COMPONENTS OF MARITIME PINE SUSCEPTIBILITY TO TWISTING RUST - A PATH COEFFICIENT ANALYSIS

Data collected in a three-year field experiment on infection and damag e by Melampsora pinitorqua on maritime pines showing contrasting growt h were analysed by means of path analysis. The severity of infection i n 2 year-old pines could be mainly explained by the amount of suscepti ble tissues (elongating shoots). In contrast, two years later, 'intrin sic susceptibility', i.e. expressed after inoculation, was shown to ha ve the most important direct effect on the amount of infection. Intrin sic susceptibility could be satisfactorily assessed by artificial inoc ulations on excised shoots under controlled conditions. Shoot potassiu m and phosphorous contents were shown to have marked positive effects on susceptibility. The hypothesis of an age effect on the relative imp ortance of different resistance mechanisms, i.e. avoidance and active resistance, in maritime pine interaction with twisting rust is discuss ed.