**Tamil Nadu Agricultural University, Madurai.
***Horticultural Research Station, Periyakulam-626501.
T. N. Agricultural University, Coimbatore
*Formed part of thesis approved by Tamil Nadu Agricultural University for the award of M. Sc. (Ag) degree in plant Pathology to the senior author.
The rate of respiration in downy mildew infected grape leaves of grades 1 and 2 was higher than in healthy leaves. After that, it was less than that of healthy leaves. Among infected leaves of three colours the maximum increase in oxygen uptake was in viridian green leaves and least increase was in emerald green leaves. In infected leaves the contents of reducing, non-reducing and total sugars were reduced. The maximum reduction was found to be in viridian green leaves. Among the respiratory enzymes, activity of catalase was reduced while that of ascorbic acid oxidase increased in infected leaves. The maximum reduction of catalase activity was in leaves showing infection grade 1. In infection of grade 3 the activity of peroxidase was less than in healthy leaves but in infection grades 1 and 2 it was higher than in healthy leaves.
