reduced development of white mold lesions on bean and cucumber detached leaves pretreated concentrations up to 600 g/ml. Also, it has reduced white mold disease severity on bean and cucumber plants at concentrations up to 500 g/ml. This effect may be attributed to short-lived partial resistance induced in plant tissues treated with NAA.