Changes in pigment composition, acid metabolism, etc. In pedilanthus tithymaloides leaf following powdery mildew infection

Abstract

The development of powdery mildew in the leaf of Pedilanthus tithymaloides var. caculatus by Oidium mangiferae and associated changes in the host were studied. The disease development occurred through a series of distinct changes in symptoms. Initial whitish, powdery patches of mildew infections became gradually colonized by a mycoparasite, Ampelomyces quisqualis (Syn. Cicinnobolus) giving the affected areas a greyish appearance. Further advancement of the disease rendered the tissue chlorotic which subsequently transformed into a bright pinkish-red colour. Towards the end of the dry season, certain infected areas became corky and suberized. Higher rainfall reduced both the intensity of powdery mildew and the growth of the mycoparasite. Typical powdery mildew symptoms were induced by artificial inoculation of young leaves with conidia of O. mangiferae. A significant alteration of leaf pigment composition occurred when the infected areas turned to a pinkish-red colour. These areas lacked chlorophyll-a but contained other photosynthetic pigments plus several additional anthocyanins that were absent in healthy tissues. The healthy leaves of P. tithymaloides showed a clear pattern of diurnal fluctuation of acidity and stomatal aperture on a scale similar to a known CAM plant, Kalanchoe sp. The major acid was found to be malic. The infection by O. mangiferae, however, reduced the morning acidity in the leaf by about 35% and altered the normal acid fluctuation pattern. The overall results indicated that powdery mildew infection induces a range of disturbances to leaf metabolism, the most obvious being the activation of synthesis of new anthocyanins, degradation of chlorophyll-a and disturbance to malic acid metabolism.