The effect of leaf necrosis caused by Ascochyta caulina (P. Karst.) v.d. Aa & v Kest. on photosynthesis of leaves of Chenopodium album L. was studied in growth chambers. Fully expanded leaves of young plants were sprayed with a suspension of pycnidiospores, or with a spore-free control solution. Subsequently, leaves were exposed to high humidity for 6, 18, or 24 h to promote different levels of necrosis. Individual leaves were assessed for proportion of necrotic leaf area and net photosynthetic rate (npr) 3, 4 and 7 d after treatment. The npr of leaves decreased with increasing proportion of necrotic leaf area. The relationship between the two was linear in five of six regression analyses carried out. The observed linearity indicates that necrotic leaf tissue hardly affected the surrounding healthy leaf tissue. The proportion of necrotic leaf area that caused a npr of zero was on average 0.4, ranging from 0.2 to 0.5.
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