Department of Plant Pathology, University of Florida, 1453 Fifield Hall, Gainesville 32611
The effects of rust (caused by Uromyces appendiculatus) and anthracnose (caused by Colletotrichum lindemuthianum) and their interaction on the photosynthetic rates of healthy and diseased bean (Phaseolus vulgaris) leaves were determined by gas-exchange analysis, in plants with each disease, grown under controlled conditions. The equation Px/P0 = (1 - x)β was used to relate relative photosynthetic rate (Px/P0) to proportional disease severity (x), where β represents the ratio between virtual and visual lesion. The β values obtained for rust were near one, indicating that the effect of the pathogen on the remaining green leaf area was minimal. The high values of β obtained for anthracnose (8.46 and 12.18) indicated that the photosynthesis in the green area beyond the necrotic symptoms of anthracnose was severely impaired. The impact of anthracnose on bean leaf photosynthesis should be considered in assessments of the proportion of healthy tissue in diseased leaves. The accurate assessment of the healthy portion of the leaf could improve the use of concepts such as healthy leaf area duration and healthy leaf area absorption, which are valuable predictors of crop yield. The equation used to analyze the interaction between rust and anthracnose on the same leaf was Pz = P0 (1 - x)βx × (1 - y)βy, where Pz is the relative photosynthetic rate of any given leaf, P0 is the maximum relative photosynthetic rate, x is anthracnose severity, y is rust severity, βx is the β value for anthracnose in the presence of rust, and βy is the β value for rust in the presence of anthracnose. From the resulting response surface, no interaction of the two diseases was observed. Dark respiration rate increased on diseased leaves compared with control leaves. The remaining green leaf area of leaves with both diseases was not a good source to estimate net photosynthetic rate because the effect of anthracnose extended far beyond the visual lesions, whereas the effect of rust on photosynthesis was essentially limited to the pustule plus halo.