SHTIENBERG, D., Department of Plant Pathology and Microbiology, Hebrew University of Jerusalem, Faculty of Agriculture, PO Box 12, Rehovot, 76100, Israel

The effects of Septoria leaf blotch (caused by Mycosphaerella graminicola) and leaf rust (caused by Puccinia recondita) on the physiological processes affecting yield were studied under semi‐arid conditions in Israel. In the absence of water stress, photosynthetic activity decreased as the rate of disease development increased. Transpirational activity was reduced in leaves infected by Septoria leaf blotch, but intensified in leaves with slight leaf rust infections (0.5‐5% severity). When leaf rust seventy exceeded 5%, the transpirational water loss declined. Both diseases were associated with a decrease in the maximum rate of grain‐weight accumulation, but did not affect the duration of weight accumulation. An inverse linear relationship between disease and final grain weight was observed. Under conditions of water stress, photosynthetic activity of leaves slightly infected by Septoria leaf blotch (1‐5”., severity) was greater than that of uninfected leaves. Severely infected leaves (” 25% severity) photosynthesized the least. Both diseases were associated with a decrease in the maximum rate of grain weight accumulation, but the duration of accumulation was longer in tillers with a slow rate of disease development than in uninfected tillers. The relationship between disease and final grain weight in these cases was parabolic; yield was increased at low levels of infection. Theoretical explanations and practical implications of these phenomena are presented and discussed. Copyright © 1991, Wiley Blackwell. All rights reserved