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Bastiaans, L. 1991. Ratio between virtual and visual lesion size as a measure to describe reduction in leaf photosynthesis of rice due to leaf blast. Phytopathology 81:611-615.The effect of Pyricularia oryzae, the causal organism of blast in rice, on net photosynthetic rate of rice leaves was measured in the field and in a greenhouse experiment. Leaf blast reduced photosynthesis not only through a reduction in green leaf area, but also through an effect on photosynthesis of the remaining green leaf tissue. A function was derived to relate the net photosynthetic rate of diseased leaf area (Px) to the photosynthesis of comparable healthy leaf area (P 0 ) and disease severity (x): Px = P 0 (1 -x) 13 • This function is based on the assumption that Additional keywords: Oryza sativa.After infection of rice leaves with Pyricularia oryzae Cavara, the causal organism of blast in rice ( Oryza sativa L.), ellipticalshaped lesions appear on leaves of susceptible cultivars. Goto (6) reported that yield loss due to leaf blast exceeded yield loss caused by cutting off of a percentage of leaf area equal to the percentage leaf covered by blast. This extra reduction meant that leaf blast influenced the host plant more than just the loss in leaf area. For various pathosystems, an effect of the pathogen on photosynthesis of the remaining green leaf area has been reported (for example, Erysiphe graminis on wheat [18] and Alternaria alternata on cotton [5]). Pathosystems in which the pathogen did not impair photosynthesis of healthy leaf tissue also have been reported (for example, Puccinia recondita on wheat [19] and Phytophthora infestans on potato [22]). Obviously, the effect of a pathogen on host photosynthesis varies according to the pathosystem under consideration.As far as the effect of P. oryzae on photosynthesis of rice is concerned, only observations on canopy photosynthesis of seedlings have been reported. Burrell and Rees (3) and Padhi et al (16) observed a marked reduction in canopy photosynthesis of inoculated seedlings when compared with the performance of healthy plants. Whether the reduction was solely attributable to a reduction in green leaf area was not determined.In this study, the effect of P. oryzae on leaf photosynthesis was measured, and a function was derived to describe the dependence of leaf photosynthetic rate on the fraction of leaf area with blast lesions. MATERIALS AND METHODSIn an experimental field at the International Rice Research Institute, Los Banos, The Philippines, net rates of leaf photosynthesis were measured to determine the effect of leaf blast on photosynthesis of rice leaves. Similar observations were made © 1991 The American Phytopathological Society the visual lesion is part of a virtual lesion in which photosynthesis is negligible. The parameter f3 expresses the ratio between virtual and visual lesion size and characterizes the effect of the pathogen on leaf photosynthesis for the entire range of measured disease severities. A value of f3 between 3 and 4 gave a good descript...

Mathematical modelling is a commonly used tool for studying the long-term dynamics of weed populations in agriculture. This was reflected in our review by the large number of scientific papers (134 original publications) and the continuing need to gain an overview over this fast developing field (20 previous review papers were found). In this article, we provide a more comprehensive review than earlier seen, striving to include all relevant publications. Thus, we cover models of the population dynamics of 60 weed species in 40 crops. An online, accompanying database provides an indexed bibliography. Despite the large variation in crops, weeds and geography, the models were surprisingly similar in their approach: structured around the weed life cycle, excluding environmental factors and giving little attention to validation or even documentation of model construction. In addition, their application was similar, limited mostly to strategic decision making. We hope that the overview provided by this review will inspire weed modellers and that it will serve as a basis for discussion and as a frame of reference when we proceed to advance the modelling of weed populations to a new level, developing new approaches and tackling new application domains.

SummaryDespite increased concerns regarding the heavy reliance of many cropping systems on chemical weed control, adoption of ecological weed management practices is only steadily progressing. For this reason, this paper reflects on both the possibilities and limitations of cultural weed control practices. Cultural weed control utilises a number of principles, predominantly: (i) a reduced recruitment of weed seedlings from the soil seedbank, (ii) an alteration of crop-weed competitive relations to the benefit of the crop and (iii) a gradual reduction of the size of the weed seedbank. Compared with chemical control, the general applicability, reliability and efficacy of most measures is only moderate, and consequently, cultural control strategies need to consist of a combination of measures, resulting in increased systems complexity. Combined with the trade-offs connected to some of the measures, this hampers large-scale implementation. It is argued that tailoring cultural weed management strategies to the needs and skills of individual farmers would be an important step forward. Research can aid in improving the utilisation of cultural weed control strategies by focussing on a broadening of the range of available measures and by providing clear quantitative insight in efficacy, variability in outcome and trade-offs of these measures.