Salicylic Acid and Acetylsalicylic Induced Resistance to Powdery Mildew Disease of Milk thistle (Syllibium marianum)

Microscopic examination showed that Erysiphe cichoracaerum is the causal agent of Milk thistle powdery mildew and its symptoms begin to appear in the middle of March. Plants sprayed with Neem (5%) had no effect on the percentage of chlorophyll, but reduced the severity percentage infection to 2.2, 50.0% respectively. The use of different concentrations of salicylic acid (SA) acetylsalicylic acid (ASA) led to an increase in wet and dry weight and higher plants hieght. One spray with 1 mmole ASA increased the wet weight to10.9 g. In the meantime, the dry weight differed significantly from other treatments except of 1 mmole SA once spray. The better concentration was (24%) for increasing the percentage of chlorophyll in infected plants with powdery mildew was observed with 5 mmole SA once spray and significantly different from the rest of the treatments other than 5 mmol SA sprayed twice which was 21%. The concentration of 1 mmol. SA also was found the best in inducing resistance resulting in a reduction of the infection severity to 0.2 and percentage of infection severity to 95%. Spraying of Milk thistle plants with SA increased the activity of peroxidase enzyme (60%) in plants sprayed twice with SA 5 m mole (0.089) comparing to the control (0.053). Also, the enzyme activity of phenylalanine ammonia lyase was increased (0.618) in plants sprayed twice with 5 mmole SA (53%) and hence significantly different from other treatments other than SA 10 mmole sprayed twice.